Liver cell carcinomas in the medaka (Oryzias latipes) induced by methylazoxymethanol-acetate

Medaka (Oryzias latipes) were treated with 0.3 ppm methylazoxymethanol-acetate (MAM-A) for 3 days (group I) and with 0.1 ppm MAM-A for 2 weeks (group II) and the effects on the medaka liver cells up to 6 weeks after commencement of the treatment and the induction of liver tumours at the 18th or 24th weeks were observed by light and electron microscopy. As an early effect, PAS-positive granules and basophilic liver cells appeared at the fourth or fifth week in group II. In group I, PAS-positive granules, basophilic liver cells and eosinophilic liver cells were observed from the second week. They tended to increase in number and formed aggregate bodies and basophilic liver cell foci. The aggregate bodies of PAS-positive granules resembled melano-macrophage centres (MMC). However, there were ultrastructural differences between these aggregate bodies and MMC. Basophilic liver cells contained much rough-surfaced endoplasmic reticulum and eosinophilic liver cells contained much smooth-surfaced endoplasmic retriculum and mitochondria by electron microscopical observation. Liver cell nodules appeared in 3 out of 21 fish but liver tumours were not found within 24 weeks in group II. In group I, basophilic liver cell foci progressed to liver cell nodules in 5 out of 9 fish. Furthermore, they progressed to liver cell adenomas in 3 and carcinomas in 3 of 9 fish at 18 weeks. The sequential morphological alterations observed in the present experiment were similar to those obtained in rats. Basophilic liver cell foci appeared to be an early aspect of hepatocarcinogenesis in medaka. Considering the susceptibility of medaka to the carcinogenic effect of MAM-A in a relatively short time and the low concentration of this material required for induction of tumours, medaka seems to be a useful animal for research on neoplasms and pre-screening carcinogenicity of chemicals.