Primary epididymis squamous cell carcinoma in a CB6F1-Tg rasH2 mouse.

In a 26-week carcinogenicity study in rasH2-Tg mice, squamous cell carcinoma on the epididymis was observed in a male mouse in the positive control group treated with N-methyl-N-nitrosourea. A 29-week-old male rasH2-Tg mouse that was euthanized 21 weeks after the administration of N-methyl-N-nitrosourea had a white-grayish mass on the left caput epididymis. The mass was nodular and consisted of pleomorphic tumor cells forming alveolar, sheeted, and trabecular structures suggesting epithelial tumor growth. These cells presented a cobblestone-like arrangement and formed intercellular bridges. Keratinization was infrequently observed. Periodic acid-methenamine-silver staining revealed argyrophilic fibrous structures around the alveolar structure of the tumor cells. Immunohistochemically, the tumor cells were positive for cytokeratin AE1/AE3 and cytokeratin 14 and negative for cytokeratin 5, p63, uroplakin III, vimentin, desmin, and αSMA. These immunohistochemical results suggested the tumor cells originated from the epididymal ducts. Metastatic lesions were observed in the mesenteric, inguinal, and pancreaticoduodenal lymph nodes. Based on these results, this tumor was diagnosed to be a primary squamous cell carcinoma of the epididymis. This is the first report of primary squamous cell carcinoma of the epididymis in a rasH2-Tg mouse.