TURNOVER OF CELL‐RENEWING POPULATIONS UNDERGOING CIRCADIAN RHYTHMS IN CELL PROLIFERATION

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Abstract
Evidence is presented in support of the concept of partial synchrony of cells as the cause for circadian rhythms in DNA synthesis and mitotic activity. The nonuniform age distribution of cells in cycle indicated that equations based on total asynchrony were not applicable for calculation of cytokinetic parameters in cellrenewing populations undergoing circadian rhythms. The integration of the circadian mitotic curve is introduced as a simple and accurate method for determination of proliferation rate and turnover time. An approximately linear increase in the labeling index following repeated injections of 3H‐thymidine demonstrated that nearly 100% basal cells in hamster cheek pouch epithelia were in cycle during a turnover time. These experiments suggest that if there is a G2 phase in cellrenewing tissues, this is short with respect to turnover time and that it may be a specific compartment where the control of cell proliferation operates.