The classical definition of a<unk>  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<unk> s 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<unk> s. Apart from this, it is said that<unk>  function is regulated in a feedback mechanism. Self-renewal Two mechanisms ensure that a<unk>  population is maintained (doesn't shrink in size): 1. Asymmetric cell division: a<unk>  divides into one mother cell, which is identical to the original<unk> , and another daughter cell, which is differentiated. When a<unk>  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<unk> . 2. Stochastic differentiation: when one<unk>  grows and divides into two differentiated daughter cells, another<unk>  undergoes mitosis and produces two<unk> s identical to the original.<unk> s use telomerase, a protein that restores telomeres, to protect their DNA and extend their cell division limit (the Hayflick limit). Potency meaning Pluripotent, embryonic<unk> s originate as inner cell mass (ICM) cells within a blastocyst. These<unk> s 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<unk> s A:<unk>  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<unk> . * Totipotent (also known as omnipotent)<unk> s 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<unk> s 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<unk> s can differentiate into a number of cell types, but only those of a closely related family of cells. * Oligopotent<unk> s can differentiate into only a few cell types, such as lymphoid or myeloid<unk> s. * Unipotent cells can produce only one cell type, their own, but have the property of self-renewal, which distinguishes them from non-stem cells (e.g. progenitor cells, which cannot self-renew). Identification In practice,<unk> s are identified by whether they can regenerate tissue. For example, the defining test for bone marrow or hematopoietic<unk> s (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<unk> s from the transplanted individual, which can themselves be transplanted into another individual without HSCs, demonstrating that the<unk>  was able to self-renew. Properties of<unk> s 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.<unk> s 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<unk> s.
Stem cell
Blood cell
Progenitor cell
Hematopoietic stem cell
Neural stem cell
Lymphoblast
Cancer stem cell
Brain cell