Distribution of Interstitial Cells and Differentiating Nematocytes in Nests inHydra attenuata

We have used tissue maceration to quantitate interstitial cell nests in Hydra attenuata and thiolacetic acid-lead nitrate staining to quantitate differentiating nematocytes. Most I-cell nests have 2, 4, 8, or 16 cells. Differentiating nematocytes occur in nests of 4, 8, 16, and 32. All classes of I-cell nests and nests of dilferentiating nematocytes are abundant in the gastric region. In contrast, the hypostome, basal disk and the regions immediately adjacent to them have no nests of differentiating nematocytes, virtually no nests of 4, 8, or 16 I-cells and reduced numbers of 1 and 2 I-cell nesls. Since nests of 4 or more I-cells are found only where nematocyte differentiation takes place, these nest classes are probably nematocyte precursors. Since differentiation of nerves occurs in the hypostome and basal disk where only single or paired I-cells are found, these size classes include nerve precursors. The disproportionately large number of I-cell nests of 2 suggests that these include the autoreproductive stem cells responsible for maintaining the populations of I-cell derivatives in growing hydra.