Regulation of interstitial cell differentiation in Hydra attenuata. III. Effects of I-cell and nerve cell densities.

The interstitial cell (i-cell) of hydra, a multipotent stem cell, produces two classes of differentiated cell types, nerve cells and nematocytes, throughout asexual growth. Using a new assay, the regulation of i-cell commitment to either nerve cell or nematocyte differentiation was investigated. This assay was used to determine the fractions of i-cells differentiating into nerve cells and nematocyte precursors in a variety of in vivo cellular milieus produced by hydroxyurea treatment, differential feeding, and reaggregation of dissociated cells. Nematocyte commitment was found to be positively correlated with the size of the i-cell population and independent of the axial position of the i-cells along the body column. This indicates that i-cell commitment to nematocyte differentiation may be regulated by feedback from the i-cell population. Nerve cell commitment was found to be correlated with regions of high nerve cell density. This suggests that nerve cell commitment is regulated by feedback from the nerve cell population or is dependent on axial position. Implications of such mechanisms for the regulation of i-cell population size and distribution are discussed.