Overview of stem cells

INTRODUCTION — Stem cells are cells that are capable of self-renewal and differentiation. Embryonic stem (ES) cells can form the entire human body. Somatic (adult) stem cells can form an entire organ or tissue; they were first identified in the hematopoietic system and are believed to be present in many other tissues.

Stem cell-based therapies have altered the care of individuals with hematologic, oncologic, dermatologic, ophthalmologic, and orthopedic conditions.

This topic reviews stem cell biology, clinical and research applications, and ethical considerations. Separate topics discuss the biology and clinical use of hematopoietic stem cells. (See "Overview of hematopoietic stem cells" and "Hematopoietic cell transplantation (HCT): Sources of hematopoietic stem/progenitor cells".)

STEM CELL BIOLOGY

What defines a stem cell? — All stem cells share two cardinal features: they are capable of self-renewal and they can differentiate.

●Self-renewal – Self-renewal is the ability of a cell to proliferate and produce daughter stem cells that retain stem cell potential, without the loss of differentiation potential and without undergoing senescence (biologic aging). In principle, stem cell renewal is indefinite. Other types of cells that are not stem cells may be able to self-renew for a limited number of cell divisions.

●Differentiation – Differentiation is the ability of a cell to develop into another cell type with a dedicated, specific function that cannot be executed in the nondifferentiated state. Differentiation capacity is not limited to stem cells, but the combination of self-renewal and differentiation potential is unique to stem cells.

A variety of cell types are not stem cells. Progenitor or precursor cells are distinct in that they are able to proliferate and differentiate, contributing to tissue regeneration, but they lack the potential for self-renewal. Progenitor cells arise from stem cells and typically have a limited lifespan and a limited number of possible cell divisions.

Symmetric versus asymmetric stem cell division — Cell division occurs through the process of DNA replication and mitosis. (See "Basic genetics concepts: Chromosomes and cell division", section on 'Mitosis'.)

Stem cells can divide symmetrically or asymmetrically (figure 1); the daughter cells may be stem cells, more differentiated cells, or one of each [1,2].

●Symmetric stem cell division – Produces two daughter cells that are either both stem cells or both differentiated cells. Symmetric stem cell division that produces two daughter stem cells will increase the stem cell pool. Symmetric stem cell division that produces two differentiated daughter cells will deplete the stem cell pool.

●Asymmetric stem cell division – Produces one replacement stem cell and one cell that is more differentiated.

Degrees of stem cell potency — The potency of a stem cell is defined by the types of differentiated cells (or other stem cells) it can produce (table 1). Classically, the potency of cells has been thought to be determined by the stage of embryonic development at which they are created.

●Totipotent – Totipotent stem cells have the capability to produce all cell types of the developing organism, including both embryonic and extraembryonic (eg, placental) tissues. Cells that arise from the first few cell divisions following fertilization of the egg are generally the only cells thought to have totipotency.

●Pluripotent – Pluripotent stem cells can only make cells of the embryo proper, but they can make all cells of the embryo including germ cells and cells from any of the germ layers. Therefore, they can make any cell of the body.

Pluripotency is thought to be limited to stem cells derived from either the inner cell mass of the blastocyst (a preimplantation stage of development occurring approximately 7 to 10 days after fertilization) or nascent germ cells in the embryo. Pluripotent stem cells are created from divisions of totipotent stem cells.

Cells cultured and cell lines established from these structures are called embryonic stem (ES) cells and embryonic germ cells, respectively. Pluripotent cells can be created in the laboratory from other cell types as well. (See 'Induced pluripotent stem (iPS) cells' below.)

Once the primitive streak forms in an embryo (day 10 to 14 postfertilization), most stem cells in that embryo are thought to be restricted to multipotency or unipotency. These have often been called somatic or "adult" stem cells. (See 'Somatic stem cells' below.)

Stem cells derived from umbilical cord blood are sometimes mistakenly considered equivalent to ES cells in the lay press, but they are actually multipotent somatic stem cells; they are also called hematopoietic stem cells because they give rise to the hematopoietic system.

Adult (somatic) stem cells can be multipotent or unipotent.

●Multipotent – Multipotent stem cells can produce different types of cells within a given germ layer. For example, multipotent stem cells from a mesodermal tissue like the blood (hematopoietic stem cells) can make all the blood cell types, including white blood cells (WBCs), red blood cells (RBCs), and platelets, but they cannot make cells of a different germ layer such as neural cells (ectoderm) or liver cells (endoderm).

●Unipotent – Unipotent stem cells make a single cell type. An example is a stem cell in the skin that can contribute to maintenance of the epidermis [3].

Cancer stem cells — A model that is gaining support in animal models hypothesizes that cancers, including solid tumors and hematologic malignancies, contain a definable population of malignant cells that can be demonstrated to have the two cardinal features of stem cells, self-renewal and differentiation capacity. (See 'What defines a stem cell?' above.)

