title: DNA synthesis in mouse epidermis: S phase cells that remain unlabeled after pulse labeling with DNA precursors progress slowly through S.
abstract: 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.
mesh_terms: D000818:Animals; D002469:Cell Separation; D004247:DNA / Q000096:biosynthesis*; D000078404:Epidermal Cells; D004817:Epidermis / Q000378:metabolism* / Q000502:physiology; D005260:Female; D005434:Flow Cytometry; D007399:Interphase; D008297:Male; D051379:Mice; D008812:Mice, Hairless; D012038:Regeneration; D013936:Thymidine / Q000378:metabolism; D014316:Tritium
chemical_list: D014316:Tritium; D004247:DNA; D013936:Thymidine