Changes in cell population kinetics during epidermal carcinogenesis

The changes in cell population kinetics in response to a mechanical stimulation by wounding were studied in a hyperplastic epidermis, papilloma and a squamous cell carcinoma of the mouse dorsal skin induced by repeated treatments with 20-methylcholanthrene. The G0 cell population, which consists of about 66% of the basal cells in the dorsal epidermis of the normal mouse, disappeared in hyperplastic epidermis, papilloma and carcinoma. The zone of proliferating cells enlarged in the papilloma, including several supra-basal layers, in contrast to its strict localization in the basal layer in hyperplastic epidermis. Proliferating cells were distributed in almost all areas in the carcinoma without localization. The normal responses to wounding were preserved in the hyperplastic epidermis, except for the absence of G0 cells, while the papilloma and carcinoma showed no increase in proliferating cells or shortening of the cell cycle time. From the view-point of cell population kinetics, papilloma and carcinoma are regarded as the tissues in which the flexible control mechanisms of irreversible cell differentiation to mature cells and of the length of cell cycle time are lacking.