These cells, like stem cells in nonmalignant tissues, could continually replace more mature cells of the tumor that have a limited lifespan. They are therefore hypothesized to be the cells that enable persistence and perhaps metastasis of tumors.

Experimental support for the cancer stem cell hypothesis has been generated in immunodeficient mice engrafted with human cancer stem cells [4,5]. In these animals, a subset of tumor cells is capable of engrafting the tumor, while other subsets cannot. Further, the tumor-initiating subset can be sequentially transplanted to initiate new tumors and can yield the full diversity of cells observed in the original malignancy, thereby fulfilling the experimental definition of stem cells.

A subpopulation of cancer cells that behave as cancer stem cells has been demonstrated in several tumor types [4-10]:

●Acute myelogenous leukemia

●Breast cancer

●Colon cancer

●Ovarian cancer

●Pancreatic cancer

●Head and neck cancer

●Malignant glioma

Whether all tumors have a subset of cancer stem cells is controversial. Further, much of the definition of whether cancer stem cells exist depends on engraftment in the immunodeficient mice, which is not necessarily a good surrogate for behavior in the person from whom the cancer cells were derived. However, the model is beginning to impact development and testing of therapies in oncology. (See 'Research applications' below.)

SOURCES OF STEM CELLS FOR CLINICAL USE OR RESEARCH — Stem cells can be derived from human embryos or somatic tissues, or they can be created in the laboratory by inducing greater potency in an already differentiated somatic cell, sometimes referred to as "de-differentiation."

Embryonic stem (ES) cells — ES cells are typically derived from a preimplantation blastocyst (7 to 10 days postfertilization). They have the capacity to become an embryo.

●Clinical use – Preimplantation genetic testing is done by removing a single ES cell from a developing embryo produced by in vitro fertilization (IVF). The cell can be tested for genetic abnormalities. If the remaining embryo (minus the cell that was removed) is transferred to the uterus and implants, it can develop normally into a fetus. Testing of ES cells from different embryos can allow selection of those embryos that lack a specific genetic abnormality for transfer. (See "Preimplantation genetic testing".)

Therapeutic uses of ES cells are discussed below. (See 'Embryonic stem (ES) cell-based replacement therapy (investigational)' below.)

●Research use – ES cells are a valuable source of cells to study developmental processes and related disorders because they have pluripotent capacity, with the versatility to give rise to virtually any cell type or tissue [11]. The most frequent techniques used to derive ES cells for research is to disrupt the blastocyst from which the cells are derived.

Ethical considerations have prompted research into other stem cell sources. (See 'Ethical considerations' below.)

Stem cells obtained from umbilical cord blood are not ES cells; they are a type of multipotent somatic stem cell. (See 'Degrees of stem cell potency' above.)

Somatic stem cells — Somatic stem cells (sometimes referred to as tissue-specific stem cells or adult stem cells) are thought to be present in most tissues and to persist throughout life (figure 2). One of the few organs thought to lack stem cells is the pancreas, although failure to demonstrate pancreatic stem cells may be the result of technical limitations rather than their true absence.

Tissue-specific stem cells are thought to provide the basis for tissue maintenance and response to injury. It is possible that stem cells are a durable source of replacement cells in some tissues but not in others.

●Tissues with high cell turnover – Stem cells are especially important for tissues where there is high cell turnover, such as the blood, skin, and intestine. Stem cells in these tissues have been clearly defined experimentally [12-14]. Hematopoietic stem cells are thought to account for the complete restoration of blood cell production and bone marrow regeneration following extensive cell damage to bone marrow with cancer chemotherapy.

●Tissues with less turnover – There is also evidence for a possible stem cell population for some tissues where the rate of cell turnover is lower, such as muscle, brain, and kidney [15-18].

For some tissues, including the islet cells of the pancreas, it is not clear whether adult stem cells exist [19,20]. A report suggesting the possibility of lung stem cells was retracted [21,22].

In tissues lacking stem cells, proliferation of more mature cells, including progenitor cells, precursor cells, or terminally differentiated cells, may be the basis for replacement when needed. The main implication is that mature cells are thought to have a limited number of cell divisions, as opposed to stem cells, which can self-renew indefinitely. (See 'What defines a stem cell?' above.)

When tissue injury is extensive, a tissue that depends upon residual mature cells to replace the injured cells may be more limited in its regenerative capacity than tissues that can rely on resident stem cells. This is one hypothesis for the failure of pancreatic islet damage to be repaired in type 1 diabetes mellitus. (See "Pathogenesis of type 1 diabetes mellitus".)

Induced pluripotent stem (iPS) cells — The idea that stem cell potential mirrors the stage of development at which the cells were harvested, and that potency is progressively reduced during development, dramatically changed in 2006.

