stem cell stem cell in multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. they are the earliest type of cell in a cell lineage. they are found in both embryonic and adult organisms, but they have slightly different properties in each. they are usually distinguished from progenitor cells, which cannot divide indefinitely, and precursor or blast cells, which are usually committed to differentiating into one cell type. this article is about the cell type. for the medical therapy, see cell therapy. stem celltransmission electron micrograph of a mesenchymal stem cell displaying typical ultrastructural characteristicsdetailsidentifierslatincellula praecursoriameshd013234thh1.00.01.0.00028, h2.00.01.0.00001 fma63368anatomical terminology in mammals, roughly 50–150 cells make up the inner cell mass during the blastocyst stage of embryonic development, around days 5–14. these have stem-cell capability. in vivo, they eventually differentiate into all of the body's cell types (making them pluripotent). this process starts with the differentiation into the three germ layers – the ectoderm, mesoderm and endoderm – at the gastrulation stage. however, when they are isolated and cultured in vitro, they can be kept in the stem-cell stage and are known as embryonic stem cells (escs). adult stem cells are found in a few select locations in the body, known as niches, such as those in the bone marrow or gonads. they exist to replenish rapidly lost cell types and are multipotent or unipotent, meaning they only differentiate into a few cell types or one type of cell. in mammals, they include, among others, hematopoietic stem cells, which replenish blood and immune cells, basal cells, which maintain the skin epithelium, and mesenchymal stem cells, which maintain bone, cartilage, muscle and fat cells. adult stem cells are a small minority of cells; they are vastly outnumbered by the progenitor cells and terminally differentiated cells that they differentiate into. research into stem cells grew out of findings by canadian biologists ernest mcculloch, james till and andrew j. becker at the university of toronto and the ontario cancer institute in the 1960s. as of 2016, the only established medical therapy using stem cells is hematopoietic stem cell transplantation, first performed in 1958 by french oncologist georges mathé. since 1998 however, it has been possible to culture and differentiate human embryonic stem cells (in stem-cell lines). the process of isolating these cells has been controversial, because it typically results in the destruction of the embryo. sources for isolating escs have been restricted in some european countries and canada, but others such as the uk and china have promoted the research. somatic cell nuclear transfer is a cloning method that can be used to create a cloned embryo for the use of its embryonic stem cells in stem cell therapy. in 2006, a japanese team led by shinya yamanaka discovered a method to convert mature body cells back into stem cells. these were termed induced pluripotent stem cells (ipscs). history the term stem cell was coined by theodor boveri and valentin haecker in late 19th century. pioneering works in theory of blood stem cell were conducted in the beginning of 20th century by artur pappenheim, alexander maximow, franz ernst christian neumann. the key properties of a stem cell were first defined by ernest mcculloch and james till at the university of toronto and the ontario cancer institute in the early 1960s. they discovered the blood-forming stem cell, the hematopoietic stem cell (hsc), through their pioneering work in mice. mcculloch and till began a series of experiments in which bone marrow cells were injected into irradiated mice. they observed lumps in the spleens of the mice that were linearly proportional to the number of bone marrow cells injected. they hypothesized that each lump (colony) was a clone arising from a single marrow cell (stem cell). in subsequent work, mcculloch and till, joined by graduate student andrew john becker and senior scientist louis siminovitch, confirmed that each lump did in fact arise from a single cell. their results were published in nature in 1963. in that same year, siminovitch was a lead investigator for studies that found colony-forming cells were capable of self-renewal, which is a key defining property of stem cells that till and mcculloch had theorized. the first therapy using stem cells was a bone marrow transplant performed by french oncologist georges mathé in 1958 on five workers at the vinča nuclear institute in yugoslavia who had been affected by a criticality accident. the workers all survived. in 1981, embryonic stem (es) cells were first isolated and successfully cultured using mouse blastocysts by british biologists martin evans and matthew kaufman. this allowed the formation of murine genetic models, a system in which the genes of mice are deleted or altered in order to study their function in pathology. by 1998, embryonic stem cells were first isolated by american biologist james thomson, which made it possible to have new transplantation methods or various cell types for testing new treatments. in 2006, shinya yamanaka's team in kyoto, japan converted fibroblasts into pluripotent stem cells by modifying the expression of only four genes. the feat represents the origin of induced pluripotent stem cells, known as ips cells. in 2011, a female maned wolf, run over by a truck, underwent stem cell treatment at the zoo brasília, this being the first recorded case of the use of stem cells to heal injuries in a wild animal. properties the classical definition of a stem cell requires that it possesses two properties: self-renewal: the ability to go through numerous cycles of cell growth and cell division, known as cell proliferation, while maintaining the undifferentiated state. potency: the capacity to differentiate into specialized cell types. in the strictest sense, this requires stem cells to be either totipotent or pluripotent—to be able to give rise to any mature cell type, although multipotent or unipotent progenitor cells are sometimes referred to as stem cells. apart from this, it is said that stem cell function is regulated in a feedback mechanism. self-renewal two mechanisms ensure that a stem cell population