Early cell kinetic effects of a single dose of monochromatic ultraviolet B irradiation on hairless mouse epidermis.

Hairless mice were exposed to a single erythemic (25 mJ/cm2) or suberythemic dose (12.5 mJ/cm2) of ultraviolet B (UVB) irradiation at 297 nm. The cell kinetic changes were observed at several times during the first 7 d after the irradiation. The mitotic count, the mitotic rate (stathmokinetic method), and the number of suprabasal and basal cells were scored in histologic sections. The incorporation of [3H]thymidine was measured after pulse labeling, and the DNA distribution pattern was studied by flow cytometry. Initially, both UVB-doses induced a block or delay in the cell proliferation. The rate of entrance of cells into mitosis and the uptake of [3H]thymidine were reduced, and cells accumulated in the S phase of the cell cycle. Hence, during the first period after irradiation, UVB seemed to interfere with the DNA synthesis by inducing a prolonged S phase duration. The DNA synthesis rate was reduced to the same degree after both UVB-doses. From 24 h after irradiation rapid regenerative proliferation took place, most pronounced after the highest UVB-dose. Waves of proliferation seemed to arise from partially synchronized cohorts of cells proceeding through the cell cycle at a higher speed than normal. Thus, the present study indicates that UVB irradiation is comparable with the cell kinetic effects following both chemical skin carcinogens and non-carcinogenic skin irritants. UVB induces an inhibitory effect on the DNA synthesis activity, in addition to regenerative cell proliferation subsequent to cell toxicity.