N-METHYL-N'-NITRO-N-NITROSOGUANIDINE-INDUCED NEOPLASMS IN MEDAKA (ORYZIAS-LATIPES)

To test the sensitivity of the small fish species Oryzias latipes to t he direct-acting carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG ), medaka were exposed at 15 days of age to 30 mg/L for 1 hr and follo wed for up to 16 mo. One hundred neoplasms were diagnosed in 84 of 213 exposed fish, with approximately equal percentages in males and femal es. Many neoplasms (62%) were of mesenchymal origin and were categoriz ed as blood vascular neoplasms (hemangioma and hemangiosarcoma), invas ive sarcomas, and scale-associated neoplasms. Invasive sarcomas includ ed rhabdomyosarcoma, fascial sarcoma, hemangiopericytoma, and undiffer en tiated sarcoma. A scale-associated neoplasm, termed lepidocytoma, w as an unusual neoplasm of scale anlage. Thyroid follicular neoplasms, with a 100% incidence in males, and pancreatic acinar carcinoma were t he most common epithelial tumors. Neoplasms of the gills, swim bladder , and olfactory epithelium were also seen as well as teratoma with mix ed epithelial and mesenchymal components. The study showed a broad ran ge of neoplasms induced by MNNG in medaka, with a tissue distribution that might support direct action on exposed tissues.