In a remarkable set of experiments, genes encoding transcription factors expressed in pluripotent ES cells were introduced into mature cells [23]. This was done in a way that the genes would be "ectopically" expressed rather than suppressed by the normal genetic machinery. A small number of these mature cells reverted back (de-differentiated) into a highly immature state resembling an ES cell.

This process, referred to as reprogramming, induces a pluripotent state in a previously differentiated cell type (figure 3). These cells are therefore called iPS cells.

The ability to induce pluripotency has resulted in new insights and implications for therapy and research.

●Insight – The ability to create an iPS cell from a differentiated cell indicates that a cell's state of differentiation, and thus its function, can be manipulated. Cells have a plasticity that is far greater than previously recognized. As an example, a keratinocyte derived from the skin can be induced to become a pluripotent stem cell, essentially rewinding the cell's history to revert to an embryonic-like state.

●Therapeutic implications – A cell taken from an individual can be induced to be capable of forming any other cell type in that individual's body. As an example, a skin or blood sample obtained from a patient with a degenerative brain disorder could in principle be converted into a pluripotent stem cell that in turn could become a source for generating the neural cells affected by the disease. This would bypass the transplantation barrier by which neural cells from another individual can induce an immune response requiring potent immunosuppression. Creation of iPS cells to treat disease is at an early investigational stage and is not in clinical use. (See 'Induced pluripotent stem (iPS) cell-based therapies (investigational)' below.)

●Research implications

•Disease models – iPS cells allow investigators to generate disease models in the laboratory using easily obtained human cells for disorders where obtaining primary tissue, or developing reliable animal models, has been difficult. This includes heritable neurologic disorders, in which all of the body's cells contain the pathogenic genetic variants and for which neural tissue is challenging to obtain.

•Personalized medicine – An iPS from a given individual represents a highly personalized source of cells. While technologies to generate iPS involve genetic manipulations, it is anticipated that other methods will be developed to generate iPS cells that are genetically identical to the individual and have less risk of genetic abnormalities. Such cells could be used to assess "personalized" drug therapies and may represent a source of immunologically identical cells for drug testing and/or transplantation.

Further modifications of the iPS methodology are under active development. As an example, treatment of somatic cells with a cocktail of small molecule compounds was shown to alter gene expression in a similar pattern to that induced by ectopic expression of embryonic genes [24]. These advances raise the prospect of future use of iPS for therapeutic purposes. (See 'Induced pluripotent stem (iPS) cell-based therapies (investigational)' below.)

•Epigenetic regulation – As cells in the body differentiate, the genes that give them stem cell features are not removed; they are merely repressed by the genetic machinery, often via epigenetic mechanisms. Epigenetic changes involve modifications to the DNA that do not change the fundamental nucleic acid sequence. Research on altering stem cell potency may shed light on epigenetic regulation. (See "Principles of epigenetics", section on 'Overview of regulation'.)

CLINICAL APPLICATIONS — The evolving role of stem cells and range of possible applications in clinical medicine is developing along several lines (table 2).

Examples include:

●Stem cells as therapy (to replace endogenous cells that have been damaged or destroyed, or to modify the behavior of other cells). (See 'Stem cells as cell replacement therapy' below and 'Use of non-engrafting mesenchymal stem cells to modify disease course' below.)

●Stem cells as targets of drug therapy. (See 'Endogenous stem cells as targets of drug therapy' below.)

Stem cells as cell replacement therapy — Stem cells that have been taken out of the body and modified (autologous) or harvested from another individual (allogeneic) can be used to replace the person's own cells that either carry a pathogenic variant or have been destroyed.

●Pathogenic variant – Changes to stem cells causing them to produce abnormal progeny may be due to a heritable/genetic condition such as sickle cell disease or thalassemia or to an acquired condition such as leukemia or a neurodegenerative disorder.

●Destruction – Loss of stem cells may be due to an acquired disorder (autoimmune destruction, radiation exposure) or done therapeutically, such as bone marrow ablation using chemotherapy and radiation prior to hematopoietic stem cell transplantation.

The use of stem cells to replace abnormal or missing cells is conceptually compelling, and the identification of stem cells in a number of tissue types offers great promise for expanded clinical applications. There is the additional potential that patient-specific induced pluripotent cells may be a source of virtually any cell type. However, several issues remain to be addressed. (See 'Challenges with stem cells as replacement' below.)

Somatic stem cell-based replacement therapy — The paradigm for stem cells as a means of replacing diseased or injured tissue was first explored in response to the threat of nuclear war after World War II. Research on overcoming the effects of radiation injury led to the first demonstration of stem cells, the existence of which had been hypothesized since the early twentieth century.

Subsequently, it has been shown that both manipulated and unmanipulated stem cells can have a tremendous role in otherwise potentially fatal conditions.

