Editing Developmental biology (section)

=== 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.