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Stem cells: what they are and what they do.

Stem cells offer promise for new medical treatments. Learn about stem cell types, current and possible uses, and the state of research and practice.

You've heard about stem cells in the news, and perhaps you've wondered if they might help you or a loved one with a serious disease. Here are some answers to frequently asked questions about stem cells.

What are stem cells?

Stem cells: The body's master cells

Stem cells are the body's master cells. All other cells arise from stem cells, including blood cells, nerve cells and other cells.

Stem cells are a special type of cells that have two important properties. They are able to make more cells like themselves. That is, they self-renew. And they can become other cells that do different things in a process known as differentiation. Stem cells are found in almost all tissues of the body. And they are needed for the maintenance of tissue as well as for repair after injury.

Depending on where the stem cells are, they can develop into different tissues. For example, hematopoietic stem cells reside in the bone marrow and can produce all the cells that function in the blood. Stem cells also can become brain cells, heart muscle cells, bone cells or other cell types.

There are various types of stem cells. Embryonic stem cells are the most versatile since they can develop into all the cells of the developing fetus. The majority of stem cells in the body have fewer abilities to give rise to cells and may only help maintain and repair the tissues and organs in which they reside.

No other cell in the body has the natural ability to generate new cell types.

Why is there such an interest in stem cells?

Researchers are studying stem cells to see if they can help to:

• Increase understanding of how diseases occur. By watching stem cells mature into cells in bones, heart muscle, nerves, and other organs and tissue, researchers may better understand how diseases and conditions develop.

Generate healthy cells to replace cells affected by disease (regenerative medicine). Stem cells can be guided into becoming specific cells that can be used in people to regenerate and repair tissues that have been damaged or affected by disease.

People who might benefit from stem cell therapies include those with leukemia, Hodgkin disease, non-Hodgkin lymphoma and some solid tumor cancers. Stem cell therapies also might benefit people who have aplastic anemia, immunodeficiencies and inherited conditions of metabolism.

Stem cells are being studied to treat type 1 diabetes, Parkinson's disease, amyotrophic lateral sclerosis, heart failure, osteoarthritis and other conditions.

Stem cells may have the potential to be grown to become new tissue for use in transplant and regenerative medicine. Researchers continue to advance the knowledge on stem cells and their applications in transplant and regenerative medicine.

Test new drugs for safety and effectiveness. Before giving drugs in development to people, researchers can use some types of stem cells to test the drugs for safety and quality. This type of testing may help assess drugs in development for toxicity to the heart.

New areas of study include the effectiveness of using human stem cells that have been programmed into tissue-specific cells to test new drugs. For the testing of new drugs to be accurate, the cells must be programmed to acquire properties of the type of cells targeted by the drug. Techniques to program cells into specific cells are under study.

Where do stem cells come from?

There are several sources of stem cells:

Embryonic stem cells. These stem cells come from embryos that are 3 to 5 days old. At this stage, an embryo is called a blastocyst and has about 150 cells.

These are pluripotent (ploo-RIP-uh-tunt) stem cells, meaning they can divide into more stem cells or can become any type of cell in the body. This allows embryonic stem cells to be used to regenerate or repair diseased tissue and organs.

• Adult stem cells. These stem cells are found in small numbers in most adult tissues, such as bone marrow or fat. Compared with embryonic stem cells, adult stem cells have a more limited ability to give rise to various cells of the body.

Adult cells altered to have properties of embryonic stem cells. Scientists have transformed regular adult cells into stem cells using genetic reprogramming. By altering the genes in the adult cells, researchers can make the cells act similarly to embryonic stem cells. These cells are called induced pluripotent stem cells (iPSCs).

This new technique may allow use of reprogrammed cells instead of embryonic stem cells and prevent immune system rejection of the new stem cells. However, scientists don't yet know whether using altered adult cells will cause adverse effects in humans.

Researchers have been able to take regular connective tissue cells and reprogram them to become functional heart cells. In studies, animals with heart failure that were injected with new heart cells had better heart function and survival time.

Perinatal stem cells. Researchers have discovered stem cells in amniotic fluid as well as umbilical cord blood. These stem cells can change into specialized cells.

Amniotic fluid fills the sac that surrounds and protects a developing fetus in the uterus. Researchers have identified stem cells in samples of amniotic fluid drawn from pregnant women for testing or treatment — a procedure called amniocentesis.

Why is there controversy about using embryonic stem cells?

The National Institutes of Health created guidelines for human stem cell research in 2009. The guidelines define embryonic stem cells and how they may be used in research and include recommendations for the donation of embryonic stem cells. Also, the guidelines state that embryonic stem cells from embryos created by in vitro fertilization can be used only when the embryo is no longer needed.

Where do these embryos come from?

The embryos being used in embryonic stem cell research come from eggs that were fertilized at in vitro fertilization clinics but never implanted in women's uteruses. The stem cells are donated with informed consent from donors. The stem cells can live and grow in special solutions in test tubes or petri dishes in laboratories.

Why can't researchers use adult stem cells instead?

Progress in cell reprogramming and the formation of iPSCs has greatly enhanced research in this field. However, reprogramming is an inefficient process. When possible, iPSCs are used instead of embryonic stem cells since this avoids the ethical issues about use of embryonic stem cells that may be morally objectionable for some people.

Although research into adult stem cells is promising, adult stem cells may not be as versatile and durable as are embryonic stem cells. Adult stem cells may not be able to be manipulated to produce all cell types, which limits how adult stem cells can be used to treat diseases.

Adult stem cells are also more likely to contain irregularities due to environmental hazards, such as toxins, or from errors acquired by the cells during replication. However, researchers have found that adult stem cells are more adaptable than was first thought.

What are stem cell lines, and why do researchers want to use them?

A stem cell line is a group of cells that all descend from a single original stem cell and are grown in a lab. Cells in a stem cell line keep growing but don't become specialized cells. Ideally, they remain free of genetic defects and continue to create more stem cells. Clusters of cells can be taken from a stem cell line and frozen for storage or shared with other researchers.

What is stem cell therapy (regenerative medicine), and how does it work?

Stem cell therapy, also known as regenerative medicine, promotes the repair response of diseased, dysfunctional or injured tissue using stem cells or their derivatives. It is the next chapter in organ transplantation and uses cells instead of donor organs, which are limited in supply.

Researchers grow stem cells in a lab. These stem cells are manipulated to specialize into specific types of cells, such as heart muscle cells, blood cells or nerve cells.

The specialized cells can then be implanted into a person. For example, if the person has heart disease, the cells could be injected into the heart muscle. The healthy transplanted heart muscle cells could then contribute to repairing the injured heart muscle.

Researchers have already shown that adult bone marrow cells guided to become heart-like cells can repair heart tissue in people, and more research is ongoing.

Have stem cells already been used to treat diseases?

Yes. Doctors have performed stem cell transplants, also known as bone marrow transplants, for many decades. In hematopoietic stem cell transplants, stem cells replace cells damaged by chemotherapy or disease or serve as a way for the donor's immune system to fight some types of cancer and blood-related diseases. Leukemia, lymphoma, neuroblastoma and multiple myeloma often are treated this way. These transplants use adult stem cells or umbilical cord blood.

Researchers are testing adult stem cells to treat other conditions, including some degenerative diseases such as heart failure.

What are the potential problems with using embryonic stem cells in humans?

For embryonic stem cells to be useful, researchers must be certain that the stem cells will differentiate into the specific cell types desired.

Researchers have discovered ways to direct stem cells to become specific types of cells, such as directing embryonic stem cells to become heart cells. Research is ongoing in this area.

Embryonic stem cells also can grow irregularly or specialize in different cell types spontaneously. Researchers are studying how to control the growth and development of embryonic stem cells.

Embryonic stem cells also might trigger an immune response in which the recipient's body attacks the stem cells as foreign invaders, or the stem cells might simply fail to function as expected, with unknown consequences. Researchers continue to study how to avoid these possible complications.

What is therapeutic cloning, and what benefits might it offer?

Therapeutic cloning, also called somatic cell nuclear transfer, is a way to create versatile stem cells independent of fertilized eggs. In this technique, the nucleus is removed from an unfertilized egg. This nucleus contains the genetic material. The nucleus also is removed from the cell of a donor.

This donor nucleus is then injected into the egg, replacing the nucleus that was removed, in a process called nuclear transfer. The egg is allowed to divide and soon forms a blastocyst. This process creates a line of stem cells that is genetically identical to the donor's cells — in essence, a clone.

Some researchers believe that stem cells derived from therapeutic cloning may offer benefits over those from fertilized eggs because cloned cells are less likely to be rejected once transplanted back into the donor. And it may allow researchers to see exactly how a disease develops.

Has therapeutic cloning in people been successful?

No. Researchers haven't been able to successfully perform therapeutic cloning with humans despite success in a number of other species.

Researchers continue to study the potential of therapeutic cloning in people.

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• Stem cell basics. National Institutes of Health. https://stemcells.nih.gov/info/basics/stc-basics/#stc-I. Accessed March 21, 2024.
• Lovell-Badge R, et al. ISSCR guidelines for stem cell research and clinical translation: The 2021 update. Stem Cell Reports. 2021; doi:10.1016/j.stemcr.2021.05.012.
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• Stem cell transplants in cancer treatment. National Cancer Institute. https://www.cancer.gov/about-cancer/treatment/types/stem-cell-transplant/. Accessed March 21, 2024.
• Townsend CM Jr, et al. Regenerative medicine. In: Sabiston Textbook of Surgery: The Biological Basis of Modern Surgical Practice. 21st ed. Elsevier; 2022. https://www.clinicalkey.com. Accessed March 21, 2024.
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• NIH guidelines for human stem cell research. National Institutes of Health. https://stemcells.nih.gov/research-policy/guidelines-for-human-stem-cell-research. Accessed March 21, 2024.
• De la Torre P, et al. Current status and future prospects of perinatal stem cells. Genes. 2020; doi:10.3390/genes12010006.
• Yen Ling Wang A. Human induced pluripotent stem cell-derived exosomes as a new therapeutic strategy for various diseases. International Journal of Molecular Sciences. 2021; doi:10.3390/ijms22041769.
• Alessandrini M, et al. Stem cell therapy for neurological disorders. South African Medical Journal. 2019; doi:10.7196/SAMJ.2019.v109i8b.14009.
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• Open access
• Published: 26 February 2019

Stem cells: past, present, and future

• Wojciech Zakrzewski 1 ,
• Maciej Dobrzyński 2 ,
• Maria Szymonowicz 1 &
• Zbigniew Rybak 1  

Stem Cell Research & Therapy volume  10 , Article number:  68 ( 2019 ) Cite this article

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In recent years, stem cell therapy has become a very promising and advanced scientific research topic. The development of treatment methods has evoked great expectations. This paper is a review focused on the discovery of different stem cells and the potential therapies based on these cells. The genesis of stem cells is followed by laboratory steps of controlled stem cell culturing and derivation. Quality control and teratoma formation assays are important procedures in assessing the properties of the stem cells tested. Derivation methods and the utilization of culturing media are crucial to set proper environmental conditions for controlled differentiation. Among many types of stem tissue applications, the use of graphene scaffolds and the potential of extracellular vesicle-based therapies require attention due to their versatility. The review is summarized by challenges that stem cell therapy must overcome to be accepted worldwide. A wide variety of possibilities makes this cutting edge therapy a turning point in modern medicine, providing hope for untreatable diseases.

Stem cell classification

Stem cells are unspecialized cells of the human body. They are able to differentiate into any cell of an organism and have the ability of self-renewal. Stem cells exist both in embryos and adult cells. There are several steps of specialization. Developmental potency is reduced with each step, which means that a unipotent stem cell is not able to differentiate into as many types of cells as a pluripotent one. This chapter will focus on stem cell classification to make it easier for the reader to comprehend the following chapters.

Totipotent stem cells are able to divide and differentiate into cells of the whole organism. Totipotency has the highest differentiation potential and allows cells to form both embryo and extra-embryonic structures. One example of a totipotent cell is a zygote, which is formed after a sperm fertilizes an egg. These cells can later develop either into any of the three germ layers or form a placenta. After approximately 4 days, the blastocyst’s inner cell mass becomes pluripotent. This structure is the source of pluripotent cells.

Pluripotent stem cells (PSCs) form cells of all germ layers but not extraembryonic structures, such as the placenta. Embryonic stem cells (ESCs) are an example. ESCs are derived from the inner cell mass of preimplantation embryos. Another example is induced pluripotent stem cells (iPSCs) derived from the epiblast layer of implanted embryos. Their pluripotency is a continuum, starting from completely pluripotent cells such as ESCs and iPSCs and ending on representatives with less potency—multi-, oligo- or unipotent cells. One of the methods to assess their activity and spectrum is the teratoma formation assay. iPSCs are artificially generated from somatic cells, and they function similarly to PSCs. Their culturing and utilization are very promising for present and future regenerative medicine.

Multipotent stem cells have a narrower spectrum of differentiation than PSCs, but they can specialize in discrete cells of specific cell lineages. One example is a haematopoietic stem cell, which can develop into several types of blood cells. After differentiation, a haematopoietic stem cell becomes an oligopotent cell. Its differentiation abilities are then restricted to cells of its lineage. However, some multipotent cells are capable of conversion into unrelated cell types, which suggests naming them pluripotent cells.

Oligopotent stem cells can differentiate into several cell types. A myeloid stem cell is an example that can divide into white blood cells but not red blood cells.

Unipotent stem cells are characterized by the narrowest differentiation capabilities and a special property of dividing repeatedly. Their latter feature makes them a promising candidate for therapeutic use in regenerative medicine. These cells are only able to form one cell type, e.g. dermatocytes.

Stem cell biology

A blastocyst is formed after the fusion of sperm and ovum fertilization. Its inner wall is lined with short-lived stem cells, namely, embryonic stem cells. Blastocysts are composed of two distinct cell types: the inner cell mass (ICM), which develops into epiblasts and induces the development of a foetus, and the trophectoderm (TE). Blastocysts are responsible for the regulation of the ICM microenvironment. The TE continues to develop and forms the extraembryonic support structures needed for the successful origin of the embryo, such as the placenta. As the TE begins to form a specialized support structure, the ICM cells remain undifferentiated, fully pluripotent and proliferative [ 1 ]. The pluripotency of stem cells allows them to form any cell of the organism. Human embryonic stem cells (hESCs) are derived from the ICM. During the process of embryogenesis, cells form aggregations called germ layers: endoderm, mesoderm and ectoderm (Fig.  1 ), each eventually giving rise to differentiated cells and tissues of the foetus and, later on, the adult organism [ 2 ]. After hESCs differentiate into one of the germ layers, they become multipotent stem cells, whose potency is limited to only the cells of the germ layer. This process is short in human development. After that, pluripotent stem cells occur all over the organism as undifferentiated cells, and their key abilities are proliferation by the formation of the next generation of stem cells and differentiation into specialized cells under certain physiological conditions.

Oocyte development and formation of stem cells: the blastocoel, which is formed from oocytes, consists of embryonic stem cells that later differentiate into mesodermal, ectodermal, or endodermal cells. Blastocoel develops into the gastrula

Signals that influence the stem cell specialization process can be divided into external, such as physical contact between cells or chemical secretion by surrounding tissue, and internal, which are signals controlled by genes in DNA.

Stem cells also act as internal repair systems of the body. The replenishment and formation of new cells are unlimited as long as an organism is alive. Stem cell activity depends on the organ in which they are in; for example, in bone marrow, their division is constant, although in organs such as the pancreas, division only occurs under special physiological conditions.

Stem cell functional division

Whole-body development.

During division, the presence of different stem cells depends on organism development. Somatic stem cell ESCs can be distinguished. Although the derivation of ESCs without separation from the TE is possible, such a combination has growth limits. Because proliferating actions are limited, co-culture of these is usually avoided.

ESCs are derived from the inner cell mass of the blastocyst, which is a stage of pre-implantation embryo ca. 4 days after fertilization. After that, these cells are placed in a culture dish filled with culture medium. Passage is an inefficient but popular process of sub-culturing cells to other dishes. These cells can be described as pluripotent because they are able to eventually differentiate into every cell type in the organism. Since the beginning of their studies, there have been ethical restrictions connected to the medical use of ESCs in therapies. Most embryonic stem cells are developed from eggs that have been fertilized in an in vitro clinic, not from eggs fertilized in vivo.

Somatic or adult stem cells are undifferentiated and found among differentiated cells in the whole body after development. The function of these cells is to enable the healing, growth, and replacement of cells that are lost each day. These cells have a restricted range of differentiation options. Among many types, there are the following:

Mesenchymal stem cells are present in many tissues. In bone marrow, these cells differentiate mainly into the bone, cartilage, and fat cells. As stem cells, they are an exception because they act pluripotently and can specialize in the cells of any germ layer.

Neural cells give rise to nerve cells and their supporting cells—oligodendrocytes and astrocytes.

Haematopoietic stem cells form all kinds of blood cells: red, white, and platelets.

Skin stem cells form, for example, keratinocytes, which form a protective layer of skin.

The proliferation time of somatic stem cells is longer than that of ESCs. It is possible to reprogram adult stem cells back to their pluripotent state. This can be performed by transferring the adult nucleus into the cytoplasm of an oocyte or by fusion with the pluripotent cell. The same technique was used during cloning of the famous Dolly sheep.

hESCs are involved in whole-body development. They can differentiate into pluripotent, totipotent, multipotent, and unipotent cells (Fig.  2 ) [ 2 ].

Changes in the potency of stem cells in human body development. Potency ranges from pluripotent cells of the blastocyst to unipotent cells of a specific tissue in a human body such as the skin, CNS, or bone marrow. Reversed pluripotency can be achieved by the formation of induced pluripotent stem cells using either octamer-binding transcription factor (Oct4), sex-determining region Y (Sox2), Kruppel-like factor 4 (Klf4), or the Myc gene

Pluripotent cells can be named totipotent if they can additionally form extraembryonic tissues of the embryo. Multipotent cells are restricted in differentiating to each cell type of given tissue. When tissue contains only one lineage of cells, stem cells that form them are called either called oligo- or unipotent.

iPSC quality control and recognition by morphological differences

The comparability of stem cell lines from different individuals is needed for iPSC lines to be used in therapeutics [ 3 ]. Among critical quality procedures, the following can be distinguished:

Short tandem repeat analysis—This is the comparison of specific loci on the DNA of the samples. It is used in measuring an exact number of repeating units. One unit consists of 2 to 13 nucleotides repeating many times on the DNA strand. A polymerase chain reaction is used to check the lengths of short tandem repeats. The genotyping procedure of source tissue, cells, and iPSC seed and master cell banks is recommended.

Identity analysis—The unintentional switching of lines, resulting in other stem cell line contamination, requires rigorous assay for cell line identification.

