A MORPHOLOGICAL-STUDY OF A THYROID-CARCINOMA IN A MEDAKA, ORYZIAS-LATIPES (TEMMINCK-AND-SCHLEGEL)

A large mass protruding beyond the left operculum was found in an adul t male medaka, