Ultraviolet radiation-induced skin lesions in laboratory opossums (Monodelphis domestica) exposed from the weanling stage.

After repeated exposure to ultraviolet radiation, adult laboratory opossums (Monodelphis domestica) may develop hyperplasia and neoplasia of the shaved skin. We exposed opossums from the weanling stage (8-10 weeks after birth) and compared the incidence of lesions at designated time points with those observed following introduction of adults (around 5 months old) to the same irradiation protocol (125 J/m2 of UVB three times each week for up to 40 weeks). The overall incidence of hyperplasia and neoplasia among opossums exposed from the weanling stage was markedly lower than among animals first exposed as adults, and hyperkeratosis and sarcoma were not observed. Although freckling was widespread, cutaneous melanocytic nevi were rare among animals first exposed as weanlings; however, one animal without freckling developed malignant melanoma with presumptive metastasis to the spleen. The basis of the lowered lesion incidence among weanling-introduced animals is not clear; it may be that cutaneous immunity to ultraviolet radiation damage is more efficient in juveniles than in adults.