Editing T cell (section)

===Origin, early development and migration to the thymus===
All T cells originate from c-kit<sup>+</sup>Sca1<sup>+</sup> [[haematopoietic stem cell]]s (HSC) which reside in the bone marrow. In some cases, the origin might be the foetal [[liver]] during [[embryonic development]]. The HSC then differentiate into multipotent progenitors (MPP) which retain the potential to become both [[Myeloid Cell|myeloid]] and [[lymphoid cells]]. The process of differentiation then proceeds to a common lymphoid progenitor (CLP), which can only differentiate into T, B or NK cells.<ref>{{cite journal|vauthors = Kondo M|title = One Niche to Rule Both Maintenance and Loss of Stemness in HSCs|journal = Immunity|volume = 45|issue = 6|pages = 1177–1179|date = December 2016|pmid = 28002722|doi = 10.1016/j.immuni.2016.12.003|doi-access = free}}</ref> These CLP cells then migrate via the blood to the thymus, where they [[wiktionary:engraft|engraft:]]. Henceforth they are known as [[thymocyte]]s, the immature stage of a T cell.

The earliest cells which arrived in the thymus are commonly termed ''double-negative'', as they express neither the [[CD4]] nor [[CD8]] co-receptor. The newly arrived CLP cells are CD4<sup>−</sup>CD8<sup>−</sup>CD44<sup>+</sup>CD25<sup>−</sup>ckit<sup>+</sup> cells, and are termed early thymic progenitor (ETP) cells.<ref>{{cite journal|vauthors = Osborne LC, Dhanji S, Snow JW, Priatel JJ, Ma MC, Miners MJ, Teh HS, Goldsmith MA, Abraham N|display-authors = 6|title = Impaired CD8 T cell memory and CD4 T cell primary responses in IL-7R alpha mutant mice|journal = The Journal of Experimental Medicine|volume = 204|issue = 3|pages = 619–631|date = March 2007|pmid = 17325202|pmc = 2137912|doi = 10.1084/jem.20061871}}</ref> These cells will then undergo a round of division and [[downregulate]] c-kit and are termed ''double-negative one'' (DN1) cells. To become T cells, the thymocytes must undergo multiple DN stages as well as positive selection and negative selection.

Double negative thymocytes can be identified by the surface expression of [[CD2]], [[CD5 (protein)|CD5]] and [[CD7]]. Still during the double negative stages, [[CD34]] expression stops and [[CD1]] is expressed. Expression of both CD4 and CD8 makes them ''double positive'', and matures into either CD4<sup>+</sup> or CD8<sup>+</sup> cells.