Regenerative proliferation of mouse epidermal cells following application of a carcinogenic skin irritant (MCA). Micro-flow fluorometric DNA measurements and 3H-TdR incorporation studies of isolated basal cells.

0.025 ml of a 1% solution of the complete skin carcinogen 20-methylcholanthrene (MCA) dissolved in benzene was applied to the back skin of hairless mice. At different time intervals up to 3 days after the carcinogen application groups of animals were injected i.p. with 30 muCi 3H-TdR 30 min before they were killed. Single cell suspensions of epidermal basal cells were prepared by a combined enzymatic and mechanical separation method, and the DNA frequency distribution pattern from each cell suspension was measured by means of micro-flow fluorometry. Smears for autoradiography were made from each cell suspension and the labeling index and mean grain count assessed. After a short initial delay, MCA induced an increase in the labeling index similar to that observed after non-specific cell injury and cell loss. Thereafter, the cells were considerably delayed in their progression through the S phase, with a low exit from S resulting in a transient emptying of the G2 compartment, without indications of any significant delay of the passage through G2 phase. The cells that had been injured by the MCA application in or just before S phase proceeded into the G2 phase and mitosis more than 24 h after the initiation of DNA synthesis. The cell kinetic reaction of epidermis to a single application of MCA is thus very different from that caused by a nonspecific cell damage, e.g. application of the vesicant agent cantharidin or removal of surface cells by cellophane tape stripping.