Suckling young of opossums (Monodelphis domestica) were exposed to ultraviolet radiation (UVR, predominantly UVB: 290-320 nm) in part to determine an optimal protocol for induction and progression of melanoma in this species. In all, 620 litters were introduced to one of seven protocols. The lowest dose (175 J/m2) administered three times a week for almost three weeks led to the highest incidence of melanotic lesions with melanoma potential (8.1%) among young (5-month-old) adults. Among 101 much older animals (> 17 months at necropsy), 43% showed metastatic melanoma to the lymph nodes and almost one-third of these had progressed to widespread dissemination. Three of the latter animals, from a total of 13 obtained so far, were selected for detailed histological examination of disseminated disease. At necropsy, all three showed widespread metastases beyond the lymph nodes to the spleen, lungs, and other distant sites. Histological changes typical of malignant melanoma included junctional activity, mitotic figures, and nerve and vessel invasion. This novel finding leads us to conclude that UVR can act as a complete carcinogen for progression to widely disseminated disease and that exposure of sucklings can lead, in old age, to widespread metastatic melanoma in this model. The results are thus not inconsistent with the view that, in humans, early exposure to sunlight might act as an initiating factor in a later progression to malignant melanoma.
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