Ultraviolet light carcinogenesis in hairless mice: cell kinetics during induction and progression of squamous cell carcinoma as estimated by the double-labeling method.

The double-labeling method for studying cell cycle kinetics was applied to uninvolved epidermis of hairless mice (irradiated and unirradiated), and at various stages of ultraviolet light (UVL)-induced squamous cell carcinoma. The growth fraction (GF) was determined by continuous labeling with tritiated thymidine (3HTdR). The data indicate that the double-labeling technique is an acceptable method for studying cell kinetics in UVL-induced squamous cell carcinoma. The GF in normal epidermis (unirradiated and irradiated), hyperplastic epidermis, and lesions of squamous cell carcinoma is 1. The duration of cell generation (Tc) and DNA synthesis (Ts) periods decreased progressively with induction and progression of squamous cell carcinoma. For normal epidermis (irradiated mice) the Tc and Ts were 91 and 6.3 hr, respectively. In the hyperplastic epidermis, the Tc and Ts were reduced to 44 and 4.5 hr, respectively, while even lower values were obtained for the tumor (Tc = 16 hr, Ts = 3.1 hr). The labeling index (LI) in normal epidermis was 7% and increased progressively in hyperplastic epidermis (10%) and squamous cell carcinoma (19%). Epidermal cell differentiation in hyperplastic and tumor tissues appears to have been delayed (GF = 1), since cells above the basal layer, which in normal epidermis keratinize, retain the ability to proliferate, as evidenced by extensive incorporation of [3H]TdR in these cells. The results suggest that tumor production is associated with a progressive shortening of the cell cycle and delayed keratinization of epidermal cells.