The Properties of Nerve Cell Precursors in Hydra

Two signals, the head activator and an injury stimulus, control differentiation of nerve cells from uncommitted stem cells in hydra [Th. Holstein, H. C. Schaller, and C. N. David, (1986) Dm. Biol. 115, g-171. The time of action of these signals in the precursor cell cycle was determined. Methanol extracts of hydra containing 10-i’ M head activator cause nerve cell commitment in S phase of the precursor cell cycle. Committed precursors complete the cell cycle, divide, and arrest in Gl. Injury relieves the Gl block and precursors differentiate nerve cells. Under these conditions the time from commitment to nerve differentiation is 12 hr, the time from the end of S phase to nerve differentiation is 9 hr, and the time from the Gl block to nerve differentiation is 4 hr. Committed precursors blocked in Gl are unstable, decaying with a half-life of 12 hr if not stimulated to differentiate by an injury stimulus. o I986 Academic Press, 1nc.