Delayed induction of pigmented spots on UVB-irradiated hairless mice

Human skin exposed to solar radiation for a long time subsequently develops pigmented spots. which are named solar lentigines. Since no animal model o f this process is currently available, we attempted to induce similar spots in pigmented hairless mice. The mice were irradiated at 38 or 94 mJ/cm(2) three times/week for various periods of time (1-8 weeks) under an ultraviol et light source (Toshiba FL-SE; UVB). Skin pigmentation of irradiated mice was visually observed and skill color was determined with a colorimeter for 78 weeks. Uniform pigmentation was induced, but persisted only during expo sure, disappearing completely within 2 weeks after cessation of exposure. A t about 28 weeks after the first exposure, pigmented spots suddenly began t o appear. These pigmented spots were less than 2 mm in diameter and light b rown in color. The length of the latent period until appearance and the ext ent of development of these spots were dependent on the exposure period. Hi stological examination revealed increased numbers of active melanocytes and melanin granules in the affected epidermis. These pigmented spots closely resemble solar lentigines in humans, and the mice should be useful as an an imal model of solar lengtigines. (C) 2001 Elsevier Science Inland Ltd. All rights reserved.