Ultraviolet radiation-induced corneal tumours in the South American opossum, Monodelphis domestica.

Chronic exposure of the South American opossum, Monodelphis domestica, to ultraviolet radiation (UVR) induced 154 primary tumours of the cornea in 152 eyes. Tumours developed gradually; frank neoplasia was preceded by non-neoplastic proliferation of corneal stromal fibroblasts (keratocytes) and extensive neovascularization. Histologically, the majority of tumours (134 of 154) appeared to be fibrosarcomas arising from keratocytes, but about 12 per cent of the tumours (18 of 154) had a highly vascular appearance, suggesting haemangiosarcoma. In two eyes, squamous cell carcinomas overlay mesenchymal tumours. Ultrastructural features of UVR-induced corneal tumours were consistent with tumours, and cultured skin fibroblasts expressed high content of messenger RNA for the intermediate filament vimentin; no cytokeratin messenger RNA was detected in these cells and cell lines. Based upon their light microscopic, ultrastructural, and intermediate filament biosynthetic characteristics, the majority of UVR-induced corneal tumours in M. domestica appeared to be fibrosarcomas. Haemangiosarcomas constituted a smaller proportion of the tumours, and squamous cell carcinomas were very rare.