Proliferative lesions in swimbladder of Japanese medaka Oryzias latipes and guppy Poecilia reticulata

Thirteen cases of proliferative lesions of the swimbladder were encountered in Japanese medaka Oryzias latipes and guppy Poecilia reticulata from abou t 10 000 medaka and 5000 guppies used in carcinogenicity tests and histolog ically examined. Two of the 4 cases from medaka and 8 of the 9 from guppies occurred in untreated control specimens. The lesions affected the gas glan d epithelium and included hyperplasia, adenoma, and adenocarcinoma. One med aka had hyperplasia of the gas gland epithelium and in 1 guppy the gland wa s enlarged with an increase in the number of epithelial layers. Gas gland a denomas, 3 cases in medaka and 1 in the guppy, were typically larger than t he hyperplastic lesions, formed expansive masses up to 1 mm in greatest dim ension, and exhibited a solid or glandular growth pattern and mild cellular pleomorphism. Adenocarcinoma was the most advanced lesion and all 7 cases occurred in guppies. Adenocarcinomas sometimes filled the entire swimbladde r and measured up to 2.5 mm in diameter. Cells of adenocarcinomas were high ly pleomorphic, with atypical nuclei, and an elevated mitotic activity. Bec ause most of these tumors occurred in fish from control groups or in tests with noncarcinogenic compounds, the lesions observed here are probably spon taneous rather than chemically induced. Their rare occurrence, however, mak es swimbladder proliferative lesions in small-fish carcinogenesis models se nsitive indicators of compounds that might target cells of the gas gland.