Editing Cellular differentiation (section)

==Mammalian cell types==
{{See also|List of distinct cell types in the adult human body}}

Three basic categories of cells make up the mammalian body: [[germ cell]]s, [[somatic cell]]s, and [[stem cell]]s. Each of the approximately 37.2 trillion (3.72x10<sup>13</sup>) cells in an adult human has its own copy or copies of the [[genome]] except certain [[cell types]], such as [[red blood cell]]s, that lack nuclei in their fully differentiated state. Most cells are [[diploid]]; they have two copies of each [[chromosome]]. Such cells, called somatic cells, make up most of the human body, such as skin and muscle cells. Cells differentiate to specialize for different functions.<ref>{{Cite book|title = Molecular Cell Biology| vauthors = Lodish H |publisher = New York: W. H. Freeman|year = 2000|isbn = 978-0-7167-3136-8|url = https://archive.org/details/molecularcellbio00lodi|edition = 4th|at = Section 14.2|url-access = registration}}</ref>

Germ line cells are any line of cells that give rise to [[gametes]]—eggs and sperm—and thus are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development.<ref>{{cite journal | vauthors = Zakrzewski W, Dobrzyński M, Szymonowicz M, Rybak Z | title = Stem cells: past, present, and future | journal = Stem Cell Research & Therapy | volume = 10 | issue = 1 | pages = 68 | date = February 2019 | pmid = 30808416 | doi = 10.1186/s13287-019-1165-5 | doi-access = free | pmc = 6390367 }}</ref>
          
Development begins when a [[sperm]] fertilizes an [[egg (biology)|egg]] and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a [[blastocyst]].<ref>{{cite book | vauthors = Kumar R | title = Textbook of Human Embryology | publisher = I.K. International Publishing House | isbn = 9788190675710 | year = 2008 | page = 22 | url = https://books.google.com/books?id=MhmT-avx3DUC&pg=PA22 }}</ref> The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the [[inner cell mass]]. The cells of the inner cell mass go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. These cells are referred to as [[pluripotent]].<ref>{{Cite book|title = Encyclopedia of Neuroscience| vauthors = Binder MD, Hirokawa N, Windhorst U |publisher = Springer|year = 2009|isbn = 978-3540237358 }}</ref>

Pluripotent stem cells undergo further specialization into [[multipotent]] [[progenitor cell]]s that then give rise to functional cells. Examples of stem and progenitor cells include:<ref>{{cite journal | vauthors = Tesche LJ, Gerber DA | title = Tissue-derived stem and progenitor cells | journal = Stem Cells International | volume = 2010 | pages = 824876 | date = October 2009 | doi = 10.4061/2010/824876 | doi-access = free | pmid = 21048854 | pmc = 2963308 }}</ref><ref>{{cite journal | vauthors = Kriegstein A, Alvarez-Buylla A | title = The glial nature of embryonic and adult neural stem cells | journal = Annual Review of Neuroscience | volume = 32 | issue =  | pages = 149–184 | date = 21 July 2009 | pmid = 19555289 | pmc = 3086722 | doi = 10.1146/annurev.neuro.051508.135600 }}</ref><ref>{{cite journal | vauthors = Morgan JE, Partridge TA | title = Muscle satellite cells | journal = The International Journal of Biochemistry & Cell Biology | volume = 35 | issue = 8 | pages = 1151–1156 | date = August 2003 | pmid = 12757751 | doi = 10.1016/s1357-2725(03)00042-6 }}</ref>
* ''[[Radial glial cell]]s'' (embryonic neural stem cells) that give rise to excitatory neurons in the fetal brain through the process of [[neurogenesis]].<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 = Lui JH, Hansen DV, Kriegstein AR | title = Development and evolution of the human neocortex | journal = Cell | volume = 146 | issue = 1 | pages = 18–36 | date = July 2011 | pmid = 21729779 | pmc = 3610574 | doi = 10.1016/j.cell.2011.06.030 }}</ref><ref>{{cite journal | vauthors = Rash BG, Ackman JB, Rakic P | title = Bidirectional radial Ca(2+) activity regulates neurogenesis and migration during early cortical column formation | journal = Science Advances | volume = 2 | issue = 2 | pages = e1501733 | date = February 2016 | pmid = 26933693 | pmc = 4771444 | doi = 10.1126/sciadv.1501733 | bibcode = 2016SciA....2E1733R }}</ref>
* ''[[Hematopoietic stem cells]]'' (adult stem cells) from the [[bone marrow]] that give rise to red blood cells, [[white blood cell]]s, and [[platelet]]s.
* ''[[Mesenchymal stem cells]]'' (adult stem cells) from the bone marrow that give rise to stromal cells, fat cells, and types of bone cells
* ''[[Epithelia]]l stem cells'' (progenitor cells) that give rise to the various types of skin cells
* ''Muscle [[satellite cell]]s'' (progenitor cells) that contribute to differentiated [[muscle tissue]].
A pathway that is guided by the cell adhesion molecules consisting of four amino acids, [[arginine]], [[glycine]], [[asparagine]], and [[serine]], is created as the cellular blastomere [[gastrulation|differentiates]] from the single-layered [[blastula]] to the three primary [[germ layer|layers of germ cells]] in mammals, namely the [[ectoderm]], [[mesoderm]] and [[endoderm]] (listed from most distal (exterior) to proximal (interior)). The ectoderm ends up forming the skin and the nervous system, the mesoderm forms the bones and muscular tissue, and the endoderm forms the internal organ tissues.