is maintained (doesn't shrink in size): 1. asymmetric cell division: a stem cell divides into one mother cell, which is identical to the original stem cell, and another daughter cell, which is differentiated. when a stem cell self-renews, it divides and does not disrupt the undifferentiated state. this self-renewal demands control of cell cycle as well as upkeep of multipotency or pluripotency, which all depends on the stem cell. 2. stochastic differentiation: when one stem cell grows and divides into two differentiated daughter cells, another stem cell undergoes mitosis and produces two stem cells identical to the original. stem cells use telomerase, a protein that restores telomeres, to protect their dna and extend their cell division limit (the hayflick limit). potency meaning main article: cell potency pluripotent, embryonic stem cells originate as inner cell mass (icm) cells within a blastocyst. these stem cells can become any tissue in the body, excluding a placenta. only cells from an earlier stage of the embryo, known as the morula, are totipotent, able to become all tissues in the body and the extraembryonic placenta. human embryonic stem cells a: stem cell colonies that are not yet differentiated. b: nerve cells, an example of a cell type after differentiation. potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell. totipotent (also known as omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. such cells can construct a complete, viable organism. these cells are produced from the fusion of an egg and sperm cell. cells produced by the first few divisions of the fertilized egg are also totipotent. pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers. multipotent stem cells can differentiate into a number of cell types, but only those of a closely related family of cells. oligopotent stem cells can differentiate into only a few cell types, such as lymphoid or myeloid stem cells. unipotent cells can produce only one cell type, their own, but have the property of self-renewal, which distinguishes them from non-stem cells identification in practice, stem cells are identified by whether they can regenerate tissue. for example, the defining test for bone marrow or hematopoietic stem cells (hscs) is the ability to transplant the cells and save an individual without hscs. this demonstrates that the cells can produce new blood cells over a long term. it should also be possible to isolate stem cells from the transplanted individual, which can themselves be transplanted into another individual without hscs, demonstrating that the stem cell was able to self-renew. properties of stem cells can be illustrated in vitro, using methods such as clonogenic assays, in which single cells are assessed for their ability to differentiate and self-renew. stem cells can also be isolated by their possession of a distinctive set of cell surface markers. however, in vitro culture conditions can alter the behavior of cells, making it unclear whether the cells shall behave in a similar manner in vivo. there is considerable debate as to whether some proposed adult cell populations are truly stem cells. embryonic embryonic stem cells (escs) are the cells of the inner cell mass of a blastocyst, formed prior to implantation in the uterus. in human embryonic development the blastocyst stage is reached 4–5 days after fertilization, at which time it consists of 50–150 cells. escs are pluripotent and give rise during development to all derivatives of the three germ layers: ectoderm, endoderm and mesoderm. in other words, they can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type. they do not contribute to the extraembryonic membranes or to the placenta. during embryonic development the cells of the inner cell mass continuously divide and become more specialized. for example, a portion of the ectoderm in the dorsal part of the embryo specializes as 'neurectoderm', which will become the future central nervous system. later in development, neurulation causes the neurectoderm to form the neural tube. at the neural tube stage, the anterior portion undergoes encephalization to generate or 'pattern' the basic form of the brain. at this stage of development, the principal cell type of the cns is considered a neural stem cell. the neural stem cells self-renew and at some point transition into radial glial progenitor cells (rgps). early-formed rgps self-renew by symmetrical division to form a reservoir group of progenitor cells. these cells transition to a neurogenic state and start to divide asymmetrically to produce a large diversity of many different neuron types, each with unique gene expression, morphological, and functional characteristics. the process of generating neurons from radial glial cells is called neurogenesis. the radial glial cell, has a distinctive bipolar morphology with highly elongated processes spanning the thickness of the neural tube wall. it shares some glial characteristics, most notably the expression of glial fibrillary acidic protein (gfap). the radial glial cell is the primary neural stem cell of the developing vertebrate cns, and its cell body resides in the ventricular zone, adjacent to the developing ventricular system. neural stem cells are committed to the neuronal lineages (neurons, astrocytes, and oligodendrocytes), and thus their potency is restricted. nearly all research to date has made use of mouse embryonic stem cells (mes) or human embryonic stem cells (hes) derived from the early inner cell mass. both have the essential stem cell characteristics, yet they require very different environments in order to maintain an undifferentiated state. mouse es cells are grown on a layer of gelatin as an extracellular matrix (for support) and require the presence of leukemia inhibitory factor (lif) in serum media. a drug cocktail containing inhibitors to gsk3b and the mapk/erk pathway, called 2i, has also been shown to maintain pluripotency in stem cell culture. human escs are grown on a feeder layer of mouse embryonic fibroblasts and require the presence of basic fibroblast growth factor (bfgf or fgf-2). without optimal culture conditions or genetic manipulation, embryonic stem cells will rapidly differentiate. a human embryonic stem cell is also defined by the expression of several