Nephroblastomas were observed in 50 Japanese eel reared at a farm for 5 to 9 months from 1989 after collection in the wild. The tumors, arising from the trunk kidney near the anus, were noted externally as abdominal swellings and varied in size from 30 to 75 mm in maximum diameter. Most were elastic, solid, and well encapsulated. Histologically, the nephroblastomas were composed of combinations of three main tissue elements. Spindle- or oval-shaped cells resembling human blastema cells were observed in most tumors to some larger or smaller degree. Although variation was evident from tumor to tumor, and even within the same tumor, the most common histological type was epithelial with formation of alveolar nests, the cells sometimes being arranged in tubular structures simulating normal renal tubules. A muscle tissue element with distinct cross-striations was also observed. Liver metastases were found in one case. Histological examination of apparently normal kidneys from 100 eels revealed one early-stage nephroblastoma.

The cause of these tumors is unclear, although they have been discovered with increasing incidence after the spread of indoor eel culture with raised water temperatures (26–27°C) in Japan. Environmental factor(s) associated with the new aquaculture method may thus play a role in their genesis.

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Supported by Grants-in-Aid for Cancer Research from the Ministry of Education, Science and Culture and Ministry of Health and Welfare, Japan.

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