Differential susceptibility of a fish, tilapia Oreochromis mossambicus (Teleostei, Cichlidae) to hepatocarcinogenesis by diethylnitrosamine and methylazoxymethanol acetate.
Oreochromis mossambicus, commonly known as tilapia, is a freshwater teleost with a wide tolerance to environmental conditions. Multiple focal lesions in the liver were observed 2 months after cessation of a one-month long treatment with 100 p.p.m. diethylnitrosamine. Cells were small and compact and arranged in sheets. Ultrastructurally, these cells have abundant endoplasmic reticulum, round mitochondria, less conspicuous golgi apparatus and fat droplets. Other organs like the intestines, spleen, kidneys, ovaries and pituitary appeared normal. Two liver inducible enzymes, gamma-glutamyl transferase and tyrosine aminotransferase were elevated by 5- and 3-fold respectively. Aggressive migration of hepatocytes was observed in tumorigenic liver explants. Vitellogenesis and early embryological development appeared unaffected as the female fish spawned during hepatocarcinogenesis. However, their fry were stunted and short-lived. To compare the susceptibility of tilapia to another hepatocarcinogen, the fish were also treated with methylazoxymethanol acetate at 10 p.p.m. for 0.5-1 h. However, methylazoxymethanol acetate was too toxic and 75% of the fish succumbed 1 day after treatment. Moreover, after 2 months post-treatment, neither tumors nor change in enzyme activities were observed in any organ. These results suggest that tilapia could be a useful model for screening and differentiating carcinogens since they could develop liver tumors within only 2 months after treatment with diethylnitrosamine.