DNA synthesis in mouse epidermis: S phase cells that remain unlabeled after pulse labeling with DNA precursors progress slowly through S.

Epidermal basal cells from hairless mice were isolated after pulse labeling with tritiated DNA precursors and subjected to DNA flow cytometry combined with cell sorting. Cells were sorted from a window in the middle of the S phase, collected on glass slides, and subjected to autoradiography. Unlabeled cells in the middle of the S phase were found in normal mouse epidermis after optimal pulse labeling with tritiated thymidine [( 3H]dThd), in accordance with previous results. The proportion of unlabeled S phase cells was considerably increased among basal cells from mice treated with growth-inhibitory epidermal extracts. Reanalysis and re-sorting of cells previously sorted from mid S showed that unlabeled cells could not be accounted for by G1 contamination. Furthermore, labeling with precursors incorporated into DNA by "de novo" metabolic pathway [( 3H]Urd) did not reduce the proportion of unlabeled S phase cells, either when given alone or when given in combination with the precursor for DNA incorporated by the "salvage" pathway [( 3H]dThd). This strongly indicates that the unlabeled S phase cells do not synthesize DNA continuously, or are synthesizing DNA at a rate below the level of detection. A reduced proportion of unlabeled S phase cells was found in regenerating epidermis. This may be explained by a dilution effect caused by the 3-fold increase in the total number of cells within S phase at this condition. The observation that essentially all cells in mid S phase were labeled during 4 days of continuous labeling with [3H]dThd, indicates that cells in S phase that remain unlabeled after optimal pulse labeling are cycling, albeit slowly. Two-parameter sorting based on DNA and light scatter indicated that slowly cycling cells are larger than the average. These cells may represent a subpopulation of basal cells going through their last division cycle before differentiation.