Evidence of rapid and slow progression of cells through G2 phase in mouse epidermis: a comparison between phase durations measured by different methods.

Percentage labelled mitosis (PLM) measurements were initiated at four different times during a 24-hr period and continued for 24 hr in hairless mouse epidermis. Estimates of G2 and S phase durations (mean TG2 and mean TS) were calculated. A significant number of labelled mitoses (10--20%) was seen after 30 min in all four PLM measurements and the estimated mean TG2 varied from 1.4 to 2.5 hr and was in agreement with values from PLM measurements in other epithelial tissues. These mean TG2 values were much shorter than expected from [3H]TdR double labelling 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 two G2 cell populations; one with a rapid and another with a slow rate of cell cycle progression. The cells with the higher rate are mainly registered by the PLM method, whereas those with the lower rate largely escape detection by this method. TG2 estimates from PLM measurements in mouse epidermis therefore do not reflect the phase duration of the entire G2 population. It is also concluded that circadian variations in TS can not be accurately registered by the PLM method.