EVIDENCE OF RAPID AND SLOW PROGRESSION OF CELLS THROUGH G2PHASE IN MOUSE EPIDERMIS: A COMPARISON BETWEEN PHASE DURATIONS MEASURED BY DIFFERENT METHODS

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Abstract
Percentage labeled mitosis (PLM) measurements were initiated at 4 different times during a 24-h period and continued for 24 h in hairless mouse epidermis. Estimates of G2 and S phase durations (mean TG2 and mean TS) were calculated. A significant number of labeled mitoses (10-20%) was seen after 30 min in all 4 PLM measurements, and the estimated mean TG2 varied from 1.4-2.5 h and was in agreement with values from PLM measurements in other epithelial tissues. Mean TG2 values were shorter than expected from [3H]TdR [thymidine] double labeling experiments and from a multiparameter cell kinetic study in hairless mouse epidermis and did not reflect the circadian variations seen in these studies. The differences in estimates of phase durations can be explained by postulating 2 G2 cell populations, one with a rapid and another with a slow rate of cell cycle progression. The cells with the higher rate were mainly registered by the PLM method; those with the lower rate largely escaped detection by this method. TG2 estimates from PLM measurements in mouse epidermis did not reflect the phase duration of the entire G2 population. Circadian variations in TS can not be accurately registered by the PLM method.

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