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 patter n is formed by migration of interstitial cell precursors from the gast ric region to sites of nerve cell differentiation in the head and basa l disk. Migration occurs early in the final cell cycle of precursors a nd appears to be coincident with commitment to nerve cell differentiat ion. Following migration, each precursor divides and gives rise to a p air of nerve cells. Treatment of whole hydra with a neuropeptide stimu lates formation of nerve cell precursors; differentiation of these pre cursors is regulated by a second signal, which is provided in vivo by budding or in vitro by injuring tissue.