Editing Developmental biology (section)

== Developmental processes ==

=== Cell differentiation ===

[[File:Slack Essential Dev Biol Fig 14.12a.jpg|thumb|The Notch-delta system in neurogenesis (Slack Essential Dev Biol Fig 14.12a)]]

[[Cell differentiation]] is the process whereby different functional cell types arise in development. For example, neurons, muscle fibers and hepatocytes (liver cells) are well known types of differentiated cells. Differentiated cells usually produce large amounts of a few proteins that are required for their specific function and this gives them the characteristic appearance that enables them to be recognized under the light microscope. The genes encoding these proteins are highly active. Typically their [[chromatin]] structure is very open, allowing access for the transcription enzymes, and specific transcription factors bind to regulatory sequences in the DNA in order to activate gene expression.<ref>{{cite journal | vauthors = Li B, Carey M, Workman JL | title = The role of chromatin during transcription | journal = Cell | volume = 128 | issue = 4 | pages = 707–19 | date = February 2007 | pmid = 17320508 | doi = 10.1016/j.cell.2007.01.015 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Heintzman ND, Stuart RK, Hon G, Fu Y, Ching CW, Hawkins RD, Barrera LO, Van Calcar S, Qu C, Ching KA, Wang W, Weng Z, Green RD, Crawford GE, Ren B | display-authors = 6 | title = Distinct and predictive chromatin signatures of transcriptional promoters and enhancers in the human genome | journal = Nature Genetics | volume = 39 | issue = 3 | pages = 311–8 | date = March 2007 | pmid = 17277777 | doi = 10.1038/ng1966 | s2cid = 1595885 }}</ref> For example, [[NeuroD]] is a key transcription factor for neuronal differentiation, [[myogenin]] for muscle differentiation, and [[HNF4]] for hepatocyte differentiation.
Cell differentiation is usually the final stage of development, preceded by several states of commitment which are not visibly differentiated. A single tissue, formed from a single type of progenitor cell or stem cell, often consists of several differentiated cell types. Control of their formation involves a process of lateral inhibition,<ref>{{cite journal | vauthors = Meinhardt H, Gierer A | year = 2000 | title = Pattern formation by local self-activation and lateral inhibition | url = http://www.me.ucsb.edu/~moehlis/APC514/2002_1.pdf | journal = BioEssays | volume = 22 | issue = 8| pages = 753–760 | doi = 10.1002/1521-1878(200008)22:8<753::aid-bies9>3.0.co;2-z | pmid = 10918306 | url-status = live | archive-url = https://web.archive.org/web/20171027025156/https://me.ucsb.edu/~moehlis/APC514/2002_1.pdf | archive-date = 2017-10-27 | citeseerx = 10.1.1.477.439 }}</ref> based on the properties of the [[Notch signaling pathway]].<ref>{{cite journal | vauthors = Sprinzak D, Lakhanpal A, Lebon L, Santat LA, Fontes ME, Anderson GA, Garcia-Ojalvo J, Elowitz MB | display-authors = 6 | title = Cis-interactions between Notch and Delta generate mutually exclusive signalling states | journal = Nature | volume = 465 | issue = 7294 | pages = 86–90 | date = May 2010 | pmid = 20418862 | pmc = 2886601 | doi = 10.1038/nature08959 | bibcode = 2010Natur.465...86S }}</ref> For example, in the neural plate of the embryo this system operates to generate a population of neuronal precursor cells in which NeuroD is highly expressed.

=== Regeneration ===
[[Regeneration (biology)|Regeneration]] indicates the ability to regrow a missing part.<ref>{{cite book | vauthors = Carlson BM | date = 2007 | title = Principles of Regenerative Biology. | publisher = Academic Press | location = Burlington MA }}</ref> This is very prevalent amongst plants, which show continuous growth, and also among colonial animals such as hydroids and ascidians. But most interest by developmental biologists has been shown in the regeneration of parts in free living animals. In particular four models have been the subject of much investigation. Two of these have the ability to regenerate whole bodies: ''[[Hydra (genus)|Hydra]]'', which can regenerate any part of the polyp from a small fragment,<ref>{{cite journal | vauthors = Bosch TC | title = Why polyps regenerate and we don't: towards a cellular and molecular framework for Hydra regeneration | journal = Developmental Biology | volume = 303 | issue = 2 | pages = 421–33 | date = March 2007 | pmid = 17234176 | doi = 10.1016/j.ydbio.2006.12.012 | doi-access = free }}</ref> and [[planarian]] worms, which can usually regenerate both heads and tails.<ref name="Reddien P.W., Alvarado A.S. 2004 725–757">{{cite journal | vauthors = Reddien PW, Sánchez Alvarado A | s2cid = 1320382 | title = Fundamentals of planarian regeneration | journal = Annual Review of Cell and Developmental Biology | volume = 20 | pages = 725–57 | year = 2004 | pmid = 15473858 | doi = 10.1146/annurev.cellbio.20.010403.095114 }}</ref> Both of these examples have continuous cell turnover fed by [[stem cells]] and, at least in planaria, at least some of the stem cells have been shown to be [[cell potency|pluripotent]].<ref>{{cite journal | vauthors = Wagner DE, Wang IE, Reddien PW | title = Clonogenic neoblasts are pluripotent adult stem cells that underlie planarian regeneration | journal = Science | volume = 332 | issue = 6031 | pages = 811–6 | date = May 2011 | pmid = 21566185 | pmc = 3338249 | doi = 10.1126/science.1203983 | bibcode = 2011Sci...332..811W }}</ref> The other two models show only distal regeneration of appendages. These are the insect appendages, usually the legs of hemimetabolous insects such as the cricket,<ref>{{cite journal | vauthors = Nakamura T, Mito T, Bando T, Ohuchi H, Noji S | title = Dissecting insect leg regeneration through RNA interference | journal = Cellular and Molecular Life Sciences | volume = 65 | issue = 1 | pages = 64–72 | date = January 2008 | pmid = 18030418 | doi = 10.1007/s00018-007-7432-0 | pmc = 11131907 }}</ref> and the limbs of [[urodele amphibians]].<ref>{{cite journal | vauthors = Simon A, Tanaka EM | title = Limb regeneration | journal = Wiley Interdisciplinary Reviews. Developmental Biology | volume = 2 | issue = 2 | pages = 291–300 | year = 2013 | pmid = 24009038 | doi = 10.1002/wdev.73 | s2cid = 13158705 }}</ref> Considerable information is now available about amphibian limb regeneration and it is known that each cell type regenerates itself, except for connective tissues where there is considerable interconversion between cartilage, dermis and tendons. In terms of the pattern of structures, this is controlled by a re-activation of signals active in the embryo.
There is still debate about the old question of whether regeneration is a "pristine" or an "adaptive" property.<ref>{{cite book | vauthors = Slack JM | date = 2013 | title = Essential Developmental Biology | chapter = Chapter 20 | publisher = Wiley-Blackwell | location = Oxford }}</ref> If the former is the case, with improved knowledge, we might expect to be able to improve regenerative ability in humans. If the latter, then each instance of regeneration is presumed to have arisen by natural selection in circumstances particular to the species, so no general rules would be expected.