Summary

The cell cycle parameters of interstitial cells in Hydra attenuata have been determined. Interstitial cells were classified according to cluster size in which they occur (1, 2, 4, 8 or 16 cells) and morphology using maceration preparations and histological sections. The lengths of G₁, S, G₂ and M were determined by standard methods of cell cycle analysis using pulse-chase and continuous labelling with [³H]- and [¹⁴C]thymidine. Nuclear DNA contents were measured microfluorimetrically. All classes of interstitial cells proliferate but the cell cycle of large interstitial cells occurring singly or in pairs is longer than that of interstitial cells occurring in clusters of 4, 8 and 16 cells. The S-phase is 11-12 h long and G₁ is less than 1 h for all classes of interstitial cells. G₂ is 3-4 h long for interstitial cells in clusters of 4, 8 and 16 cells giving these cells a total cell cycle duration of 16-17 h. In contrast, large interstitial cells occurring as singles and in clusters of 2 have G₂ durations ranging from 4 to 22 h. Two subpopulations can be discerned among these cells, one having a G₁ of about 6 h and a total cell cycle of about 19 h, the other having an average G₂ of 14 h and a total cell cycle of about 27 h. The differences in cell cycle duration appear to be associated with interstitial cell function. Cells having a short cell cycle are probably committed to nematocyte differentiation, while large interstitial cells having long and variable cell cycles appear to be undetermined stem cells responsible for proliferating further interstitial cells. The variable length of G₂ in these cells suggest it as a possible control point.