●Hematopoietic system – In 1963, researchers demonstrated that a bone marrow-derived cell could replace all the blood elements and rescue an otherwise lethally irradiated animal by simple infusion of donor cells into the blood [25]. (See "Clinical manifestations, evaluation, and diagnosis of acute radiation exposure", section on 'Acute radiation syndrome'.)

Over the ensuing decades, hematopoietic stem cell transplantation became a standard means of treating hematologic malignancies as well as certain heritable and acquired bone marrow failure states, including radiation injury and other hematopoietic cell disorders such as thalassemia.

Subsequent work has also investigated the use of gene therapy with modified autologous hematopoietic cells to treat disorders such as adrenoleukodystrophy, sickle cell disease, and thalassemia.

●Skin – In 1975, it was reported that cultured cells from the skin could generate large numbers of cells sufficient to provide an autologous cutaneous barrier in patients with severe burns [26].

In another study in a seven-year-old child with the severe genetic disease junctional epidermolysis bullosa, genetic correction of the disease-causing variant in autologous skin stem cells, followed by transplantation of cultured skin grafts, resulted in regeneration of the skin [27]. (See "Overview of the management of epidermolysis bullosa", section on 'Experimental therapies'.)

●Bone grafting – Mesenchymal stem cells are used for bone grafting in orthopedics [28,29]. (See "Basic principles of bone grafts and bone substitutes".)

●Cornea – Limbal stem cells (from the region between the cornea and the bulbar conjunctiva) are used for corneal generation in ophthalmology practice [30].

●Other tissues – The identification of stem cells in other tissue types has led to the preclinical testing of stem cell therapies for other disorders, including those involving muscle and nerve tissue.

The potential use of stem cells to restore missing, lost, or damaged tissues plays a central role in the field of regenerative medicine, which seeks to improve organ function by regenerating tissue [31].

Embryonic stem (ES) cell-based replacement therapy (investigational)

●Clinical studies – Human ES cells have been successfully differentiated in vitro into multiple cell types with the potential for therapeutic use, including retinal cells, oligodendrocytes, pancreatic cells, cardiomyocytes, motor and dopaminergic neurons, and hematopoietic precursor cells [32]. Unlike somatic stem cells, which can be harvested from the individual and used for autologous transplantation, ES cells used for cell replacement therapy are allogeneic (from another individual).

•Retinal disease – Stem cell-based therapies are under investigation for retinal disease. (See "Retinitis pigmentosa: Treatment", section on 'Experimental therapies'.)

A 2015 report described use of ES cells to treat 18 patients with retinal disease (dry age-related macular degeneration or Stargardt's macular dystrophy) [33]. This involved direct injection of pure retinal pigment epithelium (RPE) cells into the subretinal space, using 50,000 to 150,000 RPE cells (with >9 percent purity) differentiated from ES cells. Graft survival was demonstrated by an increasing subretinal tissue in most patients; visual acuity improved in the injected eye in 10 of 18 individuals (56 percent). Low-dose tacrolimus and mycophenolate mofetil were used for immunosuppression beginning one week before the procedure and continuing for six weeks, followed by six additional weeks of mycophenolate mofetil only. During a median follow-up of 22 months, there was no evidence of adverse proliferation, rejection, or serious ocular or systemic toxicity.

Subsequent small studies have evaluated use of ES cell-derived RPE cells for macular degeneration using various approaches, and the results appear to demonstrate clinical benefit and safety [34-36].

•Heart disease – Stem cell-based therapies are under investigation for heart disease. (See "Investigational therapies for management of heart failure", section on 'Stem cell therapy'.)

ES-based cellular therapy was reported in a study of six patients with severe ischemic left ventricular dysfunction [37]. Human ES cell-derived cardiovascular progenitor cells embedded in a fibrin patch were epicardially delivered through a coronary artery bypass procedure, and technical feasibility and safety were demonstrated.

•Diabetes – Stem cell-based therapies are under investigation for diabetes [38,39]. (See "Pancreas and islet transplantation in diabetes mellitus", section on 'Alternative islet sources'.)

Preliminary outcomes were reported in a cohort of 15 patients with type 1 diabetes treated with ES cell-derived pancreatic endoderm cells (PECs) implanted subcutaneously in a macroencapsulation device [40]. Increased markers of pancreatic function were observed including fasting C-peptide levels and mixed meal-stimulated C-peptide secretion, indicative of meal-regulated insulin secretion. Therapy was well tolerated, with no teratoma formation or severe graft-related adverse events under immunosuppressive treatment.

•Neurologic disorders – Clinical studies are ongoing to evaluate ES cell-based therapies for Parkinson disease, spinal cord injury, and amyotrophic lateral sclerosis [41-43]

Additional studies are needed to assess the cell dosage, efficacy, and long-term safety in a larger number of patients.

