Editing Stem cell (section)

==Embryonic==
{{Main|Embryonic stem cell}}

[[Embryonic stem cell]]s (ESCs) are the cells of the [[inner cell mass]] of a [[blastocyst]], formed prior to [[Implantation (human embryo)|implantation]] in the uterus.<ref>{{cite journal | vauthors = Thomson JA, Itskovitz-Eldor J, Shapiro SS, Waknitz MA, Swiergiel JJ, Marshall VS, Jones JM | title = Embryonic stem cell lines derived from human blastocysts | journal = Science | volume = 282 | issue = 5391 | pages = 1145–1147 | date = November 1998 | pmid = 9804556 | doi = 10.1126/science.282.5391.1145 | bibcode = 1998Sci...282.1145T | doi-access = free }}</ref> In [[human embryonic development]] the [[blastocyst]] stage is reached 4–5 days after [[Human fertilization|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 layer]]s: [[ectoderm]], [[endoderm]] and [[mesoderm]]. In other words, they can develop into each of the more than 200 cell types of the adult [[human body|body]] when given sufficient and necessary stimulation for a specific cell type. They do not contribute to the [[extraembryonic membrane]]s 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]] (CNS).<ref name="Developmental biology">{{cite book |last1=Gilbert |first1=Scott F. |title=Developmental Biology |date=2014 |publisher=Sinauer Associates |isbn=978-0-87893-978-7 }}{{Page needed|date=January 2019}}</ref> 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 cell|radial glial progenitor cells]] (RGPs). Early-formed RGPs self-renew by symmetrical division to form a reservoir group of [[progenitor cell]]s. These cells transition to a [[neurogenesis|neurogenic]] state and start to divide [[Asymmetric cell division|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).<ref>{{cite journal | vauthors = Rakic P | title = Evolution of the neocortex: a perspective from developmental biology | journal = Nature Reviews. Neuroscience | volume = 10 | issue = 10 | pages = 724–735 | date = October 2009 | pmid = 19763105 | pmc = 2913577 | doi = 10.1038/nrn2719 }}</ref><ref>{{cite journal | vauthors = Noctor SC, Flint AC, Weissman TA, Dammerman RS, Kriegstein AR | title = Neurons derived from radial glial cells establish radial units in neocortex | journal = Nature | volume = 409 | issue = 6821 | pages = 714–720 | date = February 2001 | pmid = 11217860 | doi = 10.1038/35055553 | bibcode = 2001Natur.409..714N | s2cid = 3041502 }}</ref>  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 ([[neuron]]s, [[astrocyte]]s, and [[oligodendrocyte]]s), and thus their potency is restricted.<ref name="Developmental biology"/>

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 [[Glycogen synthase kinase-3 beta|GSK3B]] and the [[MAPK/ERK pathway]], called 2i, has also been shown to maintain pluripotency in stem cell culture.<ref>{{cite journal | vauthors = Ying QL, Wray J, Nichols J, Batlle-Morera L, Doble B, Woodgett J, Cohen P, Smith A | title = The ground state of embryonic stem cell self-renewal | journal = Nature | volume = 453 | issue = 7194 | pages = 519–523 | date = May 2008 | pmid = 18497825 | pmc = 5328678 | doi = 10.1038/nature06968 | bibcode = 2008Natur.453..519Y }}</ref> 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).<ref>
{{cite web|url=http://stemcells.nih.gov/research/NIHresearch/scunit/culture.asp|archive-url=https://web.archive.org/web/20100106111652/http://stemcells.nih.gov/research/NIHresearch/scunit/culture.asp|archive-date=2010-01-06|title=Culture of Human Embryonic Stem Cells (hESC)|publisher=National Institutes of Health|access-date=2010-03-07}}</ref> Without optimal culture conditions or genetic manipulation,<ref>
{{cite journal | vauthors = Chambers I, Colby D, Robertson M, Nichols J, Lee S, Tweedie S, Smith A | title = Functional expression cloning of Nanog, a pluripotency sustaining factor in embryonic stem cells|journal = Cell|volume = 113|issue = 5 | pages = 643–655 |date = May 2003| pmid = 12787505 | doi = 10.1016/S0092-8674(03)00392-1 |hdl = 1842/843 |s2cid = 2236779 | hdl-access = free }}</ref> embryonic stem cells will rapidly differentiate.

