The influence of rate of cell entry to S phase on the labelling index of human buccal epithelium

The physiological basis of the circadian rhythm in cell kinetics in human epithelia remains unclear. A 24-h periodicity in the labelling index (LI) of human buccal epithelium has been demonstrated previously. A double-labelling technique in vitro using tritiated deoxythymidine was used here to examine fluctuations in the time spent in S phase (Ts) and the rate of cell entry to S phase (S influx). While the range in Ts values was limited (5.1–6.9 h), influx to S phase ranged from 0 to 1.26% per hour at six different time periods. An increase in the S influx was apparent around 18:00 h leading to a peak LI 4 h later. These preliminary data indicate that the most likely kinetic mechanism related to circadian variations in LI is a variable G1-S influx at this site.