Rarely occurring spontaneous metastasizing testicular tumors in rats. Histopathologic and ultrastructural features.

Testicular tumors are not uncommon in aging rats. However, metastasis from these neoplasms to several distant visceral organs has not been reported. Testicular tumors were observed in a total of 18 (1.34%) Sprague-Dawley rats out of 1340 males examined. Out of these 18 neoplasms, interstitial tumors were observed in 12 rats, of which 11 were benign and one malignant. Spermatic seminomas were observed in two (0.15%) cases (one benign and one malignant). Both rats with malignant testicular neoplasms were sacrified in extremis. Remaining four tumors were classified as mesotheliomas. Detailed gross, histopathologic and electron microscopic features of primary testicular neoplasms with metastases to several distant visceral organs are described and discussed.