Control of nerve cell formation from multipotent stem cells in Hydra.

Feeding of starved animals provides a very short signal which determines stem cells to differentiate into nerve cells after the next mitosis. Only those stem cells become determined which are just in the middle of their S-phase at the time of feeding. Stem cells of any other stage of the cycle do not become determined. Nerve cell determination is suppressed by very low concentrations of an endogenous inhibitor. The inhibitor exerts its effect only during the first half of the S-phase, not before and not after this period. Based on these finding it is proposed that stem cells are susceptible to 2 different signals during the first half of their S-phase; one signal allows the development into nerve cells, the other prevents this development. Within this period the decision whether to become a nerve cell or not is reversible. It becomes fixed at the end of this period.