Histologic and electron microscopy observations on diethylnitrosamine-induced hepatomas in small aquarium fish (Oryzias latipes).

Liver neoplasms were induced in medakas (Oryzias latipes) by the addition of diethylnitrosamine (DENA) to their aquarium water at levels of 15-135 ppm for 8 weeks. After 13 weeks, 21 to 32 fish had developed hepatomas. Medakas are useful for further studies because they are highly susceptible to the carcinogenic effect of DENA, and the time for tumor induction is relatively short. Histologic type differed in the lesions of different fish and also within individual tumors. Some were typical trabecular hepatomas, others were anaplastic hepatomas or cholangiomas, or mixtures of these. Electron microscopy revealed an extensive rough-surfaced endoplasmic reticulum in a lamellar pattern, many mitochondria, and several round lysosomes in tumor cells. A few fat droplets with occasional crystalline ghosts were sometimes in the cytoplasm. The Golgi apparatus was not conspicuous. Some cells had highly developed microvilli that showed differentiation toward structures resembling bile capillaries.