●Preclinical studies – ES cell-derived somatic stem cells have also been evaluated in preclinical animal models of various disorders [44-52].

Induced pluripotent stem (iPS) cell-based therapies (investigational) — The recognition that a somatic cell taken from an individual can be induced to become a pluripotent stem cell capable of forming any other cell type in the body provides additional opportunities for regenerative medicine because these methods allow easily accessible autologous cells such as skin fibroblasts or hematopoietic cells to be reprogrammed into other cell types via an iPS cell intermediate. The cells can theoretically be used for tissue regeneration in the same patient (figure 4). (See 'Induced pluripotent stem (iPS) cells' above.)

The technology for creating iPS cells from differentiated autologous cells could overcome two important obstacles associated with human ES cells:

●Immune rejection after transplantation

●Ethical concerns regarding the use of human embryos

The use of iPS cell-derived cellular replacement products would be particularly powerful in monogenic diseases, where "patient-specific" iPS can be generated, the diseased gene in those cells potentially corrected, and the gene-corrected cells transplanted to restore organ function. The field of gene editing, which facilitates gene correction, is rapidly evolving [53]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing'.)

Investigation is in the early stages:

●Preclinical studies – Many of the studies to evaluate iPS cells as a source of cell replacement are being conducted in rodents. Examples of animal models for which iPS cells have been generated and used to differentiate cells that could then be transplanted for therapy include:

•Parkinson disease [54,55]

•Muscular dystrophy [56]

•Spinal cord injury [57]

•Type 1 diabetes

•Sickle cell disease [58]

Feasibility in a nonhuman primate model of diabetes was demonstrated by generating pancreatic islets from human iPS cells and transplanting them by intraportal infusion. This strategy restored endogenous insulin secretion and improved glycemic control. Long-term follow-up demonstrated improvement in average hemoglobin A1C values and a reduction in exogenous insulin requirement by 49 percent 15 weeks after transplantation [59].

The approach of using iPS cells to correct a genetic disorder has also been tested using human cells. As examples:

•Fanconi anemia (FA) – A normal version of an abnormal FA gene was introduced into fibroblasts from patients with FA, and the gene-corrected cells were reprogrammed to generate patient-specific iPS cells that could generate blood cell types similar to cells from unaffected individuals [60]. (See "Management and prognosis of Fanconi anemia".)

•Huntington disease (HD) – The abnormally expanded cytosine-adenine-guanine (CAG) trinucleotide repeat region of the huntingtin gene was corrected to normal length in fibroblasts derived from a patient with HD, and the fibroblasts were reprogrammed to generate iPS cells, which were then differentiated into gamma-aminobutyric acid (GABA)-producing neurons [61]. When transplanted into mice, the neurons were able to populate the basal ganglia and express GABA. In cell culture, the neurons showed normalization of signaling pathways, reduced susceptibility to cell death, and improved mitochondrial function, all of which are thought to be involved in the pathogenesis of HD. (See "Huntington disease: Genetics and pathogenesis".)

•Duchenne muscular dystrophy – Human iPS cell lines derived from individuals with Duchenne muscular dystrophy have undergone gene editing and then were directed to differentiate into skeletal muscle progenitor cells [56]. These cells displayed in vivo myogenic potential upon transplantation into immunodeficient mice and restored dystrophin.

●Clinical studies – Clinical studies using iPS cells to regenerate tissues have been initiated or designed in the areas of Parkinson disease, macular degeneration, retinitis pigmentosa, heart failure, spinal cord injury, cartilage defects, platelet transfusion, graft-versus-host disease (GVHD), and cancer immunotherapy [41,62].

iPS cell-derived retinal pigment epithelium (RPE) cells have been transplanted in a clinical study in a single patient with age-related macular degeneration, from whom autologous fibroblasts were used to generate iPS cells that were then differentiated into RPE and transplanted back to the patient [63]. The graft was well tolerated and visual acuity had not changed one year after the intervention. Before transplantation, the iPS cell-derived cells were screened for acquired mutations. A second patient who was selected for this clinical trial did not receive the transplant after testing revealed DNA alterations in the iPS cell-derived RPE cells. The efficacy and safety of iPS cell-derived transplants, therefore, remains unproven.

Challenges with stem cells as replacement — Several complexities must be addressed before widespread application of cell replacement using stem cell-based therapies becomes feasible [41]. These hurdles are not likely to be prohibitive, but each will require considerable effort and attention.

●Tissue integration – The ability of the transplanted cells to integrate into surrounding tissues to achieve a physiologically beneficial effect is of particular relevance where coordination of complex networks of cells is essential, such as in the heart and brain. Aberrant electrical circuits in these tissues can result in serious adverse events.