Residual vector testing—An appearance of reprogramming vectors integrated into the host genome is hazardous, and testing their presence is a mandatory procedure. It is a commonly used procedure for generating high-quality iPSC lines. An acceptable threshold in high-quality research-grade iPSC line collections is ≤ 1 plasmid copies per 100 cells. During the procedure, 2 different regions, common to all plasmids, should be used as specific targets, such as EBNA and CAG sequences [ 3 ]. To accurately represent the test reactions, a standard curve needs to be prepared in a carrier of gDNA from a well-characterized hPSC line. For calculations of plasmid copies per cell, it is crucial to incorporate internal reference gDNA sequences to allow the quantification of, for example, ribonuclease P (RNaseP) or human telomerase reverse transcriptase (hTERT).

Karyotype—A long-term culture of hESCs can accumulate culture-driven mutations [ 4 ]. Because of that, it is crucial to pay additional attention to genomic integrity. Karyotype tests can be performed by resuscitating representative aliquots and culturing them for 48–72 h before harvesting cells for karyotypic analysis. If abnormalities are found within the first 20 karyotypes, the analysis must be repeated on a fresh sample. When this situation is repeated, the line is evaluated as abnormal. Repeated abnormalities must be recorded. Although karyology is a crucial procedure in stem cell quality control, the single nucleotide polymorphism (SNP) array, discussed later, has approximately 50 times higher resolution.

Viral testing—When assessing the quality of stem cells, all tests for harmful human adventitious agents must be performed (e.g. hepatitis C or human immunodeficiency virus). This procedure must be performed in the case of non-xeno-free culture agents.

Bacteriology—Bacterial or fungal sterility tests can be divided into culture- or broth-based tests. All the procedures must be recommended by pharmacopoeia for the jurisdiction in which the work is performed.

Single nucleotide polymorphism arrays—This procedure is a type of DNA microarray that detects population polymorphisms by enabling the detection of subchromosomal changes and the copy-neutral loss of heterozygosity, as well as an indication of cellular transformation. The SNP assay consists of three components. The first is labelling fragmented nucleic acid sequences with fluorescent dyes. The second is an array that contains immobilized allele-specific oligonucleotide (ASO) probes. The last component detects, records, and eventually interprets the signal.

Flow cytometry—This is a technique that utilizes light to count and profile cells in a heterogeneous fluid mixture. It allows researchers to accurately and rapidly collect data from heterogeneous fluid mixtures with live cells. Cells are passed through a narrow channel one by one. During light illumination, sensors detect light emitted or refracted from the cells. The last step is data analysis, compilation and integration into a comprehensive picture of the sample.

Phenotypic pluripotency assays—Recognizing undifferentiated cells is crucial in successful stem cell therapy. Among other characteristics, stem cells appear to have a distinct morphology with a high nucleus to cytoplasm ratio and a prominent nucleolus. Cells appear to be flat with defined borders, in contrast to differentiating colonies, which appear as loosely located cells with rough borders [ 5 ]. It is important that images of ideal and poor quality colonies for each cell line are kept in laboratories, so whenever there is doubt about the quality of culture, it can always be checked according to the representative image. Embryoid body formation or directed differentiation of monolayer cultures to produce cell types representative of all three embryonic germ layers must be performed. It is important to note that colonies cultured under different conditions may have different morphologies [ 6 ].

Histone modification and DNA methylation—Quality control can be achieved by using epigenetic analysis tools such as histone modification or DNA methylation. When stem cells differentiate, the methylation process silences pluripotency genes, which reduces differentiation potential, although other genes may undergo demethylation to become expressed [ 7 ]. It is important to emphasize that stem cell identity, together with its morphological characteristics, is also related to its epigenetic profile [ 8 , 9 ]. According to Brindley [ 10 ], there is a relationship between epigenetic changes, pluripotency, and cell expansion conditions, which emphasizes that unmethylated regions appear to be serum-dependent.

hESC derivation and media

hESCs can be derived using a variety of methods, from classic culturing to laser-assisted methodologies or microsurgery [ 11 ]. hESC differentiation must be specified to avoid teratoma formation (see Fig.  3 ).

Spontaneous differentiation of hESCs causes the formation of a heterogeneous cell population. There is a different result, however, when commitment signals (in forms of soluble factors and culture conditions) are applied and enable the selection of progenitor cells

hESCs spontaneously differentiate into embryonic bodies (EBs) [ 12 ]. EBs can be studied instead of embryos or animals to predict their effects on early human development. There are many different methods for acquiring EBs, such as bioreactor culture [ 13 ], hanging drop culture [ 12 ], or microwell technology [ 14 , 15 ]. These methods allow specific precursors to form in vitro [ 16 ].

The essential part of these culturing procedures is a separation of inner cell mass to culture future hESCs (Fig.  4 ) [ 17 ]. Rosowski et al. [ 18 ] emphasizes that particular attention must be taken in controlling spontaneous differentiation. When the colony reaches the appropriate size, cells must be separated. The occurrence of pluripotent cells lasts for 1–2 days. Because the classical utilization of hESCs caused ethical concerns about gastrulas used during procedures, Chung et al. [ 19 ] found out that it is also possible to obtain hESCs from four cell embryos, leaving a higher probability of embryo survival. Additionally, Zhang et al. [ 20 ] used only in vitro fertilization growth-arrested cells.

Culturing of pluripotent stem cells in vitro. Three days after fertilization, totipotent cells are formed. Blastocysts with ICM are formed on the sixth day after fertilization. Pluripotent stem cells from ICM can then be successfully transmitted on a dish

Cell passaging is used to form smaller clusters of cells on a new culture surface [ 21 ]. There are four important passaging procedures.

Enzymatic dissociation is a cutting action of enzymes on proteins and adhesion domains that bind the colony. It is a gentler method than the manual passage. It is crucial to not leave hESCs alone after passaging. Solitary cells are more sensitive and can easily undergo cell death; collagenase type IV is an example [ 22 , 23 ].

Manual passage , on the other hand, focuses on using cell scratchers. The selection of certain cells is not necessary. This should be done in the early stages of cell line derivation [ 24 ].

Trypsin utilization allows a healthy, automated hESC passage. Good Manufacturing Practice (GMP)-grade recombinant trypsin is widely available in this procedure [ 24 ]. However, there is a risk of decreasing the pluripotency and viability of stem cells [ 25 ]. Trypsin utilization can be halted with an inhibitor of the protein rho-associated protein kinase (ROCK) [ 26 ].

Ethylenediaminetetraacetic acid ( EDTA ) indirectly suppresses cell-to-cell connections by chelating divalent cations. Their suppression promotes cell dissociation [ 27 ].

Stem cells require a mixture of growth factors and nutrients to differentiate and develop. The medium should be changed each day.

Traditional culture methods used for hESCs are mouse embryonic fibroblasts (MEFs) as a feeder layer and bovine serum [ 28 ] as a medium. Martin et al. [ 29 ] demonstrated that hESCs cultured in the presence of animal products express the non-human sialic acid, N -glycolylneuraminic acid (NeuGc). Feeder layers prevent uncontrolled proliferation with factors such as leukaemia inhibitory factor (LIF) [ 30 ].

First feeder layer-free culture can be supplemented with serum replacement, combined with laminin [ 31 ]. This causes stable karyotypes of stem cells and pluripotency lasting for over a year.

Initial culturing media can be serum (e.g. foetal calf serum FCS), artificial replacement such as synthetic serum substitute (SSS), knockout serum replacement (KOSR), or StemPro [ 32 ]. The simplest culture medium contains only eight essential elements: DMEM/F12 medium, selenium, NaHCO 3, l -ascorbic acid, transferrin, insulin, TGFβ1, and FGF2 [ 33 ]. It is not yet fully known whether culture systems developed for hESCs can be allowed without adaptation in iPSC cultures.

Turning point in stem cell therapy

The turning point in stem cell therapy appeared in 2006, when scientists Shinya Yamanaka, together with Kazutoshi Takahashi, discovered that it is possible to reprogram multipotent adult stem cells to the pluripotent state. This process avoided endangering the foetus’ life in the process. Retrovirus-mediated transduction of mouse fibroblasts with four transcription factors (Oct-3/4, Sox2, KLF4, and c-Myc) [ 34 ] that are mainly expressed in embryonic stem cells could induce the fibroblasts to become pluripotent (Fig.  5 ) [ 35 ]. This new form of stem cells was named iPSCs. One year later, the experiment also succeeded with human cells [ 36 ]. After this success, the method opened a new field in stem cell research with a generation of iPSC lines that can be customized and biocompatible with the patient. Recently, studies have focused on reducing carcinogenesis and improving the conduction system.

Retroviral-mediated transduction induces pluripotency in isolated patient somatic cells. Target cells lose their role as somatic cells and, once again, become pluripotent and can differentiate into any cell type of human body

The turning point was influenced by former discoveries that happened in 1962 and 1987.

The former discovery was about scientist John Gurdon successfully cloning frogs by transferring a nucleus from a frog’s somatic cells into an oocyte. This caused a complete reversion of somatic cell development [ 37 ]. The results of his experiment became an immense discovery since it was previously believed that cell differentiation is a one-way street only, but his experiment suggested the opposite and demonstrated that it is even possible for a somatic cell to again acquire pluripotency [ 38 ].

The latter was a discovery made by Davis R.L. that focused on fibroblast DNA subtraction. Three genes were found that originally appeared in myoblasts. The enforced expression of only one of the genes, named myogenic differentiation 1 (Myod1), caused the conversion of fibroblasts into myoblasts, showing that reprogramming cells is possible, and it can even be used to transform cells from one lineage to another [ 39 ].

Although pluripotency can occur naturally only in embryonic stem cells, it is possible to induce terminally differentiated cells to become pluripotent again. The process of direct reprogramming converts differentiated somatic cells into iPSC lines that can form all cell types of an organism. Reprogramming focuses on the expression of oncogenes such as Myc and Klf4 (Kruppel-like factor 4). This process is enhanced by a downregulation of genes promoting genome stability, such as p53. Additionally, cell reprogramming involves histone alteration. All these processes can cause potential mutagenic risk and later lead to an increased number of mutations. Quinlan et al. [ 40 ] checked fully pluripotent mouse iPSCs using whole genome DNA sequencing and structural variation (SV) detection algorithms. Based on those studies, it was confirmed that although there were single mutations in the non-genetic region, there were non-retrotransposon insertions. This led to the conclusion that current reprogramming methods can produce fully pluripotent iPSCs without severe genomic alterations.

During the course of development from pluripotent hESCs to differentiated somatic cells, crucial changes appear in the epigenetic structure of these cells. There is a restriction or permission of the transcription of genes relevant to each cell type. When somatic cells are being reprogrammed using transcription factors, all the epigenetic architecture has to be reconditioned to achieve iPSCs with pluripotency [ 41 ]. However, cells of each tissue undergo specific somatic genomic methylation. This influences transcription, which can further cause alterations in induced pluripotency [ 42 ].

Source of iPSCs

Because pluripotent cells can propagate indefinitely and differentiate into any kind of cell, they can be an unlimited source, either for replacing lost or diseased tissues. iPSCs bypass the need for embryos in stem cell therapy. Because they are made from the patient’s own cells, they are autologous and no longer generate any risk of immune rejection.

At first, fibroblasts were used as a source of iPSCs. Because a biopsy was needed to achieve these types of cells, the technique underwent further research. Researchers investigated whether more accessible cells could be used in the method. Further, other cells were used in the process: peripheral blood cells, keratinocytes, and renal epithelial cells found in urine. An alternative strategy to stem cell transplantation can be stimulating a patient’s endogenous stem cells to divide or differentiate, occurring naturally when skin wounds are healing. In 2008, pancreatic exocrine cells were shown to be reprogrammed to functional, insulin-producing beta cells [ 43 ].

The best stem cell source appears to be the fibroblasts, which is more tempting in the case of logistics since its stimulation can be fast and better controlled [ 44 ].

• Teratoma formation assay

The self-renewal and differentiation capabilities of iPSCs have gained significant interest and attention in regenerative medicine sciences. To study their abilities, a quality-control assay is needed, of which one of the most important is the teratoma formation assay. Teratomas are benign tumours. Teratomas are capable of rapid growth in vivo and are characteristic because of their ability to develop into tissues of all three germ layers simultaneously. Because of the high pluripotency of teratomas, this formation assay is considered an assessment of iPSC’s abilities [ 45 ].

Teratoma formation rate, for instance, was observed to be elevated in human iPSCs compared to that in hESCs [ 46 ]. This difference may be connected to different differentiation methods and cell origins. Most commonly, the teratoma assay involves an injection of examined iPSCs subcutaneously or under the testis or kidney capsule in mice, which are immune-deficient [ 47 ]. After injection, an immature but recognizable tissue can be observed, such as the kidney tubules, bone, cartilage, or neuroepithelium [ 30 ]. The injection site may have an impact on the efficiency of teratoma formation [ 48 ].

There are three groups of markers used in this assay to differentiate the cells of germ layers. For endodermal tissue, there is insulin/C-peptide and alpha-1 antitrypsin [ 49 ]. For the mesoderm, derivatives can be used, e.g. cartilage matrix protein for the bone and alcian blue for the cartilage. As ectodermal markers, class III B botulin or keratin can be used for keratinocytes.

Teratoma formation assays are considered the gold standard for demonstrating the pluripotency of human iPSCs, demonstrating their possibilities under physiological conditions. Due to their actual tissue formation, they could be used for the characterization of many cell lineages [ 50 ].

Directed differentiation

To be useful in therapy, stem cells must be converted into desired cell types as necessary or else the whole regenerative medicine process will be pointless. Differentiation of ESCs is crucial because undifferentiated ESCs can cause teratoma formation in vivo. Understanding and using signalling pathways for differentiation is an important method in successful regenerative medicine. In directed differentiation, it is likely to mimic signals that are received by cells when they undergo successive stages of development [ 51 ]. The extracellular microenvironment plays a significant role in controlling cell behaviour. By manipulating the culture conditions, it is possible to restrict specific differentiation pathways and generate cultures that are enriched in certain precursors in vitro. However, achieving a similar effect in vivo is challenging. It is crucial to develop culture conditions that will allow the promotion of homogenous and enhanced differentiation of ESCs into functional and desired tissues.

Regarding the self-renewal of embryonic stem cells, Hwang et al. [ 52 ] noted that the ideal culture method for hESC-based cell and tissue therapy would be a defined culture free of either the feeder layer or animal components. This is because cell and tissue therapy requires the maintenance of large quantities of undifferentiated hESCs, which does not make feeder cells suitable for such tasks.

Most directed differentiation protocols are formed to mimic the development of an inner cell mass during gastrulation. During this process, pluripotent stem cells differentiate into ectodermal, mesodermal, or endodermal progenitors. Mall molecules or growth factors induce the conversion of stem cells into appropriate progenitor cells, which will later give rise to the desired cell type. There is a variety of signal intensities and molecular families that may affect the establishment of germ layers in vivo, such as fibroblast growth factors (FGFs) [ 53 ]; the Wnt family [ 54 ] or superfamily of transforming growth factors—β(TGFβ); and bone morphogenic proteins (BMP) [ 55 ]. Each candidate factor must be tested on various concentrations and additionally applied to various durations because the precise concentrations and times during which developing cells in embryos are influenced during differentiation are unknown. For instance, molecular antagonists of endogenous BMP and Wnt signalling can be used for ESC formation of ectoderm [ 56 ]. However, transient Wnt and lower concentrations of the TGFβ family trigger mesodermal differentiation [ 57 ]. Regarding endoderm formation, a higher activin A concentration may be required [ 58 , 59 ].

There are numerous protocols about the methods of forming progenitors of cells of each of germ layers, such as cardiomyocytes [ 60 ], hepatocytes [ 61 ], renal cells [ 62 ], lung cells [ 63 , 64 ], motor neurons [ 65 ], intestinal cells [ 66 ], or chondrocytes [ 67 ].

Directed differentiation of either iPSCs or ESCs into, e.g. hepatocytes, could influence and develop the study of the molecular mechanisms in human liver development. In addition, it could also provide the possibility to form exogenous hepatocytes for drug toxicity testing [ 68 ].

Levels of concentration and duration of action with a specific signalling molecule can cause a variety of factors. Unfortunately, for now, a high cost of recombinant factors is likely to limit their use on a larger scale in medicine. The more promising technique focuses on the use of small molecules. These can be used for either activating or deactivating specific signalling pathways. They enhance reprogramming efficiency by creating cells that are compatible with the desired type of tissue. It is a cheaper and non-immunogenic method.

One of the successful examples of small-molecule cell therapies is antagonists and agonists of the Hedgehog pathway. They show to be very useful in motor neuron regeneration [ 69 ]. Endogenous small molecules with their function in embryonic development can also be used in in vitro methods to induce the differentiation of cells; for example, retinoic acid, which is responsible for patterning the nervous system in vivo [ 70 ], surprisingly induced retinal cell formation when the laboratory procedure involved hESCs [ 71 ].

The efficacy of differentiation factors depends on functional maturity, efficiency, and, finally, introducing produced cells to their in vivo equivalent. Topography, shear stress, and substrate rigidity are factors influencing the phenotype of future cells [ 72 ].

The control of biophysical and biochemical signals, the biophysical environment, and a proper guide of hESC differentiation are important factors in appropriately cultured stem cells.

Stem cell utilization and their manufacturing standards and culture systems

The European Medicines Agency and the Food and Drug Administration have set Good Manufacturing Practice (GMP) guidelines for safe and appropriate stem cell transplantation. In the past, protocols used for stem cell transplantation required animal-derived products [ 73 ].

The risk of introducing animal antigens or pathogens caused a restriction in their use. Due to such limitations, the technique required an obvious update [ 74 ]. Now, it is essential to use xeno-free equivalents when establishing cell lines that are derived from fresh embryos and cultured from human feeder cell lines [ 75 ]. In this method, it is crucial to replace any non-human materials with xeno-free equivalents [ 76 ].

NutriStem with LN-511, TeSR2 with human recombinant laminin (LN-511), and RegES with human foreskin fibroblasts (HFFs) are commonly used xeno-free culture systems [ 33 ]. There are many organizations and international initiatives, such as the National Stem Cell Bank, that provide stem cell lines for treatment or medical research [ 77 ].

Stem cell use in medicine

Stem cells have great potential to become one of the most important aspects of medicine. In addition to the fact that they play a large role in developing restorative medicine, their study reveals much information about the complex events that happen during human development.

The difference between a stem cell and a differentiated cell is reflected in the cells’ DNA. In the former cell, DNA is arranged loosely with working genes. When signals enter the cell and the differentiation process begins, genes that are no longer needed are shut down, but genes required for the specialized function will remain active. This process can be reversed, and it is known that such pluripotency can be achieved by interaction in gene sequences. Takahashi and Yamanaka [ 78 ] and Loh et al. [ 79 ] discovered that octamer-binding transcription factor 3 and 4 (Oct3/4), sex determining region Y (SRY)-box 2 and Nanog genes function as core transcription factors in maintaining pluripotency. Among them, Oct3/4 and Sox2 are essential for the generation of iPSCs.

Many serious medical conditions, such as birth defects or cancer, are caused by improper differentiation or cell division. Currently, several stem cell therapies are possible, among which are treatments for spinal cord injury, heart failure [ 80 ], retinal and macular degeneration [ 81 ], tendon ruptures, and diabetes type 1 [ 82 ]. Stem cell research can further help in better understanding stem cell physiology. This may result in finding new ways of treating currently incurable diseases.

