Spontaneous histogenic tumors of epididymis observed in B6C3F1 mice

Four primary histiogenic tumors of the epididymis were discovered in 110 male B6C3F1 mice used in a carcinogenicity study in our laboratory. From the results, all these tumors were considered to be spontaneous. Histologically, the tumors were characterized by compact growth of spindle to oval shape histiocytic cells with or without a cleaved nucleus. Focal hemorrhage was present in three cases, and erythrophagocytosis occurred in such lesions. The tumors had some similar histological features of histiocytic sarcomas, which had been observed with high malignancy in other male and female animals. However, systemic growth and atypsim of neoplastic cells found in the histiocytic sarcomas were not observed in these epididymal tumors. On the other hand, four histiogenic tumors of the uterus, with features characteristic of epididymal tumors, were observed in 106 female mice. The uterine histiogenic tumors were regarded as benign or precursor lesions of histiocytic sarcoma. Furthermore, it is suspected that histiocytic sarcoma arises from the epididymis.