CIRCADIAN RHYTHMS IN MOUSE EPIDERMAL BASAL CELL PROLIFERATION

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Abstract
Several kinetic parameters of basal cell proliferation in hairless [hr/hr Oslo strain] mouse epidermis were studied and all parameters clearly showed circadian fluctuations during 2 successive 24 h periods. Mitotic indices and the mitotic rate were studied in histological sections; the proportions of cells with S and G2 phase DNA content were measured by flow cytometry of isolated basal cells, and the [3H]TdR [thymine deoxyribose] labeling indices and grain densities were determined by autoradiography in smears from basal cell suspensions. The influx and efflux of cells from each cell cycle phase were calculated from sinusoidal curves adapted to the cell kinetic findings and the phase durations were determined. A peak of cells in S phase was observed at .apprx. 2400 h, and a cohort of partially synchronized cells passed from the S phase to the G2 phase and transversed the G2 phase and mitosis in the early morning. The fluctuations in the influx of cells into the S phase were small compared with the variations in efflux from the S phase and the flux through the subsequent cell cycle phases. The resulting delay in cell cycle traverse through S phase before 2400 h could account for the accumulation of cells in S phase and also the subsequent partial synchrony of cell cycle traverse through the G2 phase and mitosis. Circadian variations in the duration of the S phase, the G2 phase and mitosis were clearly demonstrated.