Editing Stem cell (section)

==Properties==
The classical definition of a stem cell requires that it possesses two properties:
* Self-renewal: the ability to go through numerous [[cell cycle|cycles]] of [[cell growth]] and [[cell division]], known as [[cell proliferation]], while maintaining the undifferentiated state.
* [[Cell potency|Potency]]: the capacity to [[Cellular differentiation|differentiate]] into specialized cell types. In the strictest sense, this requires stem cells to be either [[totipotency|totipotent]] or [[pluripotency|pluripotent]]—to be able to give rise to any mature cell type, although [[multipotent]] or [[unipotent cell|unipotent]] [[progenitor cell]]s 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 (does not 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 disrupts 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.<ref>Shenghui, H. E., Nakada, D., & Morrison, S. J. (2009). Mechanisms of stem cell self-renewal. Annual Review of Cell and Developmental, 25, 377–406.</ref>

H.

Stem cells use [[telomerase]], a protein that restores [[telomeres]], to protect their DNA and extend their cell division limit (the [[Hayflick limit]]).<ref>{{cite journal | vauthors = Cong YS, Wright WE, Shay JW | title = Human telomerase and its regulation | journal = Microbiology and Molecular Biology Reviews | volume = 66 | issue = 3 | pages = 407–425, table of contents | date = September 2002 | pmid = 12208997 | pmc = 120798 | doi = 10.1128/MMBR.66.3.407-425.2002 | doi-access = free }}</ref>

===Potency meaning===
{{Main|Cell potency}}
[[Image:Stem cells diagram.png|upright=1.5|thumb|right|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.]]
[[Image:Human embryonic stem cells.png|thumb|Human [[embryo]]nic stem cells <br /> A: Stem cell colonies that are not yet differentiated. <br /> B: [[Nerve]] cells, an example of a [[cell type]] after differentiation.]]

[[Cell potency|Potency]] specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.<ref name=Schoeler>{{cite book |title=Humanbiotechnology as Social Challenge |editor1=Nikolaus Knoepffler |editor2=Dagmar Schipanski |editor3=Stefan Lorenz Sorgner |page=28 |chapter=The Potential of Stem Cells: An Inventory | last = Schöler | first = Hans R. | name-list-style = vanc |publisher=Ashgate Publishing|year=2007 |isbn=978-0-7546-5755-2}}</ref>
* [[Totipotency|Totipotent]] (also known as omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism.<ref name=Schoeler/> These cells are produced from the [[Cell fusion|fusion]] of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.<ref>{{cite book | vauthors = Mitalipov S, Wolf D | chapter = Totipotency, pluripotency and nuclear reprogramming | volume = 114 | pages = 185–199 | year = 2009 | pmid = 19343304 | pmc = 2752493 | doi = 10.1007/10_2008_45 | isbn = 978-3-540-88805-5 | series = Advances in Biochemical Engineering/Biotechnology | bibcode = 2009esc..book..185M | title = Engineering of Stem Cells | publisher = Springer }}</ref>
* [[Pluripotency|Pluripotent]] stem cells are the descendants of totipotent cells and can differentiate into nearly all cells,<ref name=Schoeler/> i.e. cells derived from any of the three [[germ layer]]s.<ref>{{cite journal | vauthors = Ulloa-Montoya F, Verfaillie CM, Hu WS | title = Culture systems for pluripotent stem cells | journal = Journal of Bioscience and Bioengineering | volume = 100 | issue = 1 | pages = 12–27 | date = July 2005 | pmid = 16233846 | doi = 10.1263/jbb.100.12 | url = https://lirias.kuleuven.be/handle/123456789/238336 | url-access = subscription }}</ref>
* [[Multipotency|Multipotent]] stem cells can differentiate into a number of cell types, but only those of a closely related family of cells.<ref name=Schoeler/>
* [[Oligopotency|Oligopotent]] stem cells can differentiate into only a few cell types, such as lymphoid or myeloid stem cells.<ref name=Schoeler/>
* [[Unipotency|Unipotent]] cells can produce only one cell type, their own,<ref name=Schoeler/> 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 cell]]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 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 assay]]s, in which single cells are assessed for their ability to differentiate and self-renew.<ref>{{cite journal | vauthors = Friedenstein AJ, Deriglasova UF, Kulagina NN, Panasuk AF, Rudakowa SF, Luriá EA, Ruadkow IA | title = Precursors for fibroblasts in different populations of hematopoietic cells as detected by the in vitro colony assay method | journal = Experimental Hematology | volume = 2 | issue = 2 | pages = 83–92 | year = 1974 |id={{INIST|PASCAL7536501060}} {{NAID|10025700209}} | pmid = 4455512 }}</ref><ref>{{cite journal | vauthors = Friedenstein AJ, Gorskaja JF, Kulagina NN | title = Fibroblast precursors in normal and irradiated mouse hematopoietic organs | journal = Experimental Hematology | volume = 4 | issue = 5 | pages = 267–274 | date = September 1976 | pmid = 976387 }}</ref> 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.<ref>{{Cite journal | vauthors = Sekhar L, Bisht N |date=2006-09-01 |title=Stem Cell Therapy |journal=Apollo Medicine |volume=3 |issue=3 |pages=271–276 |doi= 10.1016/S0976-0016(11)60209-3 }}</ref>