DNA synthesis rate changes during the S phase in mouse epidermis

The in vivo DNA synthesis rate throughout the S phase of mouse epidermal cells was investigated. Epidermal basal cells were isolated at various times of the day from normal animals injected with [3H]TdR 30 min before sacrifice, and from pulse-labelled animals with regenerating and growth-inhibited epidermis. The cells were analysed by DNA flow cytometry combined with cell sorting. Cells from successive fractions of the S phase were sorted on glass slides and subjected to quantitative [3H]TdR autoradiography. The results confirmed the presence of unlabelled (slowly replicating) cells in the S phase, the proportion of which was circadian stage-dependent with minimum values at midnight and in the early morning. The DNA synthesis rate throughout the S phase showed a general trend with high values in the mid-fractions, a pattern which was similar in normal and in growth perturbed epidermis. In the early morning the DNA synthesis rate pattern was bimodal with maxima both in the first and second half of the S phase, with a corresponding trough in mid-S. At this time of day the cell progression rate through S is at its maximum, indicating a relationship between the overall DNA synthesis rate and the rate distribution pattern through S.