Neoplasia in Zebrafish (Danio rerio) Treated with N-methyl-N'nitro-N-nitrosoguanidine by Three Exposure Routes at ifferent Developmental Stages

We exposed embryos (83 hours postfertilizaton) and fry (3 weeks posthatch) to N-methyl- N'-nitro-N-nitrosoguanidine (MNNG) by immersion in aqueous solutions of 0-10 ppm for 1 hour (embryo) or 0-2 ppm for 24 hours (fry). Zebrafish embryos were microinjected with MNNG at levels of 0 or 96 ng/egg. Diets containing 0-2,000 ppm MNNG were fed to juvenile zebrafish for 3 months beginning at 2 months posthatch. Fish were sampled for histopathologic study at 6-12 months after initiation of carcinogen exposure. Embryos and fry were both quite responsive to MNNG; however, juvenile zebrafish were remarkably refractory to MNNG-induced neoplasia. Principal target organs in zebrafish treated as embryos with MNNG were liver and testis, with hepatocellular adenoma the most prevalent hepatic neoplasm. A variety of mesenchymal neoplasms occurred in zebrafish following embryo exposure to MNNG, including chondroma, hemangioma, hemangiosarcoma, leiomyosarcoma, and rhabdomyosarcoma. Testis and blood vessels were primary target organs for MNNG following fry exposure, with seminoma, hemangioma, hemangiosarcoma, and various other epithelial and mesenchymal neoplasms occurring. The zebrafish is a responsive, cost-effective lower vertebrate model system in which to study mechanisms of carcinogenesis.