Formation of a primitive nervous system: nerve cell differentiation in the polyp hydra.

Nerve cell differentiation in the polyp Hydra is strikingly patterned: head and basal disk contain high densities of nerve cells. The pattern is formed by migration of interstitial cell precursors from the gastric region to sites of nerve cell differentiation in the head and basal disk. Migration occurs early in the final cell cycle of precursors and appears to be coincident with commitment to nerve cell differentiation. Following migration, each precursor divides and gives rise to a pair of nerve cells. Treatment of whole hydra with a neuropeptide stimulates formation of nerve cell precursors; differentiation of these precursors is regulated by a second signal, which is provided in vivo by budding or in vitro by injuring tissue.