Diurnal variation in labelling index in human buccal epithelium

Diurnal variations in the number of DNA-synthesizing cells (labelling index) in human buccal epithelium were studied by in vitro pulse labelling with tritiated deoxythymidine. Eleven male volunteers provided 33 biopsy specimens at six time periods. The results showed a 24-h periodicity in the labelling index in this tissue. The mean peak labelling index obtained at 22:00 h (3.20 +/- 0.18) was significantly different from the means obtained at the rest of the times. The 24-h mean labelling index (labelled cells/100 cells) was 2.48 +/- 0.47 for three subjects who volunteered samples at all six time periods. The labelling index obtained around the middle part of the day for all subjects (2.44-2.59) appears to be representative of a 24-h basal or reference value.