The characterization of squamous cell carcinoma induced by ultraviolet irradiation in hairless mice.

Squamous cell carcinomas (SCCs) induced by ultraviolet irradiation in hairless mice were characterized according to their growth, gross appearance and light and transmission electron microscopic features. SCCs arose directly from irradiated skin (ab initio) or progressed from pre-existing epidermal tumours and lesions. SCCs could be graded using guidelines established for human tumours. SCCs comprised 60.8% of the tumours examined. Of these, 35.6% were designated as grade 1, 27.7% as grade 2, 7.9% as grade 3 and 28.7% as grade 4. Spindle cell tumours suspected of being SCCs were included in grade 4. Grades 1, 2 and 3 could not be distinguished on the basis of growth and gross appearance. Those arising ab initio presented as either red, ulcerated lesions or as raised, white, verrucose lesions. Grade 4 SCCs that arose ab initio presented as rapidly growing, red, spherical lesions. Those that arose from pre-existing tumours or lesions had no characteristic appearance, and variable growth. Light microscopically, grade 4 SCCs with an obvious point of origin from epidermis or other epidermal tumours, and putative grade 4 SCCs without such a point of origin, were characterized commonly by spindle cells, pleomorphic giant or multinucleated cells and individual cell reticular fibres. Ultrastructurally, spindle cells, although poorly differentiated, were distinct from flibroblastic proliferations and had few tonofilaments or desmosomes, and were inconsistently surrounded by basal lamina-like material. On the basis of these characteristics, and despite inconclusive positivity with immunoperoxidase staining for keratin and prekeratin, it was concluded that these spindle cell tumours were most probably of identical squamous cell origin.(ABSTRACT TRUNCATED AT 250 WORDS)