Haematopoietic stem cell transplantation

Haematopoietic stem cells are important because they are by far the most thoroughly characterized tissue-specific stem cell; after all, they have been experimentally studied for more than 50 years. These stem cells appear to provide an accurate paradigm model system to study tissue-specific stem cells, and they have potential in regenerative medicine.

Multipotent haematopoietic stem cell (HSC) transplantation is currently the most popular stem cell therapy. Target cells are usually derived from the bone marrow, peripheral blood, or umbilical cord blood [ 83 ]. The procedure can be autologous (when the patient’s own cells are used), allogenic (when the stem cell comes from a donor), or syngeneic (from an identical twin). HSCs are responsible for the generation of all functional haematopoietic lineages in blood, including erythrocytes, leukocytes, and platelets. HSC transplantation solves problems that are caused by inappropriate functioning of the haematopoietic system, which includes diseases such as leukaemia and anaemia. However, when conventional sources of HSC are taken into consideration, there are some important limitations. First, there is a limited number of transplantable cells, and an efficient way of gathering them has not yet been found. There is also a problem with finding a fitting antigen-matched donor for transplantation, and viral contamination or any immunoreactions also cause a reduction in efficiency in conventional HSC transplantations. Haematopoietic transplantation should be reserved for patients with life-threatening diseases because it has a multifactorial character and can be a dangerous procedure. iPSC use is crucial in this procedure. The use of a patient’s own unspecialized somatic cells as stem cells provides the greatest immunological compatibility and significantly increases the success of the procedure.

Stem cells as a target for pharmacological testing

Stem cells can be used in new drug tests. Each experiment on living tissue can be performed safely on specific differentiated cells from pluripotent cells. If any undesirable effect appears, drug formulas can be changed until they reach a sufficient level of effectiveness. The drug can enter the pharmacological market without harming any live testers. However, to test the drugs properly, the conditions must be equal when comparing the effects of two drugs. To achieve this goal, researchers need to gain full control of the differentiation process to generate pure populations of differentiated cells.

Stem cells as an alternative for arthroplasty

One of the biggest fears of professional sportsmen is getting an injury, which most often signifies the end of their professional career. This applies especially to tendon injuries, which, due to current treatment options focusing either on conservative or surgical treatment, often do not provide acceptable outcomes. Problems with the tendons start with their regeneration capabilities. Instead of functionally regenerating after an injury, tendons merely heal by forming scar tissues that lack the functionality of healthy tissues. Factors that may cause this failed healing response include hypervascularization, deposition of calcific materials, pain, or swelling [ 84 ].

Additionally, in addition to problems with tendons, there is a high probability of acquiring a pathological condition of joints called osteoarthritis (OA) [ 85 ]. OA is common due to the avascular nature of articular cartilage and its low regenerative capabilities [ 86 ]. Although arthroplasty is currently a common procedure in treating OA, it is not ideal for younger patients because they can outlive the implant and will require several surgical procedures in the future. These are situations where stem cell therapy can help by stopping the onset of OA [ 87 ]. However, these procedures are not well developed, and the long-term maintenance of hyaline cartilage requires further research.

Osteonecrosis of the femoral hip (ONFH) is a refractory disease associated with the collapse of the femoral head and risk of hip arthroplasty in younger populations [ 88 ]. Although total hip arthroplasty (THA) is clinically successful, it is not ideal for young patients, mostly due to the limited lifetime of the prosthesis. An increasing number of clinical studies have evaluated the therapeutic effect of stem cells on ONFH. Most of the authors demonstrated positive outcomes, with reduced pain, improved function, or avoidance of THA [ 89 , 90 , 91 ].

Rejuvenation by cell programming

Ageing is a reversible epigenetic process. The first cell rejuvenation study was published in 2011 [ 92 ]. Cells from aged individuals have different transcriptional signatures, high levels of oxidative stress, dysfunctional mitochondria, and shorter telomeres than in young cells [ 93 ]. There is a hypothesis that when human or mouse adult somatic cells are reprogrammed to iPSCs, their epigenetic age is virtually reset to zero [ 94 ]. This was based on an epigenetic model, which explains that at the time of fertilization, all marks of parenteral ageing are erased from the zygote’s genome and its ageing clock is reset to zero [ 95 ].

In their study, Ocampo et al. [ 96 ] used Oct4, Sox2, Klf4, and C-myc genes (OSKM genes) and affected pancreas and skeletal muscle cells, which have poor regenerative capacity. Their procedure revealed that these genes can also be used for effective regenerative treatment [ 97 ]. The main challenge of their method was the need to employ an approach that does not use transgenic animals and does not require an indefinitely long application. The first clinical approach would be preventive, focused on stopping or slowing the ageing rate. Later, progressive rejuvenation of old individuals can be attempted. In the future, this method may raise some ethical issues, such as overpopulation, leading to lower availability of food and energy.

For now, it is important to learn how to implement cell reprogramming technology in non-transgenic elder animals and humans to erase marks of ageing without removing the epigenetic marks of cell identity.

Cell-based therapies

Stem cells can be induced to become a specific cell type that is required to repair damaged or destroyed tissues (Fig.  6 ). Currently, when the need for transplantable tissues and organs outweighs the possible supply, stem cells appear to be a perfect solution for the problem. The most common conditions that benefit from such therapy are macular degenerations [ 98 ], strokes [ 99 ], osteoarthritis [ 89 , 90 ], neurodegenerative diseases, and diabetes [ 100 ]. Due to this technique, it can become possible to generate healthy heart muscle cells and later transplant them to patients with heart disease.

Stem cell experiments on animals. These experiments are one of the many procedures that proved stem cells to be a crucial factor in future regenerative medicine

In the case of type 1 diabetes, insulin-producing cells in the pancreas are destroyed due to an autoimmunological reaction. As an alternative to transplantation therapy, it can be possible to induce stem cells to differentiate into insulin-producing cells [ 101 ].

Stem cells and tissue banks

iPS cells with their theoretically unlimited propagation and differentiation abilities are attractive for the present and future sciences. They can be stored in a tissue bank to be an essential source of human tissue used for medical examination. The problem with conventional differentiated tissue cells held in the laboratory is that their propagation features diminish after time. This does not occur in iPSCs.

The umbilical cord is known to be rich in mesenchymal stem cells. Due to its cryopreservation immediately after birth, its stem cells can be successfully stored and used in therapies to prevent the future life-threatening diseases of a given patient.

Stem cells of human exfoliated deciduous teeth (SHED) found in exfoliated deciduous teeth has the ability to develop into more types of body tissues than other stem cells [ 102 ] (Table  1 ). Techniques of their collection, isolation, and storage are simple and non-invasive. Among the advantages of banking, SHED cells are:

Guaranteed donor-match autologous transplant that causes no immune reaction and rejection of cells [ 103 ]

Simple and painless for both child and parent

Less than one third of the cost of cord blood storage

Not subject to the same ethical concerns as embryonic stem cells [ 104 ]

In contrast to cord blood stem cells, SHED cells are able to regenerate into solid tissues such as connective, neural, dental, or bone tissue [ 105 , 106 ]

SHED can be useful for close relatives of the donor

Fertility diseases

In 2011, two researchers, Katsuhiko Hayashi et al. [ 107 ], showed in an experiment on mice that it is possible to form sperm from iPSCs. They succeeded in delivering healthy and fertile pups in infertile mice. The experiment was also successful for female mice, where iPSCs formed fully functional eggs .

Young adults at risk of losing their spermatogonial stem cells (SSC), mostly cancer patients, are the main target group that can benefit from testicular tissue cryopreservation and autotransplantation. Effective freezing methods for adult and pre-pubertal testicular tissue are available [ 108 ].

Qiuwan et al. [ 109 ] provided important evidence that human amniotic epithelial cell (hAEC) transplantation could effectively improve ovarian function by inhibiting cell apoptosis and reducing inflammation in injured ovarian tissue of mice, and it could be a promising strategy for the management of premature ovarian failure or insufficiency in female cancer survivors.

For now, reaching successful infertility treatments in humans appears to be only a matter of time, but there are several challenges to overcome. First, the process needs to have high efficiency; second, the chances of forming tumours instead of eggs or sperm must be maximally reduced. The last barrier is how to mature human sperm and eggs in the lab without transplanting them to in vivo conditions, which could cause either a tumour risk or an invasive procedure.

Therapy for incurable neurodegenerative diseases

Thanks to stem cell therapy, it is possible not only to delay the progression of incurable neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease (AD), and Huntington disease, but also, most importantly, to remove the source of the problem. In neuroscience, the discovery of neural stem cells (NSCs) has nullified the previous idea that adult CNS were not capable of neurogenesis [ 110 , 111 ]. Neural stem cells are capable of improving cognitive function in preclinical rodent models of AD [ 112 , 113 , 114 ]. Awe et al. [ 115 ] clinically derived relevant human iPSCs from skin punch biopsies to develop a neural stem cell-based approach for treating AD. Neuronal degeneration in Parkinson’s disease (PD) is focal, and dopaminergic neurons can be efficiently generated from hESCs. PD is an ideal disease for iPSC-based cell therapy [ 116 ]. However, this therapy is still in an experimental phase ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4539501 /). Brain tissue from aborted foetuses was used on patients with Parkinson’s disease [ 117 ]. Although the results were not uniform, they showed that therapies with pure stem cells are an important and achievable therapy.

Stem cell use in dentistry

Teeth represent a very challenging material for regenerative medicine. They are difficult to recreate because of their function in aspects such as articulation, mastication, or aesthetics due to their complicated structure. Currently, there is a chance for stem cells to become more widely used than synthetic materials. Teeth have a large advantage of being the most natural and non-invasive source of stem cells.

For now, without the use of stem cells, the most common periodontological treatments are either growth factors, grafts, or surgery. For example, there are stem cells in periodontal ligament [ 118 , 119 ], which are capable of differentiating into osteoblasts or cementoblasts, and their functions were also assessed in neural cells [ 120 ]. Tissue engineering is a successful method for treating periodontal diseases. Stem cells of the root apical areas are able to recreate periodontal ligament. One of the possible methods of tissue engineering in periodontology is gene therapy performed using adenoviruses-containing growth factors [ 121 ].

As a result of animal studies, dentin regeneration is an effective process that results in the formation of dentin bridges [ 122 ].

Enamel is more difficult to regenerate than dentin. After the differentiation of ameloblastoma cells into the enamel, the former is destroyed, and reparation is impossible. Medical studies have succeeded in differentiating bone marrow stem cells into ameloblastoma [ 123 ].

Healthy dental tissue has a high amount of regular stem cells, although this number is reduced when tissue is either traumatized or inflamed [ 124 ]. There are several dental stem cell groups that can be isolated (Fig.  7 ).

Localization of stem cells in dental tissues. Dental pulp stem cells (DPSCs) and human deciduous teeth stem cells (SHED) are located in the dental pulp. Periodontal ligaments stem cells are located in the periodontal ligament. Apical papilla consists of stem cells from the apical papilla (SCAP)

Dental pulp stem cell (DPSC)

These were the first dental stem cells isolated from the human dental pulp, which were [ 125 ] located inside dental pulp (Table  2 ). They have osteogenic and chondrogenic potential. Mesenchymal stem cells (MSCs) of the dental pulp, when isolated, appear highly clonogenic; they can be isolated from adult tissue (e.g. bone marrow, adipose tissue) and foetal (e.g. umbilical cord) [ 126 ] tissue, and they are able to differentiate densely [ 127 ]. MSCs differentiate into odontoblast-like cells and osteoblasts to form dentin and bone. Their best source locations are the third molars [ 125 ]. DPSCs are the most useful dental source of tissue engineering due to their easy surgical accessibility, cryopreservation possibility, increased production of dentin tissues compared to non-dental stem cells, and their anti-inflammatory abilities. These cells have the potential to be a source for maxillofacial and orthopaedic reconstructions or reconstructions even beyond the oral cavity. DPSCs are able to generate all structures of the developed tooth [ 128 ]. In particular, beneficial results in the use of DPSCs may be achieved when combined with other new therapies, such as periodontal tissue photobiomodulation (laser stimulation), which is an efficient technique in the stimulation of proliferation and differentiation into distinct cell types [ 129 ]. DPSCs can be induced to form neural cells to help treat neurological deficits.

Stem cells of human exfoliated deciduous teeth (SHED) have a faster rate of proliferation than DPSCs and differentiate into an even greater number of cells, e.g. other mesenchymal and non-mesenchymal stem cell derivatives, such as neural cells [ 130 ]. These cells possess one major disadvantage: they form a non-complete dentin/pulp-like complex in vivo. SHED do not undergo the same ethical concerns as embryonic stem cells. Both DPSCs and SHED are able to form bone-like tissues in vivo [ 131 ] and can be used for periodontal, dentin, or pulp regeneration. DPSCs and SHED can be used in treating, for example, neural deficits [ 132 ]. DPSCs alone were tested and successfully applied for alveolar bone and mandible reconstruction [ 133 ].

Periodontal ligament stem cells (PDLSCs)

These cells are used in periodontal ligament or cementum tissue regeneration. They can differentiate into mesenchymal cell lineages to produce collagen-forming cells, adipocytes, cementum tissue, Sharpey’s fibres, and osteoblast-like cells in vitro. PDLSCs exist both on the root and alveolar bone surfaces; however, on the latter, these cells have better differentiation abilities than on the former [ 134 ]. PDLSCs have become the first treatment for periodontal regeneration therapy because of their safety and efficiency [ 135 , 136 ].

Stem cells from apical papilla (SCAP)

These cells are mesenchymal structures located within immature roots. They are isolated from human immature permanent apical papilla. SCAP are the source of odontoblasts and cause apexogenesis. These stem cells can be induced in vitro to form odontoblast-like cells, neuron-like cells, or adipocytes. SCAP have a higher capacity of proliferation than DPSCs, which makes them a better choice for tissue regeneration [ 137 , 138 ].

Dental follicle stem cells (DFCs)

These cells are loose connective tissues surrounding the developing tooth germ. DFCs contain cells that can differentiate into cementoblasts, osteoblasts, and periodontal ligament cells [ 139 , 140 ]. Additionally, these cells proliferate after even more than 30 passages [ 141 ]. DFCs are most commonly extracted from the sac of a third molar. When DFCs are combined with a treated dentin matrix, they can form a root-like tissue with a pulp-dentin complex and eventually form tooth roots [ 141 ]. When DFC sheets are induced by Hertwig’s epithelial root sheath cells, they can produce periodontal tissue; thus, DFCs represent a very promising material for tooth regeneration [ 142 ].

Pulp regeneration in endodontics

Dental pulp stem cells can differentiate into odontoblasts. There are few methods that enable the regeneration of the pulp.

The first is an ex vivo method. Proper stem cells are grown on a scaffold before they are implanted into the root channel [ 143 ].

The second is an in vivo method. This method focuses on injecting stem cells into disinfected root channels after the opening of the in vivo apex. Additionally, the use of a scaffold is necessary to prevent the movement of cells towards other tissues. For now, only pulp-like structures have been created successfully.

Methods of placing stem cells into the root channel constitute are either soft scaffolding [ 144 ] or the application of stem cells in apexogenesis or apexification. Immature teeth are the best source [ 145 ]. Nerve and blood vessel network regeneration are extremely vital to keep pulp tissue healthy.

The potential of dental stem cells is mainly regarding the regeneration of damaged dentin and pulp or the repair of any perforations; in the future, it appears to be even possible to generate the whole tooth. Such an immense success would lead to the gradual replacement of implant treatments. Mandibulary and maxillary defects can be one of the most complicated dental problems for stem cells to address.

Acquiring non-dental tissue cells by dental stem cell differentiation

In 2013, it was reported that it is possible to grow teeth from stem cells obtained extra-orally, e.g. from urine [ 146 ]. Pluripotent stem cells derived from human urine were induced and generated tooth-like structures. The physical properties of the structures were similar to natural ones except for hardness [ 127 ]. Nonetheless, it appears to be a very promising technique because it is non-invasive and relatively low-cost, and somatic cells can be used instead of embryonic cells. More importantly, stem cells derived from urine did not form any tumours, and the use of autologous cells reduces the chances of rejection [ 147 ].

Use of graphene in stem cell therapy

Over recent years, graphene and its derivatives have been increasingly used as scaffold materials to mediate stem cell growth and differentiation [ 148 ]. Both graphene and graphene oxide (GO) represent high in-plane stiffness [ 149 ]. Because graphene has carbon and aromatic network, it works either covalently or non-covalently with biomolecules; in addition to its superior mechanical properties, graphene offers versatile chemistry. Graphene exhibits biocompatibility with cells and their proper adhesion. It also tested positively for enhancing the proliferation or differentiation of stem cells [ 148 ]. After positive experiments, graphene revealed great potential as a scaffold and guide for specific lineages of stem cell differentiation [ 150 ]. Graphene has been successfully used in the transplantation of hMSCs and their guided differentiation to specific cells. The acceleration skills of graphene differentiation and division were also investigated. It was discovered that graphene can serve as a platform with increased adhesion for both growth factors and differentiation chemicals. It was also discovered that π-π binding was responsible for increased adhesion and played a crucial role in inducing hMSC differentiation [ 150 ].

Therapeutic potential of extracellular vesicle-based therapies

Extracellular vesicles (EVs) can be released by virtually every cell of an organism, including stem cells [ 151 ], and are involved in intercellular communication through the delivery of their mRNAs, lipids, and proteins. As Oh et al. [ 152 ] prove, stem cells, together with their paracrine factors—exosomes—can become potential therapeutics in the treatment of, e.g. skin ageing. Exosomes are small membrane vesicles secreted by most cells (30–120 nm in diameter) [ 153 ]. When endosomes fuse with the plasma membrane, they become exosomes that have messenger RNAs (mRNAs) and microRNAs (miRNAs), some classes of non-coding RNAs (IncRNAs) and several proteins that originate from the host cell [ 154 ]. IncRNAs can bind to specific loci and create epigenetic regulators, which leads to the formation of epigenetic modifications in recipient cells. Because of this feature, exosomes are believed to be implicated in cell-to-cell communication and the progression of diseases such as cancer [ 155 ]. Recently, many studies have also shown the therapeutic use of exosomes derived from stem cells, e.g. skin damage and renal or lung injuries [ 156 ].

In skin ageing, the most important factor is exposure to UV light, called “photoageing” [ 157 ], which causes extrinsic skin damage, characterized by dryness, roughness, irregular pigmentation, lesions, and skin cancers. In intrinsic skin ageing, on the other hand, the loss of elasticity is a characteristic feature. The skin dermis consists of fibroblasts, which are responsible for the synthesis of crucial skin elements, such as procollagen or elastic fibres. These elements form either basic framework extracellular matrix constituents of the skin dermis or play a major role in tissue elasticity. Fibroblast efficiency and abundance decrease with ageing [ 158 ]. Stem cells can promote the proliferation of dermal fibroblasts by secreting cytokines such as platelet-derived growth factor (PDGF), transforming growth factor β (TGF-β), and basic fibroblast growth factor. Huh et al. [ 159 ] mentioned that a medium of human amniotic fluid-derived stem cells (hAFSC) positively affected skin regeneration after longwave UV-induced (UVA, 315–400 nm) photoageing by increasing the proliferation and migration of dermal fibroblasts. It was discovered that, in addition to the induction of fibroblast physiology, hAFSC transplantation also improved diseases in cases of renal pathology, various cancers, or stroke [ 160 , 161 ].

