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.