Histopathological and Immunohistochemical Studies of Gonadal Undifferentiated Carcinoma in Common Carp Cyprinus carpio

We examined in detail the pathological features of common carp Cyprinus carpio (mature male and immature fish) with neoplastic lesions in the abdominal cavity. Diseased fish had abdominal enlargement, exophthalmos and scale elevation, along with a large tumor and accumulated ascitic fluid. The tumor was located where the gonad would have been in a healthy fish. Histopathologically, the tumor was composed of parenchyma, stoma and necrotic tissue, and the tumor cells had infiltrated the abdominal organs. Gonadal debris, including germ cells, was observed in the tumor parenchyma. The tumor and primary tissues were dissimilar, suggesting that the tumors were undifferentiated. Immunohistochemically, the tumor cells were positive for cytokeratin AE1/AE3 but negative for vimentin. They were positive for proliferating cell nuclear antigen (PCNA) and p53. These features indicated that tumor proliferation was uncontrolled, with a fast cell cycle, resulting in the development of large tumors. The tumor can therefore be diagnosed as an undifferentiated carcinoma originated from gonadal tissue.