Evidence of diurnal variation of human epidermal cell proliferation. I. Epidermal 3H-labeling index and serum cortisol rhythm.

Performing two longitudinal studies over 48 and 66 h, respectively, in 12 healthy male adult volunteers the epidermal LI were measured by in vitro 3H-thymidine labeling technique every 6 h and serum cortisol levels simultaneously by RIA examination. While serum cortisol showed a circadian rhythm over the whole test period, a circadian rhythmic pattern of epidermal LI yet became evident 12 h after the onset of experiments. This initial time lag in appearance of LI periodicity may be due to some temporal desynchronization in the cell cycle of keratinocytes. Circadian peaks and minima of both cortisol and LI were found to be inversely correlated, with a time-shifting of 6 h according to the experimental conditions. The present results reveal strong evidence that circadian fluctuation of endogenous cortisol secretion does influence the regulation of epidermal DNA-synthesis in man.