Oh [ 162 ] also presented another option for the treatment of skin wounds, either caused by physical damage or due to diabetic ulcers. Induced pluripotent stem cell-conditioned medium (iPSC-CM) without any animal-derived components induced dermal fibroblast proliferation and migration.

Natural cutaneous wound healing is divided into three steps: haemostasis/inflammation, proliferation, and remodelling. During the crucial step of proliferation, fibroblasts migrate and increase in number, indicating that it is a critical step in skin repair, and factors such as iPSC-CM that impact it can improve the whole cutaneous wound healing process. Paracrine actions performed by iPSCs are also important for this therapeutic effect [ 163 ]. These actions result in the secretion of cytokines such as TGF-β, interleukin (IL)-6, IL-8, monocyte chemotactic protein-1 (MCP-1), vascular endothelial growth factor (VEGF), platelet-derived growth factor-AA (PDGF-AA), and basic fibroblast growth factor (bFGF). Bae et al. [ 164 ] mentioned that TGF-β induced the migration of keratinocytes. It was also demonstrated that iPSC factors can enhance skin wound healing in vivo and in vitro when Zhou et al. [ 165 ] enhanced wound healing, even after carbon dioxide laser resurfacing in an in vivo study.

Peng et al. [ 166 ] investigated the effects of EVs derived from hESCs on in vitro cultured retinal glial, progenitor Müller cells, which are known to differentiate into retinal neurons. EVs appear heterogeneous in size and can be internalized by cultured Müller cells, and their proteins are involved in the induction and maintenance of stem cell pluripotency. These stem cell-derived vesicles were responsible for the neuronal trans-differentiation of cultured Müller cells exposed to them. However, the research article points out that the procedure was accomplished only on in vitro acquired retina.

Challenges concerning stem cell therapy

Although stem cells appear to be an ideal solution for medicine, there are still many obstacles that need to be overcome in the future. One of the first problems is ethical concern.

The most common pluripotent stem cells are ESCs. Therapies concerning their use at the beginning were, and still are, the source of ethical conflicts. The reason behind it started when, in 1998, scientists discovered the possibility of removing ESCs from human embryos. Stem cell therapy appeared to be very effective in treating many, even previously incurable, diseases. The problem was that when scientists isolated ESCs in the lab, the embryo, which had potential for becoming a human, was destroyed (Fig.  8 ). Because of this, scientists, seeing a large potential in this treatment method, focused their efforts on making it possible to isolate stem cells without endangering their source—the embryo.

Use of inner cell mass pluripotent stem cells and their stimulation to differentiate into desired cell types

For now, while hESCs still remain an ethically debatable source of cells, they are potentially powerful tools to be used for therapeutic applications of tissue regeneration. Because of the complexity of stem cell control systems, there is still much to be learned through observations in vitro. For stem cells to become a popular and widely accessible procedure, tumour risk must be assessed. The second problem is to achieve successful immunological tolerance between stem cells and the patient’s body. For now, one of the best ideas is to use the patient’s own cells and devolve them into their pluripotent stage of development.

New cells need to have the ability to fully replace lost or malfunctioning natural cells. Additionally, there is a concern about the possibility of obtaining stem cells without the risk of morbidity or pain for either the patient or the donor. Uncontrolled proliferation and differentiation of cells after implementation must also be assessed before its use in a wide variety of regenerative procedures on living patients [ 167 ].

One of the arguments that limit the use of iPSCs is their infamous role in tumourigenicity. There is a risk that the expression of oncogenes may increase when cells are being reprogrammed. In 2008, a technique was discovered that allowed scientists to remove oncogenes after a cell achieved pluripotency, although it is not efficient yet and takes a longer amount of time. The process of reprogramming may be enhanced by deletion of the tumour suppressor gene p53, but this gene also acts as a key regulator of cancer, which makes it impossible to remove in order to avoid more mutations in the reprogrammed cell. The low efficiency of the process is another problem, which is progressively becoming reduced with each year. At first, the rate of somatic cell reprogramming in Yamanaka’s study was up to 0.1%. The use of transcription factors creates a risk of genomic insertion and further mutation of the target cell genome. For now, the only ethically acceptable operation is an injection of hESCs into mouse embryos in the case of pluripotency evaluation [ 168 ].

Stem cell obstacles in the future

Pioneering scientific and medical advances always have to be carefully policed in order to make sure they are both ethical and safe. Because stem cell therapy already has a large impact on many aspects of life, it should not be treated differently.

Currently, there are several challenges concerning stem cells. First, the most important one is about fully understanding the mechanism by which stem cells function first in animal models. This step cannot be avoided. For the widespread, global acceptance of the procedure, fear of the unknown is the greatest challenge to overcome.

The efficiency of stem cell-directed differentiation must be improved to make stem cells more reliable and trustworthy for a regular patient. The scale of the procedure is another challenge. Future stem cell therapies may be a significant obstacle. Transplanting new, fully functional organs made by stem cell therapy would require the creation of millions of working and biologically accurate cooperating cells. Bringing such complicated procedures into general, widespread regenerative medicine will require interdisciplinary and international collaboration.

The identification and proper isolation of stem cells from a patient’s tissues is another challenge. Immunological rejection is a major barrier to successful stem cell transplantation. With certain types of stem cells and procedures, the immune system may recognize transplanted cells as foreign bodies, triggering an immune reaction resulting in transplant or cell rejection.

One of the ideas that can make stem cells a “failsafe” is about implementing a self-destruct option if they become dangerous. Further development and versatility of stem cells may cause reduction of treatment costs for people suffering from currently incurable diseases. When facing certain organ failure, instead of undergoing extraordinarily expensive drug treatment, the patient would be able to utilize stem cell therapy. The effect of a successful operation would be immediate, and the patient would avoid chronic pharmacological treatment and its inevitable side effects.

Although these challenges facing stem cell science can be overwhelming, the field is making great advances each day. Stem cell therapy is already available for treating several diseases and conditions. Their impact on future medicine appears to be significant.

After several decades of experiments, stem cell therapy is becoming a magnificent game changer for medicine. With each experiment, the capabilities of stem cells are growing, although there are still many obstacles to overcome. Regardless, the influence of stem cells in regenerative medicine and transplantology is immense. Currently, untreatable neurodegenerative diseases have the possibility of becoming treatable with stem cell therapy. Induced pluripotency enables the use of a patient’s own cells. Tissue banks are becoming increasingly popular, as they gather cells that are the source of regenerative medicine in a struggle against present and future diseases. With stem cell therapy and all its regenerative benefits, we are better able to prolong human life than at any time in history.

Abbreviations

Basic fibroblast growth factor

Bone morphogenic proteins

Dental follicle stem cells

Dental pulp stem cells

Embryonic bodies

Embryonic stem cells

Fibroblast growth factors

Good Manufacturing Practice

Graphene oxide

Human amniotic fluid-derived stem cells

Human embryonic stem cells

Human foreskin fibroblasts

Inner cell mass

Non-coding RNA

Induced pluripotent stem cells

In vitro fertilization

Knockout serum replacement

Leukaemia inhibitory factor

Monocyte chemotactic protein-1

Fibroblasts

Messenger RNA

Mesenchymal stem cells of dental pulp

Myogenic differentiation

Osteoarthritis

Octamer-binding transcription factor 3 and 4

Platelet-derived growth factor

Platelet-derived growth factor-AA

Periodontal ligament stem cells

Rho-associated protein kinase

Stem cells from apical papilla

Stem cells of human exfoliated deciduous teeth

Synthetic Serum Substitute

Trophectoderm

Vascular endothelial growth factor

Transforming growth factors

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Zakrzewski, W., Dobrzyński, M., Szymonowicz, M. et al. Stem cells: past, present, and future. Stem Cell Res Ther 10 , 68 (2019). https://doi.org/10.1186/s13287-019-1165-5

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Stem cell research at johns hopkins institute of basic biomedical sciences.

Researchers at the IBBS are studying stem cells to figure out how they can give rise to a complex body, how cells of the body can revert back to stem cells and how this knowledge can be used to develop therapies for diseases and injuries.

Stem cells are cells that don’t have an identity but have the potential to develop into many types of cells for many purposes, liking building a complete organism, healing a wound or replacing old, worn-out cells in a tissue.

Embryonic stem cells can become any of the cells in the body and can form entire animals.

Not all stem cells come from embryos, adult stem cells are found throughout the body too. These cells don’t have the ability to become any cell in the body, but can transform into many different cell types. For instance, there are stem cells in our bone marrow that can become fat cells, cartilage cells or bone cells, but they can’t become eye cells or skin cells. Researchers have also figured out how to make adult cells, like a skin cell, turn back into cells with the properties of embryonic stem cells, called induced pluripotent stem cells or iPS cells for short.

Erika Matunis ,  in the Department of Cell Biology , studies in fruit flies how testis stem cells decide to stay stem cells and not become other cell types, like sperm. She has also discovered how cells that are turning into other cell types can revert back to stem cells if the permanent reservoir of stem cells is depleted and she is exploring the mechanism of how this happens. Her research, learning more about the most fundamental aspects of stem cell biology, helps all stem cell researchers better understand the cells they work with.

Jennifer Elisseeff ,  of the Department of Biomedical Engineering , studies the differences between embryonic stem cells and adult stem cells. She has found that embryonic stem cells are better at forming new tissues, whereas adult stem cells are better at secreting therapeutic molecules that promote healing of damaged tissue. Elisseeff is particularly interested in the factors released by stem cells that can help a tissue heal. She uses this information in the development of biosynthetic (part-natural and part-man-made) materials used for therapies. One of the materials her lab has developed is a bio-adhesive—essentially a glue that can be used in the body that is made of part synthetic and part natural components. The glue is used in conjunction with stitches to help prevent leakage of blood or fluids, but it’s flexible enough to allow cells to move in and heal the incision. Also, Elisseeff is collaborating with the military to develop a treatment for soft tissue facial reconstruction for people who have suffered severe trauma. They are developing tissue blueprints that can be transplanted in the face—or any other place in the body for that matter— that would allow a person’s own cells to move into a region to heal and restructure the tissue.

Warren Grayson ,  of the Department of Biomedical Engineering , takes stem cells from fat and bone marrow as well as stem cells that have the potential to become many different cell types, known as pluripotent stem cells, and coaxes them to regenerate bone or skeletal muscle in the lab. He does this by incubating stem cells in biosynthetic structures to give the cells a structured three-dimensional volume to grow in, and then places these either in bioreactors that provide heat, nutrients, movement, mechanical stress or control of any other condition like oxygen concentration to guide the stem cell to become a specific cell type or within a defect in animals to study the regenerative process. He hopes to one day be able to take a person’s own stem cells and grow tissues, like bone or muscle, to be implanted into their body to replace damaged tissue. Using a person’s own cells and tissues will reduce the likelihood that the transplanted tissue will be rejected by the immune system.

Related Links :  Stem Cell Research at Johns Hopkins

The future of medicine lies in understanding how the body creates itself out of a single cell and the mechanisms by which it renews itself throughout life.

When we achieve this goal, we will be able to replace damaged tissues and help the body regenerate itself, potentially curing or easing the suffering of those afflicted by disorders like heart disease, Alzheimers, Parkinsons, diabetes, spinal cord injury and cancer.

Research at the institute leverages Stanford’s many strengths in a way that promotes that goal. The institute brings together experts from a wide range of scientific and medical fields to create a fertile, multidisciplinary research environment.

There are four major research areas of emphasis at the institute:

• Mature tissue or organ stem cells:  Researchers are expanding their understanding of known stem cells that continue to function through life, the so called “adult” stem cells, the mature tissue or organ cells that include blood-forming, neural, skin and skeletal muscle stem cells. Research in this area is also aimed at understanding the clinical applications of these stem cells, such as regenerating sick or injured organs and tissues.  More »
• Human embryonic and induced-pluripotent stem (iPS) cells.  Researchers are studying how embryonic cells are created and how they specialize to become various tissues in the body. Understanding the mechanics of embryonic stem cells may well be the key to the most dramatic breakthroughs in regeneration medicine. We also now have the  capability to produce embryonic-like cells (iPS cells) from mature cells, and even to create stem cells directly from mature cells without going through the iPS stage.  More »
• New Stem Cell Lines:  The institute is exploring how stem cells can be created out of specialized cells that have grown out of the stem cell stage. This research includes the use of NT (nuclear transfer) technology and iPS (induced pluripotent stem cell) technology to create new stem cell lines, which serve as models for studying and treating disorders such as cancer, diabetes, cardiovascular disease, autoimmune disease, and neurodegenerative disorders such as Alzheimer's, Parkinson's, and Lou Gehrig's diseases.  More »
• Cancer Stem Cells:  Scientist at the institute have played a key role in discovering and studying cancer stem cells, which are believed to lie at the core of cancer’s destructive potential. The institute continues to be the global epicenter of the hunt for cancer stem cells. Researchers aim to conduct preclinical research to develop new therapeutic approaches to killing cancer stem cells, with the goal of moving these findings into clinical trials.  More »

Learn about the many ways to support the institute for Stem Cell Biology and Regenerative Medicine

Stem Cell Biology

Stem cells Cells that have the ability to differentiate into multiple types of cells and make an unlimited number of copies of themselves. Stem cells Cells that have the ability to differentiate into multiple types of cells and make an unlimited number of copies of themselves. have the remarkable potential to self-renew and differentiate into the various specialized cell types found in the body. In addition to generating all the cells and structures of the human body, they serve as a sort of internal repair system, dividing to restore or replace cells lost throughout a person’s life. Our researchers are leveraging their expertise across a wide range of disciplines and technologies to build a deep foundational understanding of stem cell function, gaining critical insights into how stem cells can be harnessed toward new therapies that regenerate damaged organs, repair injuries and prevent age-related diseases. 

Uncovering these underlying principles provides a solid framework for more applied research, yielding insights into humanity’s most complex medical mysteries and opening up avenues to enhance the body’s inherent ability to heal. Center members are developing improved methods to generate induced pluripotent stem cells iPS cells are cells taken from a patient that are reprogrammed so that they can undergo differentiation into any type of cell in the body. By maintaining the genetic code of the patient, iPS cells play a crucial role in disease modeling and regenerative medicine. induced pluripotent stem cells iPS cells are cells taken from a patient that are reprogrammed so that they can undergo differentiation into any type of cell in the body. By maintaining the genetic code of the patient, iPS cells play a crucial role in disease modeling and regenerative medicine. from patient cells and pinpointing the causes of our most intractable diseases using stem cell-derived 3D organoid 3D tissue grown from stem cells to replicate aspects of the structure and function of an organ. By modeling how multiple types of cells interact in biologically-relevant structures, these models help researchers understand how human organs develop, age and respond to disease in more detail than 2D cultures. organoid 3D tissue grown from stem cells to replicate aspects of the structure and function of an organ. By modeling how multiple types of cells interact in biologically-relevant structures, these models help researchers understand how human organs develop, age and respond to disease in more detail than 2D cultures. models. Through these approaches and a range of others, they’re uncovering the precise genetic networks that regulate the formation, maturation and function of tissue-specific cells, bringing us ever-closer to a new generation of cell-based treatments and cures. 

• Identify the genetic and molecular mechanisms that enable stem cells Cells that have the ability to differentiate into multiple types of cells and make an unlimited number of copies of themselves. stem cells Cells that have the ability to differentiate into multiple types of cells and make an unlimited number of copies of themselves. to repair or regenerate tissue following damage, injury and disease in order to develop drug and cell therapies to enhance and accelerate this process 
• Develop methods to prevent age-related diseases and prolong healthspan by studying how aging affects stem cell function and pinpointing factors that can prevent, slow or reverse the effects of aging
• Uncover how factors including genetic variation, metabolism and environmental stressors affect stem cell production and function 
• Use stem cell-derived organoid 3D tissue grown from stem cells to replicate aspects of the structure and function of an organ. By modeling how multiple types of cells interact in biologically-relevant structures, these models help researchers understand how human organs develop, age and respond to disease in more detail than 2D cultures. organoid 3D tissue grown from stem cells to replicate aspects of the structure and function of an organ. By modeling how multiple types of cells interact in biologically-relevant structures, these models help researchers understand how human organs develop, age and respond to disease in more detail than 2D cultures. models to gain critical insights into how tissues and organs — including the heart, lungs and brain — develop, age and respond to disease 
• Discover new therapies in an efficient and cost-effective manner using stem cell-derived disease-in-a-dish These models use lab-grown cell structures made from patient tissue samples to study human disease outside of the body. Cells are reprogrammed to a pluripotent state, then transformed into different cell types, allowing scientists to study disease processes in a controlled environment. They differ from organoids, which offer a more complex and organ-specific perspective, capturing cellular diversity and interactions. disease-in-a-dish These models use lab-grown cell structures made from patient tissue samples to study human disease outside of the body. Cells are reprogrammed to a pluripotent state, then transformed into different cell types, allowing scientists to study disease processes in a controlled environment. They differ from organoids, which offer a more complex and organ-specific perspective, capturing cellular diversity and interactions. models in combination with high-throughput drug screening technologies 
• Examine the molecular mechanisms that control stem cell identity and fate to identify improved methods of generating induced pluripotent stem cells iPS cells are cells taken from a patient that are reprogrammed so that they can undergo differentiation into any type of cell in the body. By maintaining the genetic code of the patient, iPS cells play a crucial role in disease modeling and regenerative medicine. induced pluripotent stem cells iPS cells are cells taken from a patient that are reprogrammed so that they can undergo differentiation into any type of cell in the body. By maintaining the genetic code of the patient, iPS cells play a crucial role in disease modeling and regenerative medicine. for use in cell therapies 
• Understand how the stem cells in plants and seaweeds influence their evolution and development — ultimately impacting our food systems, ecosystems and changing climate

Research Highlights

Uncovering the origins of blood stem cells

Center researchers develop a first-of-its-kind roadmap tracing how blood stem cells develop in the human embryo, providing a blueprint for producing fully functional blood stem cells in the lab.

Lab-grown eggs and sperm to treat infertility

UCLA study reveals timeline and pathway of germ cell development — crucial information that could help scientists generate egg and sperm cells in the lab.

Combating hair loss

Center members discover a new way to activate the stem cells in the hair follicle to make hair grow for people with baldness or alopecia, which is hair loss associated with hormonal imbalance, stress, aging or chemotherapy.

Defining stages of stem cell reprogramming

Our researchers unveil a critical new understanding of stem cell development, improving disease models and sources of patient-specific specialized cells suitable for replacement therapy.

Manufacturing blood stem cells

Center researchers lay the groundwork for generating blood stem cells in the lab that mirror their natural development, improving patient-specific therapies for blood-related diseases and cancers.