A human embryonic stem cell is also defined by the expression of several transcription factors and cell surface proteins. The transcription factors [[Oct-4]], [[Homeobox protein NANOG|Nanog]], and [[Sox2]] form the core regulatory network that ensures the suppression of genes that lead to differentiation and the maintenance of pluripotency.<ref>
{{cite journal | vauthors = Boyer LA, Lee TI, Cole MF, Johnstone SE, Levine SS, Zucker JP, Guenther MG, Kumar RM, Murray HL, Jenner RG, Gifford DK, Melton DA, Jaenisch R, Young RA | title = Core transcriptional regulatory circuitry in human embryonic stem cells | journal = Cell | volume = 122 | issue = 6 | pages = 947–956 | date = September 2005 | pmid = 16153702 | pmc = 3006442 | doi = 10.1016/j.cell.2005.08.020 }}</ref>  The cell surface antigens most commonly used to identify hES cells are the glycolipids [[stage specific embryonic antigen 3]] and 4, and the keratan sulfate antigens Tra-1-60 and Tra-1-81. The molecular definition of a stem cell includes many more proteins and continues to be a topic of research.<ref>
{{cite journal |display-authors=6 |collaboration= The International Stem Cell Initiative | vauthors = Adewumi O, Aflatoonian B, Ahrlund-Richter L, Amit M, Andrews PW, Beighton G, Bello PA, Benvenisty N, Berry LS, Bevan S, Blum B, Brooking J, Chen KG, Choo AB, Churchill GA, Corbel M, Damjanov I, Draper JS, Dvorak P, Emanuelsson K, Fleck RA, Ford A, Gertow K, Gertsenstein M, Gokhale PJ, Hamilton RS, Hampl A, Healy LE, Hovatta O, Hyllner J, Imreh MP, Itskovitz-Eldor J, Jackson J, Johnson JL, Jones M, Kee K, King BL, Knowles BB, Lako M, Lebrin F, Mallon BS, Manning D, Mayshar Y, McKay RD, Michalska AE, Mikkola M, Mileikovsky M, Minger SL, Moore HD, Mummery CL, Nagy A, Nakatsuji N, O'Brien CM, Oh SK, Olsson C, Otonkoski T, Park KY, Passier R, Patel H, Patel M, Pedersen R, Pera MF, Piekarczyk MS, Pera RA, Reubinoff BE, Robins AJ, Rossant J, Rugg-Gunn P, Schulz TC, Semb H, Sherrer ES, Siemen H, Stacey GN, Stojkovic M, Suemori H, Szatkiewicz J, Turetsky T, Tuuri T, van den Brink S, Vintersten K, Vuoristo S, Ward D, Weaver TA, Young LA, Zhang W | title = Characterization of human embryonic stem cell lines by the International Stem Cell Initiative | journal = Nature Biotechnology | volume = 25 | issue = 7 | pages = 803–816 | date = July 2007 | pmid = 17572666 | doi = 10.1038/nbt1318 |s2cid= 13780999 }}</ref>

By using human embryonic stem cells to produce specialized cells like nerve cells or heart cells in the lab, scientists can gain access to adult human cells without taking tissue from patients. They can then study these specialized adult cells in detail to try to discern complications of diseases, or to study cell reactions to proposed new drugs.