Although extensive testing is needed, one encouraging observation is that some cells appear to have an inherent capacity to incorporate into existing structures. As an example, in an animal model, human endothelial cells derived from ES cells were able to organize into tubular structures and integrate into the host vasculature when inserted as dispersed cells into a host tissue [64].

●Oncogenesis – The potential for transplanted cells to form tumors is of particular importance when using pluripotent cells, since these can form teratomas (neoplastic tumors containing cells corresponding to all three embryonic layers) in animal models [23,41,65]. Development of hematologic malignancies following transplantation of modified hematopoietic stem cells is a significant concern that may apply more to some underlying disorders than others. (See "Investigational therapies for sickle cell disease", section on 'Concern about myeloid malignancy in allogeneic transplantation' and "Hematopoietic stem cell transplantation in sickle cell disease".)

Tumorigenesis could result from several mechanisms [41]:

•For products derived from pluripotent cells, teratomas or tumors might arise from undifferentiated or immature cells retained in the final differentiated cell product.

•Somatic mutations may occur during in vitro culture that lead to tumor formation.

•For iPS cells, reprogramming factors such as c-Myc, which has oncogenic potential, may remain active [66].

•Methods of cell transduction such as retroviruses and lentiviruses have oncogenic potential by virtue of their ability to integrate into the host genome [67]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Integrating versus nonintegrating vectors'.)

This final concern has been mitigated by the introduction of efficient, integration-free methods for cell reprogramming involving the transient expression of reprogramming factors using adenoviruses, plasmids, transposons, Sendai viruses, synthetic mRNAs and recombinant proteins. These developments may mitigate the concerns about insertional mutagenesis, but will not entirely assuage concern for altered growth control of modified cells, particularly those with pluripotency.

The differentiation state of transplanted cells will need to be defined with high precision to avoid delivery of residual pluripotent cells that may differentiate aberrantly in vivo. Ruling out the presence of potentially oncogenic genetic alterations in iPS cells and their derivative cells remains critical for use in clinical cell therapies.

Evidence also suggests that oncogenesis is not restricted to pluripotent cells. In one case, a child was given cultured fetal brain cells intrathecally and subsequently developed multiple central nervous system tumors of donor origin [68]. The culture process may enable the outgrowth of genetically abnormal cell types that could be of potential danger [69]. The specifics of what types of testing will be required for pluripotent or other cells to assure genetic integrity of transplanted cells is an active area of consideration.

●Differentiation state of the cells – Generating the proper cell type from pluripotent cells remains a significant challenge for some cell types. Protocols have been devised to create some neural cell types of clear clinical importance [70]. However, for other tissues, such as the blood, the cell types created most closely resemble embryonic blood cells and are not capable of engrafting the bone marrow without further and undesirable genetic manipulation [71].

Achieving the right stage of differentiation is another consideration in development of the stem cell-derived cell therapies. It may be most desirable to generate progenitor cells (a type of cell that may be capable of self-renewing for a limited period of time and of creating a narrower range of differentiated progeny), rather than fully mature terminally differentiated cells in some tissues, so that the replaced cells do not quickly senesce and die. Uniformity and consistency of the cell product may be difficult to achieve across different donor sources.

Use of non-engrafting mesenchymal stem cells to modify disease course — Beyond use of stem cells for cell replacement, the ability of certain stem cells to alter disease without engrafting has been explored as a means of modifying cellular response to injury or aberrant immune activity. Such non-engrafting stem cells are hypothesized to provide complex signals that may affect disease outcomes without directly replacing injured cells.

Mesenchymal stem cells were first identified from bone marrow [72,73]. This population of cells is defined primarily by its ability to form colonies in tissue culture, and by the ability of single cells derived from the colonies to differentiate into osteoblasts, adipocytes or chondrocytes in vitro. Identification of these cells and understanding of their in vivo function remains controversial.

Mesenchymal stem cells have been studied in the following settings of response to injury:

●Tissue recovery – The potential for mesenchymal stem cells to form muscle led to exploration of their use in the setting of ischemic injury of the heart. While initial indications suggested that the cells might repopulate cardiac cells, it was subsequently shown that the cells do not engraft [74]. Rather, it is proposed that they provide complex paracrine signals that enhance the ability of cardiac (and other tissues) to respond to ischemic injury [75-77]. This concept remains highly contested, in part because the mechanisms by which stem cells may function in this capacity are not clear. The mesenchymal stem cell populations studied are highly variable and have been variably reported to secrete a number of factors including indoleamine 2, 3-dioxygenase, prostaglandins, interleukin-6, hepatocyte growth factor, inducible nitric oxide synthase, and tumor growth factor (TGF)-beta 1 [78].

●Immune function – It has been proposed that the mesenchymal stem cell population is capable of altering immune function and may therefore modulate injury responses as in ischemia, or mitigate the effects of immune-mediated diseases. Mesenchymal stem cells have been tested in clinical trials in humans in settings of GVHD following allogeneic hematopoietic stem cell transplantation. There has been controversy regarding the impact of the cells and the mechanism by which the cells may be acting. (See "Treatment of acute graft-versus-host disease", section on 'Mesenchymal stromal cells (MSCs)'.)

