UV radiation-induced tumors in haired mice: identification as squamous cell carcinomas

The most common tumor induced by UV radiation in haired mice is considered to be a fibrosarcoma on the basis of its presentation as a nodule in the skin and on the basis of a spindled appearance upon light microscopic examination. A squamous cell carcinoma is thought to be a much less common tumor. In the present report this concept was reevaluated in mammary tumor virus-free C3H/HeNCr (C3H-) mice. From first appearance, almost all lesions upon gross morphologic examination have an epidermal component and initially are similar to solar keratoses in humans. The lesions then become nodular and eventually develop central ulceration, often with a rolled border characteristic of squamous cell carcinomas. The morphology upon light microscopic examination ranged from well-differentiated squamous cell carcinoma to a poorly differentiated spindle cell neoplasm. Occasionally, variable patterns of squamous differentiation were seen in the same lesion. Immunoperoxidase examination with a polyclonal antikeratin serum demonstrated the presence of keratin in 84 of 87 tumors. Frequent, poorly formed desmosomes were found on ultrastructural examination. These tumors usually had a regressor phenotype upon transplantation into recipients. In conclusion, almost all UV radiation-induced tumors in C3H- mice are squamous cell carcinomas, and these tumors are usually antigenic.