Cell cycle kinetics and development of Hydra attenuata: I. epithelial cells

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Abstract
The cell cycle parameters of epithelial cells of Hydra attenuata are described. Specifically the rate of proliferation and the fraction of proliferating cells have been determined under conditions of defined growth rate. Techniques involved standard methods of cell cycle analysis using histological and tissue maceration preparations; pulse-chase and continuous labelling with [3H]thymidine followed by autoradiographic analysis, and microspectrophotometric determination of nuclear DNA content in single cells. The results indicate that more than 90% of hydra epithelial cells are actively proliferating with a cell cycle duration about equal to the tissue doubling time. In well fed hydra the average cell cycle is about 3 days long. S period is 12-15 h, G1 0-1 h, and mitosis 1.5 h. Most of the cell cycle consists of a long G2 period of variable duration (24-72 h). The results provide no evidence for a subpopulation of rapidly proliferating cells as predicted by ‘growth zone’ models of hydra morphogenesis. The results also indicate that the population of epithelial cells is self-sustaining requiring no input by differentiation from other cell types. The long and variable G2 period means that DNA synthesis and the following cell division are effectively uncoupled such that inhibitors of DNA synthesis may not stop epithelial cell division. The variable nature of the G2 period suggests it as a possible point of control of hydra growth.