Identifying differences in reprogrammed cells

UCLA researchers found that cells reprogrammed into embryonic-like cells have inherent molecular differences and distinct gene expression signatures, inspiring additional research about the role they can play in clinical therapies.

Creating induced pluripotent stem cells

UCLA scientists reprogram human skin cells into cells with embryonic stem cell-like properties without using embryos or eggs, impacting disease treatment, tissue engineering and transplantation medicine.

Rejuvenating old stem cells

UCLA researchers discover that restoring a key antioxidant rejuvenates old muscle stem cells — a finding that could help improve the body’s ability to recover from injury as we age.

Meet the Researchers

Related News

Media Coverage

Lab-grown embryo models: uk unveils first ever rules to guide research, scientists just grew the 1st-ever 'minibrains' from multiple people's cells, mouse hair turns gray when certain stem cells get stuck.

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stem cell biology • regeneration • disease modeling • pattern formation • differentiation • gametogenesis • morphogenesis • organ development • cell migration • cell-cell and cell-environment interactions • aging • cancer

Laurie A. Boyer

Eliezer calo, mary gehring, leonard p. guarente, h. robert horvitz, richard o. hynes, rudolf jaenisch, kristin knouse, jacqueline lees, ruth lehmann, harvey f. lodish, adam c. martin, elly nedivi, peter reddien, yadira soto-feliciano, robert a. weinberg, brandon (brady) weissbourd, yukiko yamashita, omer h. yilmaz, richard a. young.

Leonard P. Guarente looks at mammal, mouse, and human brains to understand the genetic underpinning of aging and age-related diseases like Alzheimer’s.

“Vaults” within germ cells offer more than safekeeping

Microscope system sharpens scientists’ view of neural circuit connections

Q&A: Pulin Li on recreating development in the lab

New findings activate a better understanding of Rett syndrome’s causes

Evolution in Action Series: Birth of a species

News brief: Calo Lab

How early-stage cancer cells hide from the immune system

How phase separation is revolutionizing biology

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• v.365(1537); 2010 Jan 12

The therapeutic potential of stem cells

In recent years, there has been an explosion of interest in stem cells, not just within the scientific and medical communities but also among politicians, religious groups and ethicists. Here, we summarize the different types of stem cells that have been described: their origins in embryonic and adult tissues and their differentiation potential in vivo and in culture. We review some current clinical applications of stem cells, highlighting the problems encountered when going from proof-of-principle in the laboratory to widespread clinical practice. While some of the key genetic and epigenetic factors that determine stem cell properties have been identified, there is still much to be learned about how these factors interact. There is a growing realization of the importance of environmental factors in regulating stem cell behaviour and this is being explored by imaging stem cells in vivo and recreating artificial niches in vitro . New therapies, based on stem cell transplantation or endogenous stem cells, are emerging areas, as is drug discovery based on patient-specific pluripotent cells and cancer stem cells. What makes stem cell research so exciting is its tremendous potential to benefit human health and the opportunities for interdisciplinary research that it presents.

1. Introduction: what are stem cells?

The human body comprises over 200 different cell types that are organized into tissues and organs to provide all the functions required for viability and reproduction. Historically, biologists have been interested primarily in the events that occur prior to birth. The second half of the twentieth century was a golden era for developmental biology, since the key regulatory pathways that control specification and morphogenesis of tissues were defined at the molecular level ( Arias 2008 ). The origins of stem cell research lie in a desire to understand how tissues are maintained in adult life, rather than how different cell types arise in the embryo. An interest in adult tissues fell, historically, within the remit of pathologists and thus tended to be considered in the context of disease, particularly cancer.

It was appreciated long ago that within a given tissue there is cellular heterogeneity: in some tissues, such as the blood, skin and intestinal epithelium, the differentiated cells have a short lifespan and are unable to self-renew. This led to the concept that such tissues are maintained by stem cells, defined as cells with extensive renewal capacity and the ability to generate daughter cells that undergo further differentiation ( Lajtha 1979 ). Such cells generate only the differentiated lineages appropriate for the tissue in which they reside and are thus referred to as multipotent or unipotent ( figure 1 ).

Origin of stem cells. Cells are described as pluripotent if they can form all the cell types of the adult organism. If, in addition, they can form the extraembryonic tissues of the embryo, they are described as totipotent. Multipotent stem cells have the ability to form all the differentiated cell types of a given tissue. In some cases, a tissue contains only one differentiated lineage and the stem cells that maintain the lineage are described as unipotent . Postnatal spermatogonial stem cells, which are unipotent in vivo but pluripotent in culture, are not shown ( Jaenisch & Young 2008 ). CNS, central nervous system; ICM, inner cell mass.

In the early days of stem cell research, a distinction was generally made between three types of tissue: those, such as epidermis, with rapid turnover of differentiated cells; those, such as brain, in which there appeared to be no self-renewal; and those, such as liver, in which cells divided to give two daughter cells that were functionally equivalent ( Leblond 1964 ; Hall & Watt 1989 ). While it remains true that different adult tissues differ in terms of the proportion of proliferative cells and the nature of the differentiation compartment, in recent years it has become apparent that some tissues that appeared to lack self-renewal ability do indeed contain stem cells ( Zhao et al . 2008 ) and others contain a previously unrecognized cellular heterogeneity ( Zaret & Grompe 2008 ). That is not to say that all tissues are maintained by stem cells; for example, in the pancreas, there is evidence against the existence of a distinct stem cell compartment ( Dor et al . 2004 ).

One reason why it took so long for stem cells to become a well-established research field is that in the early years too much time and energy were expended in trying to define stem cells and in arguing about whether or not a particular cell was truly a stem cell ( Watt 1999 ). Additional putative characteristics of stem cells, such as rarity, capacity for asymmetric division or tendency to divide infrequently, were incorporated into the definition, so that if a cell did not exhibit these additional properties it tended to be excluded from the stem cell ‘list’. Some researchers still remain anxious about the definitions and try to hedge their bets by describing a cell as a stem/progenitor cell. However, this is not useful. The use of the term progenitor, or transit amplifying, cell should be reserved for a cell that has left the stem cell compartment but still retains the ability to undergo cell division and further differentiation ( Potten & Loeffler 2008 ).

Looking back at some of the early collections of reviews written as the proceedings of stem cell conferences, one regularly finds articles on the topic of cancer stem cells ( McCulloch et al . 1988 ). However, these cells have only recently received widespread attention ( Reya et al . 2001 ; Clarke et al . 2006 ; Dick 2008 ). The concept is very similar to the concept of normal tissue stem cells, namely that cells in tumours are heterogeneous, with only some, the cancer stem cells, or tumour initiating cells, being capable of tumour maintenance or regrowth following chemotherapy. The cancer stem cell concept is important because it suggests new approaches to anti-cancer therapies ( figure 2 ).

The cancer stem cell hypothesis. The upper tumour is shown as comprising a uniform population of cells, while the lower tumour contains both cancer stem cells and more differentiated cells. Successful or unsuccessful chemotherapy is interpreted according to the behaviour of cells within the tumour.

As in the case of tissue stem cells, it is important that cancer stem cell research is not sidetracked by arguments about definitions. It is quite likely that in some tumours all the cells are functionally equivalent, and there is no doubt that tumour cells, like normal stem cells, can behave differently under different assay conditions ( Quintana et al . 2008 ). The oncogene dogma ( Hahn & Weinberg 2002 ), that tumours arise through step-wise accumulation of oncogenic mutations, does not adequately account for cellular heterogeneity, and markers of stem cells in specific cancers have already been described ( Singh et al . 2004 ; Barabé et al . 2007 ; O'Brien et al . 2007 ). While the (rediscovered) cancer stem cell field is currently in its infancy, it is already evident that a cancer stem cell is not necessarily a normal stem cell that has acquired oncogenic mutations. Indeed, there is experimental evidence that cancer initiating cells can be genetically altered progenitor cells ( Clarke et al . 2006 ).

In addition to adult tissue stem cells, stem cells can be isolated from pre-implantation mouse and human embryos and maintained in culture as undifferentiated cells ( figure 1 ). Such embryonic stem (ES) cells have the ability to generate all the differentiated cells of the adult and are thus described as being pluripotent ( figure 1 ). Mouse ES cells are derived from the inner cell mass of the blastocyst, and following their discovery in 1981 ( Evans & Kaufman 1981 ; Martin 1981 ) have been used for gene targeting, revolutionizing the field of mouse genetics. In 1998, it was first reported that stem cells could be derived from human blastocysts ( Thomson et al . 1998 ), opening up great opportunities for stem cell-based therapies, but also provoking controversy because the cells are derived from ‘spare’ in vitro fertilization embryos that have the potential to produce a human being. It is interesting to note that, just as research on adult tissue stem cells is intimately linked to research on disease states, particularly cancer, the same is true for ES cells. Many years before the development of ES cells, the in vitro differentiation of cells derived from teratocarcinomas, known as embryonal carcinoma cells, provided an important model for studying lineage selection ( Andrews et al . 2005 ).

Blastocysts are not the only source of pluripotent ES cells ( figure 1 ). Pluripotent epiblast stem cells, known as epiSC, can be derived from the post-implantation epiblast of mouse embryos ( Brons et al . 2007 ; Tesar et al . 2007 ). Recent gene expression profiling studies suggest that human ES cells are more similar to epiSC than to mouse ES cells ( Tesar et al . 2007 ). Pluripotent stem cells can also be derived from primordial germ cells (EG cells), progenitors of adult gametes, which diverge from the somatic lineage at late embryonic to early foetal development ( Kerr et al . 2006 ).

Although in the past the tendency has been to describe ES cells as pluripotent and adult stem cells as having a more restricted range of differentiation options, adult cells can, in some circumstances, produce progeny that differentiate across the three primary germ layers (ectoderm, mesoderm and endoderm). Adult cells can be reprogrammed to a pluripotent state by transfer of the adult nucleus into the cytoplasm of an oocyte ( Gurdon et al . 1958 ; Gurdon & Melton 2008 ) or by fusion with a pluripotent cell ( Miller & Ruddle 1976 ). The most famous example of cloning by transfer of a somatic nucleus into an oocyte is the creation of Dolly the sheep ( Wilmut et al . 1997 ). While the process remains inefficient, it has found some unexpected applications, such as cloning endangered species and domestic pets.

A flurry of reports almost 10 years ago suggested that adult cells from many tissues could differentiate into other cell types if placed in a new tissue environment. Such studies are now largely discredited, although there are still some bona fide examples of transdifferentiation of adult cells, such as occurs when blood cells fuse with hepatocytes during repair of damaged liver ( Anderson et al . 2001 ; Jaenisch & Young 2008 ). In addition, it has been known for many years that adult urodele amphibians can regenerate limbs or the eye lens following injury; this involves dedifferentiation and subsequent transdifferentiation steps ( Brockes & Kumar 2005 ).

The early studies involving somatic nuclear transfer indicated that adult cells can be reprogrammed to pluripotency. However, the mechanistic and practical applications of inducing pluripotency in adult cells have only become apparent in the last 2 or 3 years, with the emergence of a new research area: induced pluripotent stem cells (iPS cells). The original report demonstrated that retrovirus-mediated transduction of mouse fibroblasts with four transcription factors (Oct-3/4, Sox2, KLF4 and c-Myc; figure 1 ) that are highly expressed in ES cells could induce the fibroblasts to become pluripotent ( Takahashi & Yamanaka 2006 ). Since then, rapid progress has been made: iPS cells can be generated from adult human cells ( Takahashi et al . 2007 ; Yu et al . 2007 ; Park et al . 2008 a ); cells from a range of tissues can be reprogrammed ( Aasen et al . 2008 ; Aoi et al . 2008 ); and iPS cells can be generated from patients with specific diseases ( Dimos et al . 2008 ; Park et al . 2008 b ). The number of transcription factors required to generate iPS cells has been reduced ( Kim et al . 2008 ); the efficiency of iPS cell generation increased ( Wernig et al . 2007 ); and techniques devised that obviate the need for retroviral vectors ( Okita et al . 2008 ; Stadtfeld et al . 2008 ). These latter developments are very important for future clinical applications, since the early mice generated from iPS cells developed tumours at high frequency ( Takahashi & Yamanaka 2006 ; Yamanaka 2007 ). Without a doubt, this is currently the most exciting and rapidly moving area of stem cell research.

2. Current clinical applications of stem cells

In all the publicity that surrounds embryonic and iPS cells, people tend to forget that stem cell-based therapies are already in clinical use and have been for decades. It is instructive to think about these treatments, because they provide important caveats about the journey from proof-of-principle in the laboratory to real patient benefit in the clinic. These caveats include efficacy, patient safety, government legislation and the costs and potential profits involved in patient treatment.

Haemopoietic stem cell transplantation is the oldest stem cell therapy and is the treatment that is most widely available ( Perry & Linch 1996 ; Austin et al . 2008 ). The stem cells come from bone marrow, peripheral blood or cord blood. For some applications, the patient's own cells are engrafted. However, allogeneic stem cell transplantation is now a common procedure for the treatment of bone marrow failure and haematological malignancies, such as leukaemia. Donor stem cells are used to reconstitute immune function in such patients following radiation and/or chemotherapy. In the UK, the regulatory framework put in place for bone marrow transplantation has now an extended remit, covering the use of other tissues and organs ( Austin et al . 2008 ).

Advances in immunology research greatly increased the utility of bone marrow transplantation, allowing allograft donors to be screened for the best match in order to prevent rejection and graft-versus-host disease ( Perry & Linch 1996 ). It is worth remembering that organ transplantation programmes have also depended on an understanding of immune rejection, and drugs are available to provide effective long-term immunosuppression for recipients of donor organs. Thus, while it is obviously desirable for new stem cell treatments to involve the patient's own cells, it is certainly not essential.

Two major advantages of haemopoietic stem cell therapy are that there is no need to expand the cells in culture or to reconstitute a multicellular tissue architecture prior to transplantation. These hurdles have been overcome to generate cultured epidermis to provide autologous grafts for patients with full-thickness wounds, such as third-degree burns. Proof-of-principle was established in the mid-1970s, with clinical and commercial applications following rapidly ( Green 2008 ). Using a similar approach, limbal stem cells have been used successfully to restore vision in patients suffering from chemical destruction of the cornea ( De Luca et al . 2006 ).

Ex vivo expansion of human epidermal and corneal stem cells frequently involves culture on a feeder layer of mouse fibroblastic cells in medium containing bovine serum. While it would obviously be preferable to avoid animal products, there has been no evidence over the past 30 years that exposure to them has had adverse effects on patients receiving the grafts. The ongoing challenges posed by epithelial stem cell treatments include improved functionality of the graft (e.g. through generation of epidermal hair follicles) and improved surfaces on which to culture the cells and apply them to the patients. The need to optimize stem cell delivery is leading to close interactions between the stem cell community and bioengineers. In a recent example, a patient's trachea was repaired by transplanting a new tissue constructed in culture from donor decellularized trachea seeded with the patient's own bone marrow cells that had been differentiated into cartilage cells ( Macchiarini et al . 2008 ).

Whereas haemopoietic stem cell therapies are widely available, treatments involving cultured epidermis and cornea are not. In countries where cultured epithelial grafts are available, the number of potential patients is relatively small and the treatment costly. Commercial organizations that sell cultured epidermis for grafting have found that it is not particularly profitable, while in countries with publicly funded healthcare the need to set up a dedicated laboratory to generate the grafts tends to make the financial cost–benefit ratio too high ( Green 2008 ).

Clinical studies over the last 10 years suggest that stem cell transplantation also has potential as a therapy for neurodegenerative diseases. Clinical trials have involved grafting brain tissue from aborted foetuses into patients with Parkinson's disease and Huntington's disease ( Dunnett et al . 2001 ; Wright & Barker 2007 ). While some successes have been noted, the outcomes have not been uniform and further clinical trials will involve more refined patient selection, in an attempt to predict who will benefit and who will not. Obviously, aside from the opposition in many quarters to using foetal material, there are practical challenges associated with availability and uniformity of the grafted cells and so therapies with pure populations of stem cells are an important, and achievable ( Conti et al . 2005 ; Lowell et al . 2006 ), goal.

No consideration of currently available stem cell therapies is complete without reference to gene therapy. Here, there have been some major achievements, including the successful treatment of children with X-linked severe combined immunodeficiency. However, the entire gene therapy field stalled when several of the children developed leukaemia as a result of integration of the therapeutic retroviral vector close to the LMO2 oncogene locus ( Gaspar & Thrasher 2005 ; Pike-Overzet et al . 2007 ). Clinical trials have since restarted, and in an interesting example of combined gene/stem cell therapy, a patient with an epidermal blistering disorder received an autologous graft of cultured epidermis in which the defective gene had been corrected ex vivo ( Mavilio et al . 2006 ).

These are just some examples of treatments involving stem cells that are already in the clinic. They show how the field of stem cell transplantation is interlinked with the fields of gene therapy and bioengineering, and how it has benefited from progress in other fields, such as immunology. Stem cells undoubtedly offer tremendous potential to treat many human diseases and to repair tissue damage resulting from injury or ageing. The danger, of course, lies in the potentially lethal cocktail of desperate patients, enthusiastic scientists, ambitious clinicians and commercial pressures ( Lau et al . 2008 ). Internationally agreed, and enforced, regulations are essential in order to protect patients from the dangers of stem cell tourism, whereby treatments that have not been approved in one country are freely available in another ( Hyun et al . 2008 ).

3. What are the big questions in the field?

Three questions in stem cell research are being hotly pursued at present. What are the core genetic and epigenetic regulators of stem cells? What are the extrinsic, environmental factors that influence stem cell renewal and differentiation? And how can the answers to the first two questions be harnessed for clinical benefit?

4. Core genetic and epigenetic regulators

Considerable progress has already been made in defining the transcriptional circuitry and epigenetic modifications associated with pluripotency ( Jaenisch & Young 2008 ). This research area is moving very rapidly as a result of tremendous advances in DNA sequencing technology, bioinformatics and computational biology. Chromatin immunoprecipitation combined with microarray hybridization or DNA sequencing ( Mathur et al . 2008 ) is being used to identify transcription factor-binding sites, and bioinformatics techniques have been developed to allow integration of data obtained by the different approaches. It is clear that pluripotency is also subject to complex epigenetic regulation, and high throughput genome-scale DNA methylation profiling has been developed for epigenetic profiling of ES cells and other cell types ( Meissner et al . 2008 ).

Oct4, Nanog and Sox2 are core transcription factors that maintain pluripotency of ES cells. These factors bind to their own promoters, forming an autoregulatory loop. They occupy overlapping sets of target genes, one set being actively expressed and the other, comprising genes that positively regulate lineage selection, being actively silenced ( Jaenisch & Young 2008 ; Mathur et al . 2008 ; Silva & Smith 2008 ). Nanog stabilizes pluripotency by limiting the frequency with which cells commit to differentiation ( Chambers et al . 2007 ; Torres & Watt 2008 ). The core pluripotency transcription factors also regulate, again positively and negatively, the microRNAs that are involved in controlling ES cell self-renewal and differentiation ( Marson et al . 2008 ).