While this use of stem cell populations as disease-modifying cell platforms is an area of active investigation, the concept should not be considered proven. The prospect that mesenchymal cell populations may have the ability to arrive at sites of injury and provide paracrine signals is an exciting and potentially important application of stem cell biology, but it is not yet a defined therapy.

Endogenous stem cells as targets of drug therapy — There is emerging recognition that stem cells resident in adult tissues may be targets for pharmacologic interventions with the goal of augmenting the regenerative potential of endogenous stem cells. This approach may avoid concerns related to tissue integration of exogenous or allogeneic cells. (See 'Stem cells as cell replacement therapy' above.)

Growing understanding of the signaling processes that activate stem cells or modify their differentiation increases interest in this approach. Analogous to the use of erythropoietin to modify the activity of red blood cell (RBC) progenitor cells, agents with similar effects on stem cells might facilitate reconstitution of other blood cell types from hematopoietic stem cells or other mature cell types from other tissue stem cells.

Some studies have identified agents capable of altering stem cell activity after injury to bone or bone marrow [79,80]. As an example, a prostaglandin E2 (PGE2) derivative was identified in a chemical screen to be a potent positive regulator of hematopoietic stem cells in zebrafish, increasing self-renewal capacity and engraftment [81]. In a subsequent study in which patients with hematologic malignancies were treated by hematopoietic stem cell transplantation using umbilical cord blood stem cells, the cells were modified using the PGE2 derivative, and this was associated with accelerated neutrophil recovery and preferential long-term engraftment of the PGE2-treated cells in the recipients [82].

PGE2 similarly supports the expansion of human colon stem cells in cell culture, leading to the idea that in vivo manipulation of PGE2 metabolism may modulate stem cells and regeneration of multiple tissue types. A preclinical study in mice demonstrated that treatment with small molecules capable of increasing PGE2 levels promoted tissue regeneration after colon and liver injury [83].

RESEARCH APPLICATIONS

Disease models — Stem cells offer the possibility of creating in vitro disease models that may improve molecular understanding of disease and accelerate the development of new therapies. Human cells from individuals with diseases affecting the nervous system, for example, have been extremely difficult to obtain for in vitro analysis. With stem cell approaches, it is possible to generate sufficiently large numbers of cells to study the molecular basis of functional deficits associated with the disease or to be used in the development and testing of new drugs.

The development of induced pluripotent stem (iPS) cells has generated efforts to create in vitro disease models amenable to genetic and small molecule study. A major goal of such efforts is to identify compounds capable of altering disease progression. These efforts depend upon robust cell culture systems that resemble the in vivo context closely enough to be useful for drug evaluation. Fulfillment of this requirement still needs validation.

Two early examples depict use of iPS cells to create a disease model for further study of human disease pathogenesis and treatment.

●iPS cell lines were created from patients with familial dysautonomia (FD), a rare fatal sensory autonomic neuropathy caused by a point mutation in the IKBKAP gene, involved in transcriptional elongation [84]. The investigators demonstrated tissue-specific missplicing of IKBKAP in purified FD-iPSC-derived lineages, suggesting a mechanism for disease specificity. Functional studies revealed marked abnormalities in neurogenic differentiation and migration behavior, recapitulating disease characteristics. FD-iPSCs were subsequently used for validating the potency of candidate drugs to reverse aberrant splicing and ameliorate neuronal differentiation and migration. (See "Hereditary sensory and autonomic neuropathies", section on 'HSAN3 (Familial dysautonomia)'.)

●In another study, iPS cells were generated from skin fibroblasts from a child with spinal muscular atrophy [85]. These cells expanded robustly in culture, maintained the disease genotype, and generated motor neurons that showed selective deficits compared with those derived from the child's unaffected mother.

iPS cells have also been used to generate disease models for a wide variety of genetic diseases, including neurologic, hematologic, metabolic, and cardiovascular disorders. These studies have generated novel insights into the molecular mechanisms driving these disorders and platforms for drug screening [86].

Cancer therapy — An additional application of stem cell biology to disease modeling is in the setting of cancer. If cancer stem cells are present, then treatment must in some way target the cancer stem cells to destroy them and/or prevent growth of tumor cells. (See 'Cancer stem cells' above.)

Studies are investigating whether cancer drugs affect cancer stem cells in the same way they affect differentiated tumor cells and whether specific features of cancer stem cells can be targeted.

ETHICAL CONSIDERATIONS

●Human embryo research – The first derivation of human embryonic stem (ES) and embryonic germ cells in 1998 sparked enormous interest and controversy. The initial, and still most frequent, techniques used to derive ES cells disrupt the blastocyst from which the cells are derived, raising concern that an early human life form was being destroyed.

