Source of Cells for Regeneration
It had been considered earlier that interstitial cells constitute a reserve of undifferentiated cells that provide the cellular material for regeneration in hydra by mitotic divisions. Though recent studies have displayed that cell division in the interstitial cells can be prevented by nitrogen mustard or irradiations, and still regeneration takes place. The cells after that taking part are the differentiated cells of epidermis and gastrodermis that following amputational injury lose their specialized features, dedifferentiate and are then reorganized or moulded to from the missing structures, tentacles or the hypostome, mouth, the pedal disc as the case may be.
Interstitial cells are not necessary for regeneration, which occurs even when they are put out of action. Though, normally, the interstitial cells also participate in morphallactic regeneration in Hydra. The ectodermal and endodermal cells related to two other self-renewing lineages. Studies completed by Haynes and Burnett (1963) have shown that in hydras such as Pelmatohydra oligactis and Hydra viridis, the endoderm alone can regenerate an entire animal. In this kind of regeneration, the piece of endodermal cells of hydra first lose their differentiated features and after that re-differentiate into ectoderm, interstitial cells and endodermal components of the regenerated part.