Induction of branchial (gill) neoplasms in the medaka fish (Oryzias latipes) by N-methyl-N'-nitro-N-nitrosoguanidine.

Juvenile medaka were exposed to N-methyl-N'-nitro-N-nitrosoguanidine in water under static renewal conditions for 28 days. Two groups of 134 fish each were pulsed 3 times weekly at nominal concentrations of 1.0 and 0.5 mg/liter with N-methyl-N'-nitro-N-nitrosoguanidine dissolved in dimethylformamide. A third group of 134 fish was exposed to the solvent control, 0.01% dimethylformamide in water. Following the 28-day exposure, and during the recovery period, fish were sampled at intervals of approximately 0, 3, 6, and 9 months and examined grossly. Selected tissues were evaluated microscopically. Many tumor types developed in both N-methyl-N'-nitro-N-nitrosoguanidine exposure groups, but only the gill lesions will be discussed. Approximately 50% of the fish in both treatment groups died from gill damage in the second to third month of the recovery period. More than 90% of the surviving treated fish displayed gill lesions, which progressed from mild epithelial hyperplasia of gill filaments at 0-months recovery to epitheliomatous hyperplasia at 3 months and advanced to a more focal nodular appearance of gill filaments at 6 months. Eight to 9 months after the treatment period, at least four fish displayed branchial blastomas. The control fish had no gill lesions. Chemically induced gill tumors have not been previously observed in fish. Even gill tumors of unknown origin are very rare.