Connective tissue photodamage in the hairless mouse is partially reversible

Photodamaged connective tissue in animal and human skin is characterized by excessive accumulations of elastic fibers, loss of mature collagen, concomitant overproduction of new collagen, and greatly increased levels of glycosaminoglycans. Formerly considered irreversible changes, we recently showed in hairless mice, post irradiation, that a band of normal connective tissue was laid down subepidermally. The present studies focused on 2 aspects of this repair: whether repair would occur if animals were protected by sunscreens after dermal damage was induced and irradiation continued; whether retinoic acid could enhance the repair process. To examine the first aspect, albino hairless mice were irradiated with Westinghouse FS 20 sunlamps thrice weekly for 30 weeks. Sunscreens of high sun-protection factors were applied after 10 and 20 weeks. Not only was further damage prevented, but the damage incurred before sunscreen application was repaired. This appeared as subepidermal reconstruction zones containing normal, mature collagen and a network of fine elastic fibers. The second aspect was examined by applying 0.05% retinoic acid, topically, to animals preirradiated for 10 weeks. In contrast to controls treated with vehicle, the reconstruction zone was significantly wider in retinoic acid-treated mice. The enhanced repair was dose-related.