Evidence That a Slowly Cycling Subpopulation of Adult Murine Epidermal Cells Retains Carcinogen

The distribution and persistence of radioactively labeled benzo-(fl)pyrene [B(a)P] in the skin of adult female SENCAR mice were investigated by autoradiography of epidermal whole mounts and cross-sections at intervals following a single initiating application of 200 mnol of either H]B(a)P (2 mCi) or C]B(a)P (23 µCi). One day after treatment, the entire thickness of the skin was labeled; the grain density was greatest over hair follicles, sebaceous glands, and interfollicular epidermis. At 1 and 2 weeks, decreases in the nuclear grain density were consistent with the overall pattern of epidermal renewal. One month aftertreatment, carcinogen label-retaining cells made up approximately 2% of the interfollicular basal cells. They were also present in the hair follicles, approximately 4 and 5% of basal cells in the infundibulum and external root sheath, respectively. They were rare in the germ region and dermal papilla. Carcinogen label-retaining cells were compared with slowly cycling H]thymidine label-retaining cells and “maturing” basal cells, two distinct proliferative subsets of adult murine epidermis. Carcinogen label-retaining cells were found to have characteristics of the slowly cycling : (a) most of the carcinogen labeled nucleiwere found in the central regions of the epidermal proliferative units; (b) treatment of the carcinogen label-retaining cells with 2 µg of 12-O-tetradecanoylphorbol-13-acetate elicited labeled mitoses within 1 day, and a general decrease in grain density over basal nuclei. In contrast, maturing basal cells 4 days after a single injection of H]thymidine were found at the periphery of the epidermal proliferative units. Within 1 dayafter treatment with 2 µg of 12-O-tetradecanoylphorbol-13-acetate, maturing basal cells were displaced to the suprabasal layers. Double isotope-double emulsion autora-diographs demonstrated doubly labeled cells1 month after continuous labeling with H]thymidine and C]B(a)P and provide evidence that the radioactive carcinogen is retained by the slowly cycling H]thymidine label-retaining cells. These observations suggest that a slowly cycling population of epidermal cells may be relevant to the initiation phase of two-stage carcinogenesis.