Circadian rhythms in mouse epidermis: variation in the numbers of basal and differentiating cells, cell density, and...

In histologic sections from hairless mouse epidermis, the total epidermal cell number and the numbers of basal and differentiating cells were determined per unit length, and cell density was calculated. Sampling was made every 2 h during 2 successive 24-h spans. All parameters showed circadian changes during both 24-h spans. The cell numbers per unit length and the cell density showed both high values around 12(00), some hours subsequent to the peak of mitotic activity. The loss rate of epidermal nucleated cells was calculated from first harmonic sine curves fitted to the circadian changes in total epidermal cell number and to the mitotic rate. The loss rate showed a circadian rhythm with increased values from after 16 00 to after 00(00). This implies that there is a circadian variation in the transition of differentiating cells into horny materials, and hence in cell differentiation.