Retinoic acid provokes a regeneration-like proliferative response in murine epidermis. A bivariate DNA/bromodeoxyuridine flow cytometric study.

Retinoic acid (RA) is an inducer of epidermal proliferation by a mechanism of action which is not fully known. We examined the proliferative response of hairless mouse epidermis to a single topical application of different doses of RA (0.1-1000 nmol). The mitotic rate was assessed using the stathmokinetic method, and change in epidermal cell numbers were scored per microscopic vision field in tissue sections. Cell cycle parameters were measured by bivariate bromodeoxyuridine/DNA flow cytometry on isolated epidermal basal cells after pulse labelling up to 10 days after RA treatment. The results showed a dose-dependent increase in mitotic activity with a maximum at 3 days after RA application, and a dose-dependent hyperplasia with a maximum at 4 days after RA application. Cell-cycle analysis showed an immediate proliferative response after RA application similar to that following various skin irritants. Although differences in the G2 phase transit were seen, this indicates a similar mechanism of action of RA-induced epidermal proliferation and that associated with epidermal regeneration in general.