Evidence of diurnal variation of human epidermal cell proliferation. II. Duration of epidermal DNA synthesis.

In the epidermis of six healthy male volunteers the DNA synthesis time was measured by in vitro double labeling technique in a longitudinal study (48 h) with taking skin punch biopsies every 6 h. The external conditions of the experiments and one group of volunteers were identical to those of a previous study on LI of the epidermal keratinocytes. 14C-thymidine and 3H-thymidine were used for double labeling the DNA-synthesizing nuclei of keratinocytes. The duration of DNA replication time proved to be nearly constant, with minor fluctuation, during the entire time span of experiments (mean 6.19 +/- 0.3 h). Thus, the present results are in accordance with the conception that a periodically changing entrance rate of G1-cells into the S-phase of the generation cycle of keratinocytes is predominantly responsible for circadian variations in the number of DNA-synthesizing epidermal keratinocytes.