Evidence of Mouse Epidermal Subpopulations with Different Cell Cycle Times

In order to obtain information on the distribution of total cell cycle times in hairless mouse epidermis, basal cells were isolated and prepared for DNA flow cytometry at intervals after a pulse labeling with 50 μCi of thymidine. The DNA distributions were recorded, and cells were sorted from windows in the S, G2, and G1; phases of the cell cycle, collected on glass slides, and subjected to autoradiography. The proportions of labeled cells were scored in each fraction, and the percentage of labeled mitoses was determined in histologic sections from the same animals. Grain count distributions were recorded at selected time points over labeled cells in sorted fractions and over labeled mitoses. The movement of the labeled S-phase cohort was thus followed through all cell cycle phases. Peaks in labeled cells were observed at about 36h in S phase, G2 phase, and mitosis, and high levels of labeled G2 cells and mitoses were seen at about 80h. These results indicate the existence of one rapidly cycling subpopulation of keratinocytes with a cell cycle time slightly less than 30 h, in addition to keratinocytes with considerably longer cell cycle times. The first peak of labeled G2 cells reached only about 30%. This is consistent with earlier findings of about 30% G2 cells with a rapid traverse, and 70% with a considerably delayed traverse through G2 phase. The proportion of labeled G1 cells reached a value corresponding to twice the initial labeling index at 8h after pulse labeling. This is consistent with previously obtained phase durations, indicating an unperturbed cell cycle traverse of labeled cells from S phase through G2 and mitosis.