ULTRAVIOLET RADIATION-INDUCED SKIN-LESIONS IN LABORATORY OPOSSUMS (MONODELPHIS-DOMESTICA) EXPOSED FROM THE WEANLING STAGE

After repeated exposure to ultraviolet radiation, adult laboratory opo ssums (Monodelphis domestica) may develop hyperplasia and neoplasia of the shaved skin. We exposed opossums from the weanling stage (8-10 we eks after birth) and compared the incidence of lesions at designated t ime points with those observed following introduction of adults (aroun d 5 months old) to the same irradiation protocol (125 J/m(2) of UVB th ree times each meek for up to 40 weeks). The overall incidence of hype rplasia and neoplasia among opossums exposed from the weanling stage w as markedly lower than among animals first exposed as adults, and hype rkeratosis and sarcoma were not observed. Although freckling was wides pread, cutaneous melanocytic nevi were rare among animals first expose d as weanlings; however, one animal without freckling developed malign ant melanoma with presumptive metastasis to the spleen. The basis of t he lowered lesion incidence among weanling-introduced animals is not c lear; it may be that cutaneous immunity to ultraviolet radiation damag e is more efficient in juveniles than in adults.