Because of their combined abilities of unlimited expansion and pluripotency, embryonic stem cells remain a theoretically potential source for [[regenerative medicine]] and tissue replacement after injury or disease.,<ref>{{cite journal | vauthors = Mahla RS | title = Stem Cells Applications in Regenerative Medicine and Disease Therapeutics | journal = International Journal of Cell Biology | volume = 2016 | issue = 7 | pages = 1–24 | year = 2016 | pmid = 27516776 | pmc = 4969512 | doi = 10.1155/2016/6940283 | doi-access = free }}</ref> however, there are currently no approved treatments using ES cells. The first human trial was approved by the US Food and Drug Administration in January 2009.<ref>{{cite news | first1 = Ron | last1 = Winslow | first2 = Alicia | last2 = Mundy | name-list-style = vanc |title= First Embryonic Stem-Cell Trial Gets Approval from the FDA |newspaper= The Wall Street Journal |url= https://www.wsj.com/articles/SB123268485825709415 |date= 23 January 2009 }}</ref> However, the human trial was not initiated until October 13, 2010, in Atlanta for [[spinal cord injury research]]. On November 14, 2011, the company conducting the trial ([[Geron Corporation]]) announced that it will discontinue further development of its stem cell programs.<ref>{{cite web|url=http://www.sciencedebate.com/science-blog/embryonic-stem-cell-therapy-risk-geron-ends-clinical-trial|publisher=ScienceDebate.com|title=Embryonic Stem Cell Therapy At Risk? Geron Ends Clinical Trial|access-date=2011-12-11|archive-date=2014-08-22|archive-url=https://web.archive.org/web/20140822055210/http://www.sciencedebate.com/science-blog/embryonic-stem-cell-therapy-risk-geron-ends-clinical-trial|url-status=dead}}</ref> Differentiating ES cells into usable cells while avoiding transplant rejection are just a few of the hurdles that embryonic stem cell researchers still face.<ref>{{cite journal | vauthors = Wu DC, Boyd AS, Wood KJ | title = Embryonic stem cell transplantation: potential applicability in cell replacement therapy and regenerative medicine | journal = Frontiers in Bioscience | volume = 12 | issue = 8–12 | pages = 4525–35 | date = May 2007 | pmid = 17485394 | doi = 10.2741/2407 | s2cid = 6355307 | doi-access = free }}</ref> Embryonic stem cells, being pluripotent, require specific signals for correct differentiation – if injected directly into another body, ES cells will differentiate into many different types of cells, causing a [[teratoma]]. Ethical considerations regarding the use of unborn human tissue are another reason for the lack of approved treatments using embryonic stem cells. Many nations currently have [[moratorium (law)|moratoria]] or limitations on either human ES cell research or the production of new human ES cell lines. 
<gallery>
File:Mouse embryonic stem cells.jpg| [[Mus musculus|Mouse]] [[Mammalian embryogenesis|embryonic]] stem cells with fluorescent marker
File:Human embryonic stem cell colony phase.jpg| Human embryonic stem cell colony on mouse embryonic fibroblast feeder layer
</gallery>

=== Mesenchymal stem cells ===
{{Main|Mesenchymal stem cell}}
[[File:Human mesenchymal stem cells.gif|thumb|Human mesenchymal stem cells]]
Mesenchymal stem cells (MSC) or mesenchymal stromal cells, also known as medicinal signaling cells are known to be multipotent, which can be found in adult tissues, for example, in the muscle, liver, bone marrow and adipose tissue. Mesenchymal stem cells usually function as structural support in various organs as mentioned above, and control the movement of substances. MSC can differentiate into numerous cell categories as an illustration of adipocytes, osteocytes, and chondrocytes, derived by the mesodermal layer.<ref name="Mesenchymal and induced pluripotent">{{cite journal | vauthors = Zomer HD, Vidane AS, Gonçalves NN, Ambrósio CE | title = Mesenchymal and induced pluripotent stem cells: general insights and clinical perspectives | journal =  Stem Cells and Cloning: Advances and Applications| volume = 8 | pages = 125–134 | date = 2015-09-28 | pmid = 26451119 | pmc = 4592031 | doi = 10.2147/SCCAA.S88036 | doi-access = free }}</ref> Where the mesoderm layer provides an increase to the body's skeletal elements, such as relating to the cartilage or bone. The term "meso" means middle, infusion originated from the Greek, signifying that mesenchymal cells are able to range and travel in early embryonic growth among the ectodermal and endodermal layers. This mechanism helps with space-filling thus, key for repairing wounds in adult organisms that have to do with mesenchymal cells in the dermis (skin), bone, or muscle.<ref>{{cite journal | vauthors = Caplan AI | title = Mesenchymal stem cells | journal = Journal of Orthopaedic Research | volume = 9 | issue = 5 | pages = 641–650 | date = September 1991 | pmid = 1870029 | doi = 10.1002/jor.1100090504 | s2cid = 22606668 | doi-access = free }}</ref>

