Epidermal changes caused by chronic low-dose UV irradiation induce wrinkle formation in hairless mouse

To investigate the effects of chronic low-dose UV irradiation on the skin, hairless mice were irradiated with a 1/3 minimal erythemal dose (MED) of UV. We examined the relationship between visible changes and skin damage in the dermis and epidermis. Hairless mice were irradiated with UVB (20 mJ/cm2) and UVA (14 J/cm2) three times a week for 10 weeks, followed by a 24-week non-irradiation period. Visible fine wrinkling was present after 4 weeks of irradiation, and the wrinkling progressively worsened throughout the period of irradiation. The wrinkles remained after irradiation was discontinued. In dermal components, no significant histological changes in the collagen fibers and elastic fibers were found, and the amount of hydroxyproline was also not changed. Thus, in the epidermis, there was a significant increase in the number of stratum corneum layers and the terminal-differentiation marker, filaggrin, positive cells. The intensity of staining for the differentiation marker, keratin 1, was reduced. These changes were accompanied by wrinkle formation, and remained after discontinuance of irradiation. These findings suggested that chronic low-dose UV irradiation induces structural and quantitative changes in the epidermis as a result of keratinization impairment, and that this damage in the epidermis is an important causative factor in wrinkle formation.