Editing Hydra (genus) (section)

==Tissue regeneration==

''Hydras'' undergo [[morphallaxis]] (tissue regeneration) when injured or severed. Typically, ''Hydras'' reproduce by just budding off a whole new individual; the bud occurs around two-thirds of the way down the body axis. When a ''Hydra'' is cut in half, each half regenerates and forms into a small ''Hydra''; the "head" regenerates a "foot" and the "foot" regenerates a "head". This regeneration occurs without cell division. If the ''Hydra'' is sliced into many segments, the middle slices form both a "head" and a "foot".<ref name=":0" /> The polarity of the regeneration is explained by two pairs of positional value gradients. There is both a head and foot activation and inhibition gradient. The head activation and inhibition works in an opposite direction of the pair of foot gradients.<ref name=":1">{{cite journal | vauthors = Fujisawa T | title = Hydra regeneration and epitheliopeptides | journal = Developmental Dynamics | volume = 226 | issue = 2 | pages = 182–9 | date = February 2003 | pmid = 12557197 | doi = 10.1002/dvdy.10221 | s2cid = 26953455 | doi-access = free }}</ref> The evidence for these gradients was shown in the early 1900s with grafting experiments. The inhibitors for both gradients have shown to be important to block the bud formation. The location where the bud forms is where the gradients are low for both the head and foot.<ref name=":0" />

''Hydras'' are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates.<ref name=":1" /> This process takes place not only in the pieces of tissue excised from the body column, but also from re-aggregates of dissociated single cells. It was found that in these aggregates, cells initially distributed randomly undergo sorting and form two epithelial cell layers, in which the endodermal epithelial cells play more active roles in the process. Active mobility of these endodermal epithelial cells forms two layers in both the re-aggregate and the re-generating tip of the excised tissue. As these two layers are established, a patterning process takes place to form heads and feet.<ref name="Fujisawa-2003">{{cite journal | last=Fujisawa | first=Toshitaka | title=Hydra regeneration and epitheliopeptides | journal=[[Developmental Dynamics]] | publisher=[[American Association for Anatomy]] ([[Wiley publishing|Wiley]]) | volume=226 | issue=2 | date=2003-01-29 | issn=1058-8388 | doi=10.1002/dvdy.10221 | pages=182–189| pmid=12557197 | s2cid=26953455 | doi-access=free }}</ref>