Mesenchymal stem cells are known to be essential for regenerative medicine. They are broadly studied in [[List of countries by stem cell research trials|clinical trials]]. Since they are easily isolated and obtain high yield, high plasticity, which makes able to facilitate inflammation and encourage cell growth, cell differentiation, and restoring tissue derived from immunomodulation and immunosuppression. MSC comes from the bone marrow, which requires an aggressive procedure when it comes to isolating the quantity and quality of the isolated cell, and it varies by how old the donor. When comparing the rates of MSC in the bone marrow aspirates and bone marrow stroma, the aspirates tend to have lower rates of MSC than the stroma. MSC are known to be heterogeneous, and they express a high level of pluripotent markers when compared to other types of stem cells, such as embryonic stem cells.<ref name="Mesenchymal and induced pluripotent"/> MSCs injection leads to wound healing primarily through stimulation of angiogenesis.<ref>{{Cite journal |last1=Krasilnikova |first1=O. A. |last2=Baranovskii |first2=D. S. |last3=Lyundup |first3=A. V. |last4=Shegay |first4=P. V. |last5=Kaprin |first5=A. D. |last6=Klabukov |first6=I. D. |date=2022-04-27 |title=Stem and Somatic Cell Monotherapy for the Treatment of Diabetic Foot Ulcers: Review of Clinical Studies and Mechanisms of Action |journal=Stem Cell Reviews and Reports |volume=18 |issue=6 |pages=1974–1985 |doi=10.1007/s12015-022-10379-z |issn=2629-3277 |pmid=35476187|s2cid=248402820 }}</ref>

=== Cell cycle control ===
{{Further|Cell cycle}}
Embryonic stem cells (ESCs) have the ability to divide indefinitely while keeping their [[pluripotency]], which is made possible through specialized mechanisms of [[cell cycle]] control.<ref name=":0">{{cite journal | vauthors = Koledova Z, Krämer A, Kafkova LR, Divoky V | title = Cell-cycle regulation in embryonic stem cells: centrosomal decisions on self-renewal | journal = Stem Cells and Development | volume = 19 | issue = 11 | pages = 1663–1678 | date = November 2010 | pmid = 20594031 | doi = 10.1089/scd.2010.0136 }}</ref> Compared to proliferating [[somatic cell]]s, ESCs have unique cell cycle characteristics—such as rapid cell division caused by shortened [[G1 phase]], absent [[G2 phase|G0 phase]], and modifications in [[cell cycle checkpoint]]s—which leaves the cells mostly in [[S phase]] at any given time.<ref name=":0" /><ref name=":1">{{cite journal | vauthors = Barta T, Dolezalova D, Holubcova Z, Hampl A | title = Cell cycle regulation in human embryonic stem cells: links to adaptation to cell culture | journal = Experimental Biology and Medicine | volume = 238 | issue = 3 | pages = 271–275 | date = March 2013 | pmid = 23598972 | doi = 10.1177/1535370213480711 | s2cid = 2028793 }}</ref> ESCs' rapid division is demonstrated by their short doubling time, which ranges from 8 to 10 hours, whereas somatic cells have doubling time of approximately 20 hours or longer.<ref name=":2">{{cite journal | vauthors = Zaveri L, Dhawan J | title = Cycling to Meet Fate: Connecting Pluripotency to the Cell Cycle | language = en | journal = Frontiers in Cell and Developmental Biology | volume = 6 | pages = 57 | date = 2018 | pmid = 29974052 | pmc = 6020794 | doi = 10.3389/fcell.2018.00057 | doi-access = free }}</ref> As cells differentiate, these properties change: G1 and G2 phases lengthen, leading to longer cell division cycles. This suggests that a specific cell cycle structure may contribute to the establishment of pluripotency.<ref name=":0" />

Particularly because G1 phase is the phase in which cells have increased sensitivity to differentiation, shortened G1 is one of the key characteristics of ESCs and plays an important role in maintaining undifferentiated [[phenotype]]. Although the exact molecular mechanism remains only partially understood, several studies have shown insight on how ESCs progress through G1—and &nbsp;potentially other phases—so rapidly.<ref name=":1" />