The source of the blastocyst was generally discarded material from in vitro fertilization (IVF) clinics that was not going to be transferred. Nonetheless, the issue of using the material for research, even if that research was intended to assuage human suffering, was morally unacceptable for many. These controversies led to policy decisions among some nations that precluded ES cell research or, as in the United States, eliminated federal funding support for such research.

The advent of reprogramming to generate induced pluripotent stem (iPS) cells has provided an alternative source of pluripotent cells with less ethical concern, since they do not require destruction of embryonic or blastocyst tissue. However, iPS cells have not yet been shown to be fully equivalent to ES cells, and ES cells still represent the best-defined pluripotent cell population.

Ethical concerns about gene editing of ES cells are discussed separately. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Ethical concerns with germline genome editing'.)

●Defining safety in clinical trials – A concern regarding the use of any type of stem cells in clinical studies is how to define safety.

Safety has been explored in studies using hematopoietic stem cells for transplantation and in dermatology for keratinocyte skin grafts and mesenchymal stem cell transplantation.

However, several concerns are unique to pluripotent cells, including:

•Possible aberrant growth in tissues where this could be catastrophic, such as the central nervous system and heart.

•Possible acquisition of genetic variation that disrupts function or is oncogenic.

•Challenges in identifying when the cells have acquired tumorigenic potential.

•Challenges in determining whether the cells can integrate as functional tissue.

Clinical experience will be necessary to define some of the risks in stem cell-based therapeutics, and careful design of clinical trials with a range of parameters specific to the field are in process. (See 'Challenges with stem cells as replacement' above.)

●Potential for exploitation of patients who are looking for a cure – Stem cells have enormous potential that is well recognized by the public, and their use is a source of hope for those dealing with otherwise untreatable medical problems.

However, the field of stem cell-based therapy is in its infancy, and progress is painstaking. For those desperate to get help, unproven therapies even outside of reputable centers or clinical trials are a highly tempting option. The potential for patient exploitation raises another extremely challenging area.

•There has been a profusion of centers in many parts of the world, often offering treatment without clear definition of what cells would be used, what the source of cells is, and the scope of the clinical experience that is being used to support claims of safety and efficacy, if any.

•Services are often offered for cash payment with unclear provisions to monitor the patient following treatment or to respond to adverse events.

Patients and their doctors should ask a range of specific questions about such therapies. The International Society for Stem Cell Research (ISSCR) has provided suggested questions and other information that may be of use (www.isscr.org).

SUMMARY

●Definition and behavior – Stem cells have the capabilities of self-renewal (indefinite) and differentiation. Stem cells are classified based on the type of differentiated cell they can reproduce. They can divide to create new stem cells or more differentiated cells. Pluripotent stem cells, which include embryonic stem (ES) cells and induced pluripotent stem (iPS) cells, can make all cells of the embryo, including germ cells and cells derived from ectoderm, mesoderm, and endoderm. Somatic stem cells are more limited in repertoire. Some cancers are thought to contain a type of cancer stem cell responsible for tumor persistence and possibly metastasis. (See 'Stem cell biology' above.)

●Sources of stem cells – ES cells are typically derived from the preimplantation blastocyst (7 to 10 days postfertilization). iPS cells are created from somatic tissues in the laboratory via reprogramming. Somatic (also called adult or tissue-specific) stem cells typically derive from tissue formed beyond 10 to 14 days postfertilization. Somatic stem cells have been identified in rapidly dividing tissues such as bone marrow, skin, and gastrointestinal tract. Their presence in other tissues such as pancreatic islet cells is unclear. (See 'Sources of stem cells for clinical use or research' above.)

●Clinical uses – Stem cells have several clinical applications, although concerns about generating the right stage of differentiation, integrating accurately into target tissues, and promoting oncogenesis remain. Hematopoietic stem cell replacement is a proven therapy for certain hematologic neoplasms, bone marrow failure syndromes, and other heritable conditions such as thalassemia. Burn therapy, bone grafting, and corneal transplant tissues use stem cell-generated tissue. Tissue replacement treatment for other conditions (retinal disease, Parkinson disease, myocardial infarction) is in development. Mesenchymal stem cells may modify diseased tissue without engrafting. Stem cells may be used in the laboratory to study drug efficacy. (See 'Clinical applications' above.)

●Ethical considerations – Ethical concerns include moral objections to destroying blastocysts or embryos for stem cell research, challenges in defining safety in clinical trials, and the potential for "stem cell clinics" to use unproven therapies and exploit patients who lack other treatment options for serious conditions. (See 'Ethical considerations' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges David T Scadden, MD, and Benjamin A Raby, MD, MPH, who contributed to earlier versions of this topic review.