Research Product

Twenty-six cases of germ cell neoplasms were identified from approximately 10,000 specimens of Japanese medaka Oryzias latipes used in carcinogenesis bioassays and examined histologically. The neoplasms resembled spermatocytic seminomas and occurred in both female and male specimens. Most were confined to the peritoneal cavity but some infiltrated adjacent organs, including the kidney. The presence of neoplastic cells in the heart, gill capillaries, and orbital cavity suggested that some neoplasms had spread by way of the cardiovascular system. Histologically, the components of neoplasms resembled the cellular stages of spermatogenesis. Primary spermatocytes, secondary spermatocytes, and spermatids, but not mature spermatozoa, could be recognized. The cellular patterns of the neoplasms were solid, lobular, or a combination of those. Mitotic figures were not frequently seen in the lesions. Many of the neolasms in males contained scattered cells that resembled oocytes. The occurrence of the neoplasms appeared related to age but not to chemical exposure, because they were seen in both controls and in fish exposed to a variety of carcinogenic and noncarcinogenic chemicals. Multiple cases of the neoplasm occurred in fish maintained in the same aquarium or from the same cohort, suggesting that culture practices, genetic factors, or a virus may affect the development of the lesions.