The cell cycle is regulated by complex network of [[cyclin]]s, [[cyclin-dependent kinase]]s (Cdk), [[cyclin-dependent kinase inhibitor]]s (Cdkn), pocket proteins of the retinoblastoma (Rb) family, and other accessory factors.<ref name=":2" /> Foundational insight into the distinctive regulation of ESC cell cycle was gained by studies on mouse ESCs (mESCs).<ref name=":1" /> mESCs showed a cell cycle with highly abbreviated G1 phase, which enabled cells to rapidly alternate between M phase and S phase. In a somatic cell cycle, oscillatory activity of Cyclin-Cdk complexes is observed in sequential action, which controls crucial regulators of the cell cycle to induce unidirectional transitions between phases: [[Cyclin D]] and Cdk4/6 are active in the G1 phase, while [[Cyclin E]] and [[Cyclin-dependent kinase 2|Cdk2]] are active during the late G1 phase and S phase; and [[Cyclin A]] and Cdk2 are active in the S phase and G2, while [[Cyclin B]] and [[Cyclin-dependent kinase 1|Cdk1]] are active in G2 and M phase.<ref name=":2" /> However, in mESCs, this typically ordered and oscillatory activity of Cyclin-Cdk complexes is absent. Rather, the Cyclin E/Cdk2 complex is constitutively active throughout the cycle, keeping [[retinoblastoma protein]] (pRb) [[hyperphosphorylated]] and thus inactive. This allows for direct transition from M phase to the late G1 phase, leading to absence of D-type cyclins and therefore a shortened G1 phase.<ref name=":1" /> Cdk2 activity is crucial for both cell cycle regulation and cell-fate decisions in mESCs; downregulation of Cdk2 activity prolongs G1 phase progression, establishes a somatic cell-like cell cycle, and induces expression of differentiation markers.<ref>{{cite journal | vauthors = Koledova Z, Kafkova LR, Calabkova L, Krystof V, Dolezel P, Divoky V | title = Cdk2 inhibition prolongs G1 phase progression in mouse embryonic stem cells | journal = Stem Cells and Development | volume = 19 | issue = 2 | pages = 181–194 | date = February 2010 | pmid = 19737069 | doi = 10.1089/scd.2009.0065 }}</ref>

In human ESCs (hESCs), the duration of G1 is dramatically shortened. This has been attributed to high mRNA levels of G1-related Cyclin D2 and Cdk4 genes and low levels of cell cycle regulatory proteins that inhibit cell cycle progression at G1, such as [[P21Cip1|p21<sup>CipP1</sup>]], [[p27Kip1|p27<sup>Kip1</sup>]], and p57<sup>Kip2</sup>.<ref name=":0" /><ref>{{cite journal | vauthors = Becker KA, Ghule PN, Therrien JA, Lian JB, Stein JL, van Wijnen AJ, Stein GS | title = Self-renewal of human embryonic stem cells is supported by a shortened G1 cell cycle phase | journal = Journal of Cellular Physiology | volume = 209 | issue = 3 | pages = 883–893 | date = December 2006 | pmid = 16972248 | doi = 10.1002/jcp.20776 | s2cid = 24908771 }}</ref> Furthermore, regulators of Cdk4 and Cdk6 activity, such as members of the Ink family of inhibitors (p15, p16, p18, and p19), are expressed at low levels or not at all. Thus, similar to mESCs, hESCs show high Cdk activity, with Cdk2 exhibiting the highest kinase activity. Also similar to mESCs, hESCs demonstrate the importance of Cdk2 in G1 phase regulation by showing that G1 to S transition is delayed when Cdk2 activity is inhibited and G1 is arrest when Cdk2 is knocked down.<ref name=":0" /> However unlike mESCs, hESCs have a functional G1 phase. hESCs show that the activities of Cyclin E/Cdk2 and Cyclin A/Cdk2 complexes are cell cycle-dependent and the Rb checkpoint in G1 is functional.<ref name=":2" />

ESCs are also characterized by G1 checkpoint non-functionality, even though the G1 checkpoint is crucial for maintaining genomic stability. In response to [[DNA damage]], ESCs do not stop in G1 to repair DNA damages but instead, depend on S and G2/M checkpoints or undergo apoptosis. The absence of G1 checkpoint in ESCs allows for the removal of cells with damaged DNA, hence avoiding potential mutations from inaccurate DNA repair.<ref name=":0" /> Consistent with this idea, ESCs are hypersensitive to DNA damage to minimize mutations passed onto the next generation.<ref name=":2" />