As the basic mechanisms that maintain the pluripotent state of ES cells are delineated, there is considerable interest in understanding how pluripotency is re-established in adult stem cells. It appears that some cell types are more readily reprogrammed to iPS cells than others ( Aasen et al . 2008 ; Aoi et al . 2008 ), and it is interesting to speculate that this reflects differences in endogenous expression of the genes required for reprogramming or in responsiveness to overexpression of those genes ( Hochedlinger et al . 2005 ; Markoulaki et al . 2009 ). Another emerging area of investigation is the relationship between the epigenome of pluripotent stem cells and cancer cells ( Meissner et al . 2008 ).

Initial attempts at defining ‘stemness’ by comparing the transcriptional profiles of ES cells, neural and haemopoietic stem cells ( Ivanova et al . 2002 ; Ramalho-Santos et al . 2002 ) have paved the way for more refined comparisons. For example, by comparing the gene expression profiles of adult neural stem cells, ES-derived and iPS-derived neural stem cells and brain tumour stem cells, it should be possible both to validate the use of ES-derived stem cells for brain repair and to establish the cell of origin of brain tumour initiating cells. Furthermore, it is anticipated that new therapeutic targets will be identified from molecular profiling studies of different stem cell populations.

As gene expression profiling becomes more sophisticated, the question of ‘what is a stem cell?’ can be addressed in new ways. Several studies have used single cell expression microarrays to identify new stem cell markers ( Jensen & Watt 2006 ). Stem cells are well known to exhibit different proliferative and differentiation properties in culture, during tissue injury and in normal tissue homeostasis, raising the question of which elements of the stem cell phenotype are hard-wired versus a response to environmental conditions.

One of the growing trends in stem cell research is the contribution of mathematical modelling. This is illustrated in the concept of transcriptional noise: the hypothesis that intercellular variability is a manifestation of ‘noise’ in gene expression levels, rather than stable phenotypic variation ( Chang et al . 2008 ). Studies with clonal populations of haemopoietic progenitor cells have shown that slow fluctuations in protein levels can produce cellular heterogeneity that is sufficient to affect whether a given cell will differentiate along the myeloid or erythroid lineage ( Chang et al . 2008 ). Mathematical approaches are also used increasingly to model observed differences in cell behaviour in vivo . In studies of adult mouse interfollicular epidermis, it is observed that cells can divide to produce two undifferentiated cells, two differentiated cells or one of each ( figure 3 ); it turns out that this can be explained in terms of the stochastic behaviour of a single population of cells rather than by invoking the existence of discrete types of stem and progenitor cell ( Clayton et al . 2007 ).

The stem cell niche. Stem cells (S) are shown dividing symmetrically to produce two stem cells (1) or two differentiated cells (D) (2), or undergoing asymmetric division to produce one stem cell and one differentiated cell (3). Under some circumstances, a differentiated cell can re-enter the niche and become a stem cell (4). Different components of the stem cell niche are illustrated: extracellular matrix (ECM), cells in close proximity to stem cells (niche cells), secreted factors (such as growth factors) and physical factors (such as oxygen tension, stiffness and stretch).

5. Extrinsic regulators

There is strong evidence that the behaviour of stem cells is strongly affected by their local environment or niche ( figure 3 ). Some aspects of the stem cell environment that are known to influence self-renewal and stem cell fate are adhesion to extracellular matrix proteins, direct contact with neighbouring cells, exposure to secreted factors and physical factors, such as oxygen tension and sheer stress ( Watt & Hogan 2000 ; Morrison & Spradling 2008 ). It is important to identify the environmental signals that control stem cell expansion and differentiation in order to harness those signals to optimize delivery of stem cell therapies.

Considerable progress has been made in directing ES cells to differentiate along specific lineages in vitro ( Conti et al . 2005 ; Lowell et al . 2006 ; Izumi et al . 2007 ) and there are many in vitro and murine models of lineage selection by adult tissue stem cells (e.g. Watt & Collins 2008 ). It is clear that in many contexts the Erk and Akt pathways are key regulators of cell proliferation and survival, while pathways that were originally defined through their effects in embryonic development, such as Wnt, Notch and Shh, are reused in adult tissues to influence stem cell renewal and lineage selection. Furthermore, these core pathways are frequently deregulated in cancer ( Reya et al . 2001 ; Watt & Collins 2008 ). In investigating how differentiation is controlled, it is not only the signalling pathways themselves that need to be considered, but also the timing, level and duration of a particular signal, as these variables profoundly influence cellular responses ( Silva-Vargas et al . 2005 ). A further issue is the extent to which directed ES cell differentiation in vitro recapitulates the events that occur during normal embryogenesis and whether this affects the functionality of the differentiated cells ( Izumi et al . 2007 ).

For a more complete definition of the stem cell niche, researchers are taking two opposite and complementary approaches: recreating the niche in vitro at the single cell level and observing stem cells in vivo. In vivo tracking of cells is possible because of advances in high-resolution confocal microscopy and two-photon imaging, which have greatly increased the sensitivity of detecting cells and the depth of the tissue at which they can be observed. Studies of green fluorescent protein-labelled haemopoietic stem cells have shown that their relationship with the bone marrow niche, comprising blood vessels, osteoblasts and the inner bone surface, differs in normal, irradiated and c-Kit-receptor-deficient mice ( Lo Celso et al . 2009 ; Xie et al . 2009 ). In a different approach, in vivo bioluminescence imaging of luciferase-tagged muscle stem cells has been used to reveal their role in muscle repair in a way that is impossible when relying on retrospective analysis of fixed tissue ( Sacco et al . 2008 ).

The advantage of recreating the stem cell niche in vitro is that it is possible to precisely control individual aspects of the niche and measure responses at the single cell level. Artificial niches are constructed by plating cells on micropatterned surfaces or capturing them in three-dimensional hydrogel matrices. In this way, parameters such as cell spreading and substrate mechanics can be precisely controlled ( Watt et al . 1988 ; Théry et al . 2005 ; Chen 2008 ). Cells can be exposed to specific combinations of soluble factors or to tethered recombinant adhesive proteins. Cell behaviour can be monitored in real time by time-lapse microscopy, and activation of specific signalling pathways can be viewed using fluorescence resonance energy transfer probes and fluorescent reporters of transcriptional activity. It is also possible to recover cells from the in vitro environment, transplant them in vivo and monitor their subsequent behaviour. One of the exciting aspects of the reductionist approach to studying the niche is that it is highly interdisciplinary, bringing together stem cell researchers and bioengineers, and also offering opportunities for interactions with chemists, physicists and materials scientists.

6. Future clinical applications of stem cell research

Almost every day there are reports in the media of new stem cell therapies. There is no doubt that stem cells have the potential to treat many human afflictions, including ageing, cancer, diabetes, blindness and neurodegeneration. Nevertheless, it is essential to be realistic about the time and steps required to take new therapies into the clinic: it is exciting to be able to induce ES cells to differentiate into cardiomyocytes in a culture dish, but that is only one very small step towards effecting cardiac repair. The overriding concerns for any new treatment are the same: efficacy, safety and affordability.

In January 2009, the US Food and Drug Administration approved the first clinical trial involving human ES cells, just over 10 years after they were first isolated. In this trial, the safety of ES cell-derived oligodendrocytes in repair of spinal cord injury will be evaluated ( http://www.geron.com ). There are a large number of human ES cell lines now in existence and banking of clinical grade cells is underway, offering the opportunity for optimal immunological matching of donors and recipients. Nevertheless, one of the attractions of transplanting iPS cells is that the patient's own cells can be used, obviating the need for immunosuppression. Discovering how the pluripotent state can be efficiently and stably induced and maintained by treating cells with pharmacologically active compounds rather than by genetic manipulation is an important goal ( Silva et al . 2008 ).

An alternative strategy to stem cell transplantation is to stimulate a patient's endogenous stem cells to divide or differentiate, as happens naturally during skin wound healing. It has recently been shown that pancreatic exocrine cells in adult mice can be reprogrammed to become functional, insulin-producing beta cells by expression of transcription factors that regulate pancreatic development ( Zhou et al . 2008 ). The idea of repairing tissue through a process of cellular reprogramming in situ is an attractive paradigm to be explored further.

A range of biomaterials are already in clinical use for tissue repair, in particular to repair defects in cartilage and bone ( Kamitakahara et al . 2008 ). These can be considered as practical applications of our knowledge of the stem cell microenvironment. Advances in tissue engineering and materials science offer new opportunities to manipulate the stem niche and either facilitate expansion/differentiation of endogenous stem cells or deliver exogenous cells. Resorbable scaffolds can be exploited for controlled delivery and release of small molecules, growth factors and peptides. Conversely, scaffolds can be designed that are able to capture unwanted tissue debris that might impede repair. Hydrogels that can undergo controlled sol–gel transitions could be used to release stem cells once they have integrated within the target tissue.

Although most of the new clinical applications of stem cells have a long lead time, applications of stem cells in drug discovery are available immediately. Adult tissue stem cells, ES cells and iPS cells can all be used to screen for compounds that stimulate self-renewal or promote specific differentiation programmes. Finding drugs that selectively target cancer stem cells offers the potential to develop cancer treatments that are not only more effective, but also cause less collateral damage to the patient's normal tissues than drugs currently in use. In addition, patient-specific iPS cells provide a new tool to identify underlying disease mechanisms. Thus stem cell-based assays are already enhancing drug discovery efforts.

7. Conclusion

Amid all the hype surrounding stem cells, there are strong grounds for believing that over the next 50 years our understanding of stem cells will revolutionize medicine. One of the most exciting aspects of working in the stem cell field is that it is truly multidisciplinary and translational. It brings together biologists, clinicians and researchers across the physical sciences and mathematics, and it fosters partnerships between academics and the biotech and pharmaceutical industries. In contrast to the golden era of developmental biology, one of stem cell research's defining characteristics is the motivation to benefit human health.

Acknowledgements

We thank all members of our lab, past and present, for their energy, fearlessness and intellectual curiosity in the pursuit of stem cells. We are grateful to Cancer Research UK, the Wellcome Trust, MRC and European Union for financial support and to members of the Cambridge Stem Cell Initiative for sharing their ideas.

One contribution of 19 to a Theme Issue ‘ Personal perspectives in the life sciences for the Royal Society's 350th anniversary ’.

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USC Stem Cell study maps how genes instruct kidneys to develop differently in mice and humans

By   cristy lytal, in this section, read this next.

USC Stem Cell’s journey towards 1,000 mini-kidneys begins with $1 million from KidneyX

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How similar is kidney development in humans and in the lab mice that form the foundation of basic medical research? In a new study published in Developmental Cell , USC Stem Cell scientists probe this question by comparing the activity and regulation of the genes that drive kidney development in lab mice and humans.

“While we do have a lot in common with lab mice, our evolutionary paths diverged around 80 million years ago,” said corresponding author Andy McMahon , director of the Eli and Edythe Broad Center for Regenerative Medicine and Stem Cell Research at USC, and W.M. Keck Provost and University Professor of Stem Cell Biology and Regenerative Medicine, and Biological Sciences. “On the anatomical level, there are obvious differences in mouse and human kidneys in terms of the overall organ size, number of filtering units, and patterning of the ducts and lobes. We wanted to deepen our understanding of these fundamental differences to the level of the underlying genes and gene regulators that orchestrate kidney development.”

To accomplish this, first author Sunghyun Kim in the McMahon lab worked with his colleagues to build atlases comparing gene activity and regulation in different cell types in developing mouse and human kidneys. The scientists could then pinpoint similarities and differences between the two species and identify cell type- and species-specific genetic and gene regulatory programs relevant to kidney development and disease.

Many genes, such as the one that encodes the molecule PCDH15 , which helps cells adhere to each other, showed human-specific patterns of activity. These genes tended to be associated with cell interaction and migration, and might be necessary for building a complex, human-sized kidney during the relatively long period of embryonic development.

Other genes, including NTNG1 , a gene normally associated with nerve cell development, may be used specifically in the human kidney to guide human developmental processes.

Many gene regulators were also human-specific. Some of these have been associated with chronic kidney disease or congenital anomalies of kidney and urinary tract.

“By identifying human-specific gene regulatory regions, we were able to link these to regions previously associated with kidney disease, showing a potential for our research to provide clinical insight,” said Kim, a recent PhD graduate from USC currently pursuing postdoctoral studies at the Massachusetts General Hospital in Boston.

Additional co-authors are Kari Koppitch, Riana K. Parvez, Jinjin Guo, MaryAnne Achieng, Jack Schnell, and Nils O. Lindström from USC.

The work was federally funded by the National Institute of Diabetes and Digestive and Kidney Diseases (grants R37DK054364 and UC2DK126024), and privately funded by the Chan Zuckerberg Initiative (grant WU-20-101) as part of the Seed Network of the Human Cell Atlas consortium.

McMahon is currently or has recently been a consultant or scientific advisor to Novartis, eGENESIS, Trestle Biotherapeutics, and IVIVA Medical. All authors declare that they have no conflicts of interest.

Current Events: Bioelectrical Gradients Guide Stem Cell Morphology

Electrically conductive hydrogels may hold the power to advance the use of stem cells for neural engineering. .

Iris Kulbatski, a neuroscientist by training and word surgeon by trade, is a science editor with The Scientist's Creative Services Team. She holds a PhD in Medical Science and a Certificate in Creative Writing from the University of Toronto.

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ABOVE: Researchers recapitulate electrical gradients in vitro to help guide stem cell differentiation for neural regeneration. ©istock, Cappan

T he dance of development is electric. Bioelectrical gradients choreograph embryonic growth, signaling to stem cells what cell types they should become, where they should travel, who their neighbors should be, and what structures they should form. 1 The intensity and location of these signals serve as an electrical scaffold to map out anatomical features and guide development. Bioelectricity also shapes tissue regeneration . 2 Tapping into these mechanisms is of special interest to researchers who grapple with the challenge of regenerating injured nerves . 3 

One such curious team from Stanford University and the University of Arizona recently reported a new approach using electrically conductive hydrogels to induce differentiation of human mesenchymal stem cells into neurons and oligodendrocytes in vitro. 4 Their findings, published in the Journal of Materials Chemistry B , provide important proof of principle for future studies of biocompatible materials to electrically augment transplanted and endogenous cells after injury. 

“Our lab uses different polymers to interact with the nervous system. We think there's a window after injury that seems to mirror development,” said Paul George , a physician scientist at Stanford University and coauthor of this study. “Since a lot of development is guided by gradients and electric fields, we tried to create a hydrogel that had a gradient like you might see in the developing body that could guide stem cells to differentiate certain ways or form certain structures.”

Hydrogels are a popular biocompatible material for tissue engineers trying to mimic the native environment of cells. They retain large volumes of water, their stiffness and three-dimensional properties can be controlled, and they can be packed with electrically conductive fillers. “There are a lot of great potential applications for regenerative medicine, in vitro modeling, and potentially biomanufacturing,” said Nisha Iyer , a biomedical engineer at Tufts University, who was not involved in the study. “The idea that you could use electrical fields and 3D mechanical properties to impact stem cells without having to use different kinds of biomolecules or expensive growth factors to drive differentiation is hugely motivating.”

George and his team identified a specific differentiation pattern depending on the proximity of the stem cells to uniform versus varying electrical fields. Cells in the center of the hydrogel differentiated towards an oligodendrocyte lineage in response to a constant electric field, whereas those on the periphery tended to differentiate into neurons in response to a less intense, varying electric field. George’s study is unique because most in vitro studies of bioelectricity for neural regeneration focus on static electric fields rather than gradients. Spatial control of electrical gradients has the potential to mimic those found during development and aid neural regeneration following stem cell transplantation in future studies. 

“This is a nice proof of principle study. I think there is still quite a bit of additional work needed before we can use this practically in labs,” Iyer said. Although preliminary, this works takes the important first step for future transplantation studies of stem cells plus conductive gradient hydrogels, which could interact with the injured nervous system to potentially improve recovery. “This platform was our initial foray into trying to control those gradients and understand the developmental cues a little better,” George said. “There's so much that’s still unknown and if we can turn back the clock a little bit, maybe we can help patients who have peripheral nerve injury or stroke recover a little better.”

1. Levin M, Stevenson CG. Regulation of cell behavior and tissue patterning by bioelectrical signals: Challenges and opportunities for biomedical engineering . Annu Rev Biomed Eng . 2012;14:295-323. 2. Mathews J, Levin M. The body electric 2.0: Recent advances in developmental bioelectricity for regenerative and synthetic bioengineering. Curr Opin Biotechnol. 2018;52:134-144.  3. Oh B, et al. Modulating the electrical and mechanical microenvironment to guide neuronal stem cell differentiation . Adv Sci . 2021;8(7):2002112. 4. Song S et al. Conductive gradient hydrogels allow spatial control of adult stem cell fate . J Mater Chem B . 2024;12(7):1854-1863.

• Open access
• Published: 19 August 2024

Use of placental-derived mesenchymal stem cells to restore ovarian function and metabolic profile in a rat model of the polycystic ovarian syndrome

• Mojtaba Sarvestani 1 ,
• Alireza Rajabzadeh 2 , 3 ,
• Tahereh Mazoochi 2 , 3 ,
• Mansooreh Samimi 4 ,
• Mohsen Navari 5 , 6 &
• Faezeh Moradi 7 , 8  

BMC Endocrine Disorders volume  24 , Article number:  154 ( 2024 ) Cite this article

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Introduction

Polycystic ovary syndrome (PCOS) is an endocrine and metabolic disturbance that affects many women worldwide and is characterized by chronic anovulation, hyperandrogenism, and ovarian dysfunction. Placenta-derived mesenchymal stem cells (PDMSCs) are derived from the placenta and have advantages over other sources of MSCs in terms of availability, safety, and immunomodulation.

Materials and methods

In this experimental study, twenty female Wistar rats were assigned to four groups ( n  = 5) including control, sham, PCOS, and PCOS+PDMSCs groups. Then, PCOS was induced in the rats through administering letrozole for 21 days. PDMSCs (1 × 10 6 cells) were injected through the tail vein. Fourteen days after the cell infusion, evaluation was performed on the number of healthy follicles, corpus luteum, and cystic follicles as well as the levels of testosterone, follicle-stimulating hormone (FSH), luteinizing hormone (LH), fasting blood glucose, fasting insulin, and insulin resistance. Moreover, the serum levels of cholesterol, triglyceride (TG), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) were measured. Liver function was also determined by the evaluation of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels.

The number of corpus luteum and primordial, primary, secondary, and antral follicles was significantly elevated in the PCOS+PDMSCs group compared to the PCOS group. However, the number of cystic follicles significantly decreased in the PCOS+PDMSCs group. The LH and testosterone levels also decreased significantly, while FSH levels increased significantly in the PCOS+PDMSCs group. The levels of fasting blood glucose, fasting insulin, and insulin resistance notably decreased in the PCOS+PDMSCs group. Moreover, the lipid profile improved in the PCOS+PDMSCs group along with a significant decrease of cholesterol, LDL, and TG and an increase in HDL. The PCOS+PDMSCs group exhibited marked decreases in the AST and ALT levels as well.

The results of this study suggest that PDMSCs are a potential treatment option for PCOS because they can effectively restore folliculogenesis and correct hormonal imbalances, lipid profiles and liver dysfunction in a rat model of PCOS. However, further research is needed to establish the safety and effectiveness of PDMSCs for treating PCOS.

Peer Review reports

The polycystic ovary syndrome (PCOS) is a clinical syndrome involved in infertility disorders and associated with conditions ranging from chronic anovulation to metabolic dysfunction [ 1 ]. It also has to do with irregular menstrual cycles, hirsutism, alopecia, and acne. Long-term PCOS is accompanied by an increased risk of uterine cancer and endometrial hyperplasia [ 2 ]. Patients with these diseases present elevated testosterone and serum LH levels and diminished FSH levels [ 3 ]. In 2003, the Rotterdam Consensus established diagnostic criteria for PCOS, considered as the most commonly used clinical diagnosis and research criteria worldwide. Accordingly, the presence of at least two of the criteria for hyperandrogenism, anovulation or amenorrhea, and polycystic ovary morphology indicate PCOS. Patients with PCOS also suffer from hyperinsulinemia, insulin resistance, hypertension, dyslipidemia, and obesity [ 4 ].

PCOS is a multifactorial syndrome, and its causes are not yet well understood. Several etiological factors, including metabolic imbalances and immune system perturbations, are involved in developing heterogeneous clinical signs of PCOS. Several factors imply endocrine axis disturbance as well. Hyperandrogenism is the most prevalent biochemical perturbation in PCOS patients [ 5 ]. An increase in the pulse amplitude, the frequency of luteinizing hormone (LH) and the LH/FSH ratio increases androgen secretion from theca cells in polycystic ovaries [ 6 ].

In addition, the reduced release of sex hormone-binding globulin (SHGB) from the liver by hyperinsulinemia causes an increase in the bioavailability of free androgen [ 7 ]. Hyperinsulinemia also leads to the increased secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which results in hypersecretion of LH, increased testosterone, and decreased follicular maturation [ 8 ]. Several studies have shown that chronic inflammation affects the incidence of PCOS symptoms. Recently, specific inflammatory cytokines, namely, IL-6, TNF-α, IL-1, IL-18, and IL-17, have been found in PCOS patients [ 9 ]. Prevalent autoimmune diseases such as thyroiditis have been reported in these patients too [ 10 ]. Furthermore, there is a transition in macrophage polarization from M2 (anti-inflammatory) to M1 (proinflammatory) in PCOS. This macrophage polarization produces proinflammatory cytokines, including TNF-α and IL-6 [ 11 ].

Apart from disease management by lifestyle modifications (e.g., exercise, weight control, dietary changes), treatment options for PCOS patients include a combination of supplements and pharmacological interventions [ 12 ]. Conventional treatments for PCOS include oral contraceptives for menstrual derangements and hirsutism, antiandrogens for hyperandrogenism, and insulin sensitizers for insulin resistance and anovulation. There are newer treatment options such as statins, aromatase inhibitors, bromocriptine, and vitamin D/calcium. These therapies, however, cause a wide range of adverse metabolic and physiological effects that affect patients’ quality of life [ 13 ]. Moreover, conventional treatments for PCOS do not yield satisfactory results [ 14 ].

Recently, mesenchymal stem cells (MSCs), owing to their immunomodulatory, anti-inflammatory, and antiapoptotic properties, have been considered promising for restoring the function of damaged tissues and improving some illnesses [ 15 ]. MSCs have multiple potential functions, including self-renewal, secretion of various bioactive mediators, proliferation and differentiation into specialized cells, and migration toward damaged tissues [ 16 ]. Many previous investigations have demonstrated the therapeutic effect of MSCs or their extracellular secretome (i.e., exosomes) in various diseases, including myocardial infarction, cardiac ischemia, liver injury, and neurological, immunological and metabolic disorders [ 17 ]. Several studies have also indicated that MSCs have a strong potential to prevent ovarian dysfunction and uterus inflammation during infertility perturbations such as premature ovarian failure (POF) [ 18 ]. MSCs effectively treat other endocrine/metabolic diseases, such as diabetes, by restoring oxidative balance, relieving inflammation, and improving insulin resistance [ 19 ]. Since PCOS is a metabolic-reproductive-endocrine and even immunologic disorder, we assumed that MSCs could be a therapeutic option to decrease PCOS symptoms. Therefore, the present study was carried out to assess the ability of the mesenchymal stem cells derived from the placenta (PDMSCs) to restore ovarian function, promote follicular growth, and regulate hormone levels in a mouse model of PCOS.

Placenta-derived mesenchymal stem cells

Placenta tissue was obtained from women aged 25–30 years after an uncomplicated elective cesarean section at Shahid Beheshti Hospital (Kashan, Iran). All the participants had been informed a priori and had consented to donate. Briefly, under sterile conditions, the chorioamniotic membrane layer was removed, and 6–10 g of tissue was dissected and treated with collagenase 1 (Sigma‒Aldrich) at 37 °C in a 5% CO2 incubator for two hours until the PDMSCs crawled out of the tissue. The harvested PDMSCs were cultured at 2 × 10 5 cells/mm 3 in T25 flasks with DMEM supplemented with 10% fetal bovine serum (FBS) (Gibco, USA) and 1% penicillin-streptomycin (Gibco, USA). The PDMSCs were passaged for five generations when they reached approximately 80–90% confidence.

The phenotypic markers of the PDMSCs were evaluated using flow cytometry. The PDMSCs were also stained with R-phycoerythrin-conjugated monoclonal antibodies, which included antibodies against CD90 (Bio Legend, USA), CD105, CD73, CD45 (bioscience, USA), and CD34 (Santa Cruz. USA).

This experimental study was carried out on female Wistar rats (10-week old, weighing 200 ± 30 g). Ethical approval was received from the Ethics Committee of Kashan Medical University (IR.KAUMS.AEC.1400.001). The animals were kept at the animal breeding center of Kashan Medical University at a temperature of 24 °C, humidity of 50 ± 5%, and a 12-hour light/dark cycle. Food and water were also provided ad libitum.

Estrous cycle monitoring

Before the animals were subjected to treatment, they were checked for two sequential cycles and regular estrous cycles. Also, the PCOS induction in animals was confirmed by performing a vaginal smear test. Briefly, saline (50 µL) was injected into the vagina by means of a sampler, smeared on a slide stained with methylene blue, and examined via bright field microscopy.

Induction of PCOS and study design

Twenty female rats were divided into four groups ( n  = 5) including (a) a control group not receiving any interventions, (b) a sham group receiving oral gavage 1 ml of 0.5% carboxymethylcellulose (CMC) and a tail vein injection of 1 ml saline, (c) a PCOS group taking letrozole (1 mg/kg) (Aburaihan Pharma.co., Tehran, Iran) dissolved in 0.5% CMC, orally administered for 28 days, and (d) a PCOS+PDMSCs group in which the rats received a tail vein injection of PDMSCs (1 × 10 6 ) dissolved in 1 ml of saline.

Dosing volumes for both oral (P.O.) and intravenous (i.v.) administrations were standardized at 1 ml per 250 g body weight. This volume was selected based on universally accepted guidelines for rat studies, ensuring that dosing remained well within safe limits for both routes of administration.

Blood sampling

After two weeks of PDMSCs injection, the rats were put on a fast overnight (12 h) and then anesthetized via an IP infusion of ketamine (90 mg/kg) and xylazine (10 mg/kg). Their blood samples were directly collected from the heart, and serum was separated by centrifugation for 20 min at 3500 RPM.

Histological examination and follicle count

After the last blood collection (14 days after infusion), the rats were euthanized (IP injection of 250 mg/kg of pentobarbital sodium), and the ovaries were immediately separated, freed from the extra fat, and fixed in 10% formaldehyde. The samples were dehydrated in ascending alcohol and xylene and embedded in a paraffin block. Five-µm-thick serial sections were prepared with a microtome. One section per five serial sections was selected and ten sections from each rat were stained with hematoxylin-eosin. Only follicles with a nucleus were counted and The mean numbers of corpus luteum, primordial, primary, secondary, antral, and cystic follicles were analyzed according to the method used for counting the ovarian follicles in previse studies [ 20 , 21 ].

Follicle classification

The primordial follicle was found to have a flattened layer of granulosa cells that encompassed the oocyte. In instances where the oocyte was encircled by more than two layers of granulosa cells, it was identified as a primary follicle. Alternatively, if there was fluid accumulation between the granulosa cells, it was documented as a secondary follicle. Regardless of size, any follicle that possessed an antral cavity was classified as an antral follicle. The corpus luteum showcased distinct luteal cells with voluminous nuclei and vessels. Furthermore, a cystic entity was recognized as a large fluid-filled structure with an attenuated granulosa cell layer and a thickened theca internal cell layer.

Biochemical profile and hormonal assay

Serum fasting insulin, testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) concentrations were assessed via ELISA kits (MyBioSource, USA) following the manufacturer’s instructions. The serum levels of aspartate transaminase (AST), alanine transaminase (ALT), total cholesterol (TC), low-density lipoprotein (LDL), high-density lipoprotein (HDL), triglyceride (TG), and fasting glucose were evaluated via quantitative photometric kits (Pars Azmoon, Iran). Insulin resistance was calculated by the homeostasis model assessment of insulin resistance (HOMA-IR). The evaluation was according to the following equation [ 22 , 23 ].

Statistical analysis

The data are presented as mean ± standard deviation (SD). Kolmogorov–Smirnov test was used for evaluation of data distribution. Statistical analysis was conducted using one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison tests with SPSS vs. 22 (SPSS Inc., USA). GraphPad Prism 9(GraphPad Software, La Jolla, CA) was used to plot graphs. Statistical significance was set at P  < 0.05.

Isolation and identification of PDMSCs

The flow cytometry results showed that PDMSCs were positive for CD90 (96.7%), CD73 (99%), and CD105 (97.7%) surface markers and negative for CD45 (0.14%) and CD34 (0.32%) surface markers (Fig.  1 ).

The characterization of human PDMSCs using flow cytometry: Isolated cells were found positive for CD73, CD90 and CD105 and negative for CD34 and CD45

Estrus cycle analysis

As depicted in Fig.  2 , the rats in the control group exhibited a normal estrous cycle, whereas the rats with PCOS demonstrated a disrupted estrous cycle, primarily characterized by prolonged diestrous phase.

Representative vaginal smears from the control and PCOS rats at different stages: proestrus (P), estrous (E), metestrus (M), and diestrus (D)

Improving effect of PDMSCs treatment on ovarian morphological dysfunction

Harvested ovaries were assessed using H&E staining to explore the effect of PDMSC treatment on ovarian morphology. The control group displayed normal ovarian morphology with corpora luteum and healthy follicles. In the PCOS group, the number of healthy follicles significantly decreased compared to that in the control group. In the PDMSCs-PCOS group, the ovaries had normal morphology (Fig.  3 A), a large number of corpus luteum (5.25 ± 1.18 vs. 1.66 ± 0.53), primordial (6.93 ± 1.91 vs. 2.97 ± 0.95), primary (5.31 ± 1.71 vs. 2.74 ± 0.95), secondary (1.45 ± 0.51 vs. 0.82 ± 0.16) and antral follicles (0.68 ± 0.20 vs. 0.22 ± 0.07) versus those in the PCOS group (Fig.  3 B). Also, compared to the ovaries in the control rats, those in the PCOS rats had a significantly increased number of cystic follicles. As for the PCOS+ PDMSCs group, the number of cystic follicles (9.52 ± 1.97 vs. 19.01 ± 0.99) significantly decreased after 14 days (Fig.  3 B).

( A ) Photomicrograph of a section in the ovarian tissues in all the groups using hematoxylin and eosin staining: The ovarian tissue section of the PCOS + PDMSCs group showed a lot of healthy follicles. ( B ) Quantitative analysis of the number of follicles: The PCOS + PDMSCs group showed a reduced number of cystic follicles but increased numbers of healthy follicles and corpus luteum (mean ± SD) (* p  < 0.05, ** p  < 0.01, *** p  < 0.001, **** p  < 0.0001). CL: corpus luteum, AF: antral follicle, CF: cystic follicle

Hormone assays

The serum levels of testosterone, LH, and FSH were quantified to assess the hormonal alterations after the administration of PDMSCs (Fig.  4 ). As the results indicated, the injection of letrozole in the PCOS and PCOS+PDMSCs groups caused a substantial increase in the serum concentrations of testosterone and LH, compared to those in the control and sham groups. Furthermore, the serum levels of testosterone (9.41 ± 1.32 vs. 13.20 ± 1.27) and LH (4.74 ± 0.48 vs. 5.78 ± 0.43) were significantly lower in the PCOS+PDMSCs group than in the PCOS group following two weeks of treatment with PDMSCs. The serum concentrations of FSH were also significantly lower in the PCOS group than in the healthy control and sham groups. They were elevated in the PCOS+PDMSCs (2.83 ± 0.43 vs. 2.18 ± 0.17) group versus the PCOS group.

Evaluation of the serum hormonal levels in the control, sham, PCOS, and PCOS + PDMSCs groups: Compared to the PCOS group, PDMSCs treatment significantly decreased the serum levels of testosterone and LH and increased the FSH level (** p  < 0.01, **** p  < 0.0001). The data are shown as mean ± SD. LH: luteinizing hormone, FSH: follicle-stimulating hormone

Fasting serum glucose and insulin levels

The serum levels of fasting blood glucose and fasting insulin were assessed to evaluate insulin resistance 14 days after the treatment. The findings indicated a significant increase in the serum levels of fasting insulin, fasting glucose, and HOMA-IR in the PCOS group compared to those in the control and sham groups. However, the FBG, FINS, and HOMA-IR were significantly lower in the PDMSCs+PCOS group (166.9 ± 14.96, 14.17 ± 0.73, 5.84 ± 0.62, respectively) than in the PCOS group (188.2 ± 9.84, 15.58 ± 0.46, 7.24 ± 0.48, respectively) (Fig.  5 ).

Evaluation of the insulin resistance in the control, sham، PCOS and PCOS + PDMSCs groups: Fasting blood glucose, insulin level, and HOMA-IR index significantly decreased after PDMSCs intervention (* p  < 0.05, ** p  < 0.01, *** p  < 0.001, **** p  < 0.0001). The data are shown as mean ± SD. FBG: fasting blood glucose, FINS: fasting insulin, HOMA-IR: homeostasis model assessment of insulin resistance

Lipid profile

The serum levels of triglycerides, cholesterol, LDL, and HDL were evaluated to assess the effect of PDMSCs on the lipid profile. As Fig.  6 demonstrates, the levels of triglyceride (120.6 ± 5.66 vs. 56.11 ± 1.21), cholesterol (95.04 ± 4.48 vs. 62.49 ± 0.70), and LDL (27.40 ± 0.78 vs. 17.02 ± 0.20) in the PCOS groups were significantly elevated compared to those in the control group. However, triglyceride (120.6 ± 5.66 vs. 103.6 ± 9.13), cholesterol (95.04 ± 4.48 vs. 85.57 ± 4.39), and LDL (27.40 ± 0.78 vs. 25.64 ± 1.51) were significantly lower in the PCOS+PDMSCs group than in the PCOS group ( p  < 0.05). Additionally, the HDL levels of PCOS groups were lower than those of the control group (27.35 ± 1.71 vs. 42.28 ± 0.56). As for the HDL level in the PCOS+PDMSCsgroup, it was significantly greater than that in the PCOS group (31.48 ± 2.80 vs. 27.35 ± 1.71).

Evaluation of the lipid profile in the control, sham, PCOS, and PCOS + PDMSCs groups: After treatment with PDMSCs, the serum level of triglyceride, cholesterol, and LDL significantly decreased, but the serum level of HDL significantly increased compared to the PCOS group (* p  < 0.05, ** p  < 0.01,*** p<0.001 , **** p  < 0.0001). The data are shown as mean ± SD. TG: triglyceride, TC: total cholesterol, LDL: low-density lipoprotein, HDL: high-density lipoprotein

ALT and AST levels

The effect of PDMSCs on liver function was assessed by measuring the serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). As Fig.  7 demonstrates, compared to those in the control group, the serum ALT (127.2 ± 7.67 vs. 64.14 ± 2.01) and AST (133.0 ± 10.23 vs. 57.01 ± 1.33) levels were significantly elevated in the PCOS group ( P  < 0.01). Moreover, two weeks after the cell infusion, the serum levels of AST (117.2 ± 7.45 vs. 133.0 ± 10.23) and ALT (108.3 ± 9.56 vs.127.2 ± 7.67) in the PCOS+PDMSCs group were notably lower than those in the PCOS group.

Evaluation of the liver marker in the control, sham, PCOS, and PCOS + PDMSCs groups: Injection of PDMSCs significantly decreased the serum level of ALT and AST compared to the PCOS group (** p  < 0.01, **** p  < 0.0001). The data are shown as mean ± SD. AST: aspartate aminotransferase, ALT: alanine aminotransferase

Discussion and conclusion

The polycystic ovary syndrome (PCOS) is a medical condition that impacts both the reproductive and endocrine systems and is characterized by three critical diagnostic features, including chronic anovulation, hyperandrogenism, and the presence of polycystic ovaries. In the present study, PDMSCs were isolated from the human placenta and then infused into PCOS animals to evaluate their treatment effects. In recent years, various types of stem cells, such as human umbilical cord mesenchymal stem cells (HUMSCs), bone marrow mesenchymal stromal cells (BMSCs), and adipose mesenchymal stem cells (ADSCs), have been investigated for their ability to treat infertility diseases [ 24 ].Placenta mesenchymal stem cells have attracted widespread attention because of their low immunogenicity, accessible collection, lack of ethical issues, and high differentiation potential [ 25 ].

Similar to many previous investigations, in this study, it was observed that treating rats with letrozole could lead to significant histological changes, such as the development of cystic follicles and a decrease in healthy follicles. However, the number of healthy follicles (in all developmental stages) and the corpus luteum was increased after PDMSCs infusion. Additionally, PDMSCs could reduce the number of cystic follicles, which were significantly elevated in the PCOS group. In line with our findings, Seok et al. [ 26 ]demonstrated the substantial potential of PDMSCs in restoring the ovarian function in a rat ovariectomized (OVX) model. Another study [ 27 ]reported that injection of PDMSCs into a rat OVX model can promote primordial follicle activation.

Numerous reports have indicated that the effect of MSC transplantation on the ovarian function can occur via paracrine effects and the regulation of signaling pathways in the ovary. For example, Choi et al. [ 27 ]showed that PDMSCs activate the PI3K/AKT and ERK pathways. The PI3K/Akt pathway substantially influences follicular activation, granulosa cell development, and oocyte quality. In addition, Kim et al. [ 28 ]showed that MSCs can change the expression of circulating proteins and miRNAs associated with follicle development via bone morphogenetic protein (BMP) signaling and steroidogenesis in ovaries. BMPs are among the many growth factors secreted by MSCs. These proteins play a crucial role in female fertility and are involved in all the developmental stages of folliculogenesis.

Similarly, Kalhori et al. [ 29 ] reported that MSCs could repair damaged ovarian function by secreting several growth factors and antiapoptotic cytokines, such as TGF-β and VEGF. Angiogenesis is an essential mechanism in the recovery of the ovarian function [ 30 ]. Angiotensin, fibroblast growth factor-2 (FGF-2), and VEGF are secreted from MSCs to promote neovascularization and facilitate the blood perfusion of damaged ovarian tissues [ 31 ]. Studies have also shown that androgen promotes granulosa cell apoptosis, which causes a decrease in the number of antral follicles [ 1 ]. Wang et al. [ 32 ] reported that MSCs can inhibit GC apoptosis and enhance GC proliferation by upregulating Bcl-2 expression and releasing growth factors, including HGF, IGF-1, and VEGF.

Although the exact cause of PCOS is unclear, factors such as hyperandrogenism are essential for its occurrence. The studies published in the field have reported remarkable increases in circulating hormone levels, such as testosterone and LH, as well as lower serum FSH levels in patients with PCOS [ 33 ]. In these patients, an increase in the pulse frequency of gonadotropin-releasing hormone (GnRH) enhances LH release into the ovary. LH can directly stimulate androgen production by upregulating androgenic enzymes in theca cells. Increased expressions of CYP17A1 and CYP11A mRNAs, as well as increased activity of the 17βHSD, 3βHSD and P450c17 enzymes in theca cells, resulting in increased production of progesterone, 17OHP, and testosterone, are the persistent features of PCOS [ 34 ].

In addition, sex hormone-binding globulin (SHBG) levels are significantly lower in in the serum of PCOS individuals. A reduction in SHBG leads to elevation-free and biologically active androgens [ 35 ]. In line with these reports, we observed that the serum levels of several sex hormones, such as LH and testosterone, were significantly elevated and that the level of FSH was significantly reduced in the PCOS group. However, after PD-MSC intervention, these changes were reversed by the reduction of the serum testosterone and LH concentrations and the upregulation of the FSH level, compared to those in the PCOS group. Consistent with our results, Kalhori et al. [ 29 ] reported that MSC transplantation in PCOS models could regulate sex hormone levels, such as LH and testosterone. Moreover, Chugh et al. [ 36 ]showed that the MSC secretome could inhibit androgen production by reducing the expression of steroidogenic-related genes such as DENND1A, CYP11A1, and CYP17A1. In another in vitro study by Chugh et al. [ 37 ], BMP-2 secreted by MSCs could inhibit androgen production in the H295R cell line.

Insulin resistance is a prevalent abnormality in PCOS patients [ 38 ]. In our study, the fasting glucose level and HOMA-IR index notably increased, indicating the insulin resistance in PCOS rats. However, we found that PDMSCs improved HOMA-IR and significantly reduced FBG and FIN levels. In PCOS, the serine phosphorylation of the insulin receptor substrate (IRS) leads to the inhibited translocation of glucose transporter 4 (GLUT4) into the plasma membrane and the reduction of glucose uptake [ 39 ]. Additionally, the serine phosphorylation of IRS1 can inhibit the response to insulin receptor activation through reduced PI3K/AKT signaling [ 40 ]. As Chen et al. [ 41 ]. showed, MSCs could enhance the expression of GLUT4 and translocation activity by regulating the PI3K/AKT and MAPK pathways. Hyperandrogenism also activated the endoplasmic reticulum (ER) stress [ 42 ]. This stress can lead to the serine phosphorylation of IRS1 by activating the JNK signaling pathway [ 43 ]. As reported by Sanap et al. [ 44 ], MSCs can significantly decrease the expression of proteins related to the ER stress, such as CHOP1 and IRE1.

Several studies have indicated the interactions between hyperandrogenism and insulin resistance and liver dysfunctions such as dyslipidemia and the nonalcoholic fatty liver disease (NAFLD) in patients with PCOS [ 45 ]. Approximately 70% of women with PCOS exhibit abnormal lipid profiles [ 46 ]. Dyslipidemia, characterized by increased triglycerides, LDL, and cholesterol, is common in PCOS patients [ 46 ]. Cui et al. [ 47 ]reported that insulin resistance and elevated androgen levels contribute to hepatic steatosis and change lipid metabolism in the liver. Hyperinsulinemia through the inhibition of lipolysis results in an increase in no-esterified fatty acids. A high level of no-esterified fatty acids causes an increase in the TG level and reduces the level of HDL.

Moreover, hyperinsulinemia can elevate free testosterone (FT) levels through reduced hepatic SHBG synthesis [ 48 ]. It has also been documented that testosterone can reduce LDH levels by increasing the expression of the genes involved in LDH catabolism, such as SR-B1 [ 6 ]. Furthermore, patients with PCOS have higher alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels in the liver. Elevated ALT and AST levels reflect nonspecific hepatocellular damage and a risk of NAFLD [ 49 ].

In the present study, letrozole adversely affected liver function and significantly enhanced LDL, TC, TG, ALT, and AST in the PCOS group. Significant reductions in HDL levels were also observed in PCOS rats. However, treatment with PDMSCs decreased the levels of triglycerides, cholesterol, and LDL, which led to significantly increased HDL cholesterol levels. In addition, PCOS rats presented elevated levels of serum AST and ALT, which were reversed by PDMSCs.

Like in this study, Frodermann et al. [ 50 ]. reported that BM-MSCs significantly decreased the serum cholesterol and low-density lipoproteins (VLDLs) in LDLR-/- mice. The transplantation of Ad-MSCs also significantly improved the LDL, cholesterol, and HDL levels in patients with arteriosclerosis [ 51 ]. In this context, Shi et al. [ 52 ] showed that injecting MSCs into ApoE-/- mice could decrease total cholesterol and LDL levels. Another study [ 53 ] reported that the hepatic growth factor (HGF) secreted from MSCs has antiapoptotic effects on hepatocytes and restores liver injury. Researchers also demonstrated that the injection of UC-MSCs ameliorated NAFLD and improved lipid metabolism through upregulating the HNF4α-CES2 pathway, which plays a vital role in lipid and glucose metabolism in the liver [ 54 ].

As a result, mesenchymal stem cells improve damaged and dysfunctional tissues due to their inherent protective effects, such as anti-inflammatory, antiapoptotic, proangiogenic, and proliferative effects. In addition, PCOS, as an inflammatory, endocrine, and metabolic syndrome, can be an exciting candidate for MSC therapy. In this study, it was found that PDMSCs can significantly modify ovarian morphology, improve the imbalance of sex hormone levels, and enhance insulin sensitivity in rats with PCOS induced with letrozole. Moreover, PDMSCs had beneficial effects on liver function and lipid metabolism. However, additional investigations are needed on the molecular mechanisms of regeneration, infertility rectification, and safety concerns associated with the use of PDMSCs to treat PCOS.

A limitation of this study is the lack of direct evidence for PDMSC homing to the ovaries. While our results strongly suggest PDMSC-mediated effects based on restoring lipid profile, hormonal balance and liver and ovarian functioning markers, future studies employing molecular tracking techniques such as PCR, immunohistochemistry, or in vivo imaging would provide valuable insights into the precise localization and mechanism of action of PDMSCs in ovarian tissue repair.

Data availability

All data generated or analysed during this study are included in this published article.

Abbreviations

• Polycystic ovary syndrome

Luteinizing hormone

Follicle-stimulating hormone

Tumor Necrosis Factor-Alfa

Interleukin 6

Fasting blood glucose

Fasting insulin

Sex hormone binding globulin

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Acknowledgements

This study is financially supported by the Council for Development of Regenerative Medicine and Stem Cell Technologies in Iran.

The present study was supported by the deputy of research and technology, Kashan University of Medical Sciences, Kashan.

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Mojtaba Sarvestani

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Anatomical Sciences Research Center, Kashan University of Medical Sciences, Kashan, Iran

Department of obstetrics and gynecology, Faculty of Medicine, Kashan University of Medical Sciences, Kashan, Iran

Mansooreh Samimi

Department of Medical Biotechnology, School of Paramedical Sciences, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran

Mohsen Navari

Research Center of Advanced Technologies in Medicine, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran

Tissue Engineering and Applied Cell Sciences Division, Department of Anatomical Sciences, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran

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A.R. and T.M. designed the project. M.S. performed the experiment. M.S. and M.S. analyzed the data. A.R. and M.S. wrote the initial draft of the manuscript. F.M. and M.N. revised the manuscript. The final edit was accomplished by A.R and F.M.

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Sarvestani, M., Rajabzadeh, A., Mazoochi, T. et al. Use of placental-derived mesenchymal stem cells to restore ovarian function and metabolic profile in a rat model of the polycystic ovarian syndrome. BMC Endocr Disord 24 , 154 (2024). https://doi.org/10.1186/s12902-024-01688-0

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Will Stem Cells Help You Live Longer?

On their never-ending quest for the proverbial fountain of youth, biohackers and celebrities alike have latched onto stem cell anti-aging therapies. Do these treatments, reportedly used by longevity influencer Bryan Johnson and actors John Cleese and William Shatner, have any merit? Can they really turn back the hands of time and help you look and feel more youthful?

A harsh, but also beautiful fact of life is that we age. We say goodbye to the emotional angst of our teens and 20s, but then our joints begin to express their angst via aches, pains, and creaks.

Ultimately, aging happens because of various inflammatory processes that fall under the umbrella term inflammaging . Inflammaging affects our stem cells, the body’s innate repair system. Stem cell therapy, although still controversial as the research is in its infancy, may hold promise for reducing the impact of aging and boosting longevity.

“The challenge is that as we age the number of circulating stem cells decreases,” says stem cell scientist Christian Drapeau, M.S.c. By the age of 30, he adds, the decline reaches about 90 percent. “This reduction means that at some point in our 30s, we don’t have enough stem cells to keep up with cellular loss, and that’s when aging begins,” he says.

About the Experts

Christian Drapeau, M.S.c ., is a stem cell scientist and the founder and chief science officer at Stemregen . He is also the bestselling author of Cracking the Stem Cell Code . Drapeau is credited as first proposing and publishing the hypothesis that stem cells are the body’s repair system.

Fouad Ghaly, M.D. , is the founder of the Ghaly Center for Regenerative Medicine in California. He focuses on stem cell research and regenerative medicine therapies in his practice.

What Are Stem Cells, Anyway?

Stem cells are unique cells found in nearly all bodily tissues. They bring their A-game for keeping tissue healthy and repairing tissue that’s damaged. No other cells in the body have stem cells’ natural abilities ( 1 ).

“Stem cells are remarkable cells with the unique ability to renew and differentiate into any cell type in the body throughout a person’s lifetime,” Drapeau says. Their ability to differentiate means they can convert or morph.

Though we still have a long way to go, researchers have made substantial progress in stem cell research in the 21st century. “Historically, stem cells were primarily recognized as precursors to blood cells, including white and red blood cells, as well as platelets.”

But, as Drapeau notes, the early 2000s led to a promising discovery. Stem cells can become any other cell type. This enables them to aid in the repair and regeneration of any bodily organ ( 2 ).

“This groundbreaking finding has propelled stem cells into today’s therapeutic limelight and suggested new applications,” Drapeau says.

How stem cells function

Stem cells have two important functions: to make more stem cells and to convert into other types of cells that do different jobs ( 3 ).

Let’s say you seriously tweak your ankle on a trail run. The injured tissue sends signals to your bone marrow to release stem cells, Drapeau explains. Circulating stem cells will increase 3- to 10-fold ( 4 ). “The stem cells move to the site of damage, multiply, and transform into the necessary cells to repair the tissue,” he adds.

Stem cells also replace cells we naturally lose every day in all our organs and tissues ( 5 ). “To stay healthy, we need to constantly replace these lost cells, and that’s the role of stem cells,” Drapeau says. 

What stem cells treat now

You’ve likely heard of stem cell therapies to treat diseases. Stem cell therapy is currently approved by the U.S. Food and Drug Administration (FDA) for treating certain cancers and disorders that affect the blood and immune system.

Stem cell treatments have different processes, depending on the condition. Blood cancer treatment, for example, may involve a stem cell transplant. In these cases, healthy stem cells are either harvested from the blood or bone marrow of a donor or the patient and then transplanted into the patient ( 6 ).

Can Stem Cells Reverse Aging?

The number of circulating stem cells available to help us repair and renew tissue is crucial for our health and vitality ( 7 ). And those stem cells decline with age. Where things get tricky with this type of treatment is who gets to have access to them, as they are extremely expensive. 

“Currently, stem cell treatments for anti-aging are mostly accessible to the wealthy,” Drapeau says. However, endogenous stem cell mobilization may be more accessible. “This process involves stimulating the release of one’s own stem cells from the bone marrow, rather than relying on injections to increase circulating stem cells,” he says.

How a stem cell transplant for anti-aging works

Stem cell transplants are not currently available for anti-aging, at least not in the sense that such transplants are available in the United States to treat certain immune system conditions, blood disorders, or cancers.

However, some clinics in the country do offer therapies that harness stem cells from one’s own body. Regenerative medicine specialist Fouad Ghaly, M.D., explains one of the regenerative therapy options he offers at his clinic in California. 

“I can take fat from a man or a woman, isolate the stem cells, activate the stem cells, and inject them into the body,” he says. The stem cells go to the damaged areas and create “a field of regeneration,” he adds. (Critics of stem cell research say that stem cells may migrate to areas on the body that don’t need them.)

But not many practitioners in the U.S. offer the treatment. When you read about biohackers and celebrities getting stem cell therapies, they probably traveled elsewhere to get it. Bryan Johnson, for example, said in a post on X that he participated in a clinical trial in the Bahamas in which bone marrow-derived stem cells were injected into his knees, shoulders, and hips.

Live Longer

Age-related issues stem cell therapy may help.

Over time, inflammaging and other factors can lead to age-related diseases including type 2 diabetes, cardiovascular disease, neurodegenerative diseases, cancer, musculoskeletal disorders, sexual dysfunction, and more ( 8 ).

Age-related diseases are linked to a reduction in stem cells. “People with conditions affecting the heart, liver, kidneys, pancreas, lungs, and cardiovascular system, as well as those with high blood pressure or erectile dysfunction typically have less than 50 percent of the circulating stem cells found in healthy individuals of the same age,” Drapeau says. 

Not everyone develops these diseases as they age. The conditions are simply more prevalent in older populations. Lifestyle, environmental, and genetic factors all play a role in disease development ( 9 ).

“Harnessing the power of endogenous stem cell mobilization—releasing your own stem cells—or utilizing stem cell treatments can significantly impact age-related health issues,” Drapeau says.

Why Stem Cell Treatments Are Controversial

Over the past five years or so, the FDA, the Centers for Disease Control and Prevention (CDC), and the National Institutes of Health (NIH), have repeatedly warned the public about clinics and companies that market stem cell products or therapies ( 10 , 11 , 12 ). 

Stem cell products and treatments are not currently approved to treat any orthopedic conditions, neurological disorders, cardiopulmonary disorders, chronic pain, fatigue, macular degeneration, or autism, according to the FDA ( 10 ).

The federal agency also warns of severe health issues that have come about from unapproved therapies. “FDA has received reports of blindness, tumor formation, infections, and more … due to the use of these unapproved products,” the agency states ( 10 ). According to a 2017 report, patients who received stem cell eye injections at a Florida clinic became blind shortly after treatment ( 13 ).

And while the CDC and NIH both remain hopeful about stem cell therapy’s potential to treat various medical conditions and diseases, further research is needed to rule out acute and long-term health complications ( 11 , 12 ).

Where Stem Cell Research Is Heading

Research around stem cell therapy’s capabilities have, in many ways, only just begun ( 14 ). However, Drapeau and Ghaly say the evidence is growing around how stem cell therapies can be used strategically to combat aging and support longevity. 

“It will become clear that supporting stem cell function is the first step in addressing any type of tissue damage or degeneration,” Drapeau says. “As research progresses and clinical evidence accumulates, endogenous stem cell mobilization is likely to become standard practice for treating a wide range of conditions, from cardiovascular diseases to musculoskeletal injuries.”

But more than that, stem cells could potentially be used for organ replacement. “Can we induce a regular cell to become a liver? Yes,” Ghaly says. “Can we induce it to become a kidney? Yes. So now there will come a day when they can manufacture organs.”

Researchers have already regenerated fully functional urinary bladder tissue in a nonhuman primate by using stem cells from the animal’s bone marrow ( 15 ). The research serves as a preclinical model for humans.

Ultimately, stem cell research may shape how we treat and prevent age-related diseases, how we repair damaged tissue and organs, and how we recover from illness and injury. Such changes could help us theoretically feel and look better, despite our chronological age .

By using regenerative therapies, such as those related to stem cells, we may even be able to extend our lifespan, which is how long we live, and our healthspan, which is how long we generally feel healthy and have good quality of life.

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• Tian Z, et al. (2023. ) Introduction to stem cells.
• Wosczyna, et al. (2018.) A muscle stem cell support group: coordinated cellular responses in muscle regeneration.
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• Ajay E Kuriyan, et al. (2017.) Vision Loss after Intravitreal Injection of Autologous “Stem Cells” for AMD.
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CellCover Captures Neural Stem Cell Progression in Mammalian Neocortical Development

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Definition of cell classes across the tissues of living organisms is central in the analysis of growing atlases of single-cell RNA sequencing (scRNA-seq) data across biomedicine. Marker genes for cell classes are most often defined by differential expression (DE) methods that serially assess individual genes across landscapes of diverse cells. This serial approach has been extremely useful, but is limited because it ignores possible redundancy or complementarity across genes that can only be captured by analyzing multiple genes simultaneously. We aim to identify discriminating panels of genes. To efficiently explore the vast space of possible marker panels, leverage the large number of cells often sequenced, and overcome zero-inflation in scRNA-seq data, we propose viewing gene panel selection as a variation of the "minimal set-covering problem" in combinatorial optimization. We show that this new method, CellCover, captures cell-class-specific signals in the developing mouse neocortex that are distinct from those defined by DE methods. Transfer learning experiments across mouse, primate, and human data demonstrate that CellCover identifies markers of conserved cell classes in neurogenesis, as well as temporal progression in both progenitors and neurons. Exploring markers of human outer radial glia (oRG, or basal RG) across mammals, we show that transcriptomic elements of this key cell type in the expansion of the human cortex appeared in gliogenic precursors of the rodent before the full program emerged in the primate lineage. We have assembled the public datasets we use in this report at NeMO analytics where the expression of individual genes {NeMO Individual Genes} and marker gene panels can be freely explored {NeMO: Telley 3 Sets Covering Panels}, {NeMO: Telley 12 Sets Covering Panels}, and {NeMO: Sorted Brain Cell Covering Panels}. CellCover is available in {CellCover R} and {CellCover Python}.

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The authors have declared no competing interest.

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• 29 September 2021

Stem cells: highlights from research

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Gametes generated from scratch

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Nature 597 , S6-S7 (2021)

doi: https://doi.org/10.1038/d41586-021-02621-4

This article is part of Nature Outlook: Stem cells , an editorially independent supplement produced with the financial support of third parties. About this content .

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