Cell cycle kinetics and development of Hydra attenuata. II. Interstitial cells.

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 (i, 2,4, 8 or 16 cells) and morphology using maceration preparations and histological sections. The lengths of G,, S, Ga and M were determined by standard methods of cell cycle analysis using pulsechase and continuous labelling with PH]and [C]thymidine. Nuclear DNA contents were measured microfluorimetrically. All classes of inteistitial cells proliferate but the cell cycle of large interstitial cells occurring singly 01 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 Gt is less than 1 h for all classes of interstitial cells. G, 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 G, durations ranging from 4 to 22 h. Two subpopulations can be discerned among these cells, one having a d of about 6 h and a total cell cycle of about 19 h, the other having an average Gs 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 foi proliferating further interstitial cells. The variable length of Gt in these cells suggest it as a possible control point.