Signaling molecules in regenerating hydra.

Ever since it was discovered in hydra, regeneration has remained a stimulating question for developmental biologists. Cellular approaches have revealed that,within the first few hours of apical or basal hydra regeneration, differentiation and determination of nerve cells are the primary cellular events detectable. The head and foot activators (HA, FA), neuropeptides that are released upon injury, are signaling molecules involved in these processes. In conditions where it induces cellular differentiation or determination, HA behaves as an agonist of the cyclic AMP (cAMP) pathway involving the modulation of CREB nuclear transcription factor activity. This cascade would be required for proper regeneration,regardless of whether the polarity involved is apical or basal. Modulations of the protein kinase C pathway, which have been shown to affect apical or basal positional values, might signal to bring about this polarity; however, endogenous ligands responsible for this modulation are as yet unknown.