Growth regulation of the interstitial cell population in hydra. IV. Control of nerve cell and nematocyte differentiation by amplification of non-stem interstitial cells.

The precursors for several differentiated cell types in hydra, such as nerve cells and nematocytes, arise from the interstitial cell population. Previously, it has been suggested that the interstitial cells represent a homogeneous stem cell population, and that both the rate of growth and the amount of differentiation are regulated strictly at the level of stem cell self-renewal and commitment. However, recent evidence does not support this viewpoint. In this paper we have proposed that the interstitial cell population is complex, containing both clonable stem cells and other cells which have a reduced division capacity. In response to hydroxyurea treatment, there is an amplification in the number of divisions that the non-stem interstitial cells undergo before differentiating. This amplification model is consistent with the correlations found in the preceding report (S. Heimfeld and H.R. Bode, 1986, Dev. Biol. 115, 51-58) and fits well with previously published data. An additional experiment which tests two specific predictions of this new model is presented.