Sunscreens promote repair of ultraviolet radiation-induced dermal damage

Chronic UV irradiation profoundly damages the dermis of human and animal skin. These alterations were thought to be irreversible. Recently, we showed that substantial repair occurred in hairless mice after stopping UV exposure. A band of new connective tissue was laid down subepidermally. The present study focussed on whether repair would occur if animals were protected by sunscreens after dermal damage was induced and irradiation was continued. Albino hairless mice were exposed to Westinghouse FS20 sunlamps thrice weekly for 30 weeks. The daily dose of UV (UVB + UVA) was 0.17 J/cm2. Sunscreens of sun protection factors (SPF) 6 and 15 were applied after 10 and 20 weeks of irradiation. Biopsies were taken at 10, 20, 30, and 45 weeks of the experiment. With both sunscreens, especially SPF-15, previously damaged dermis was repaired during continued irradiation. Repair occurred in situ and, in severely damaged skin, in the novel form of subepidermal reconstruction zones of new connective tissue with parallel collagen bundles and a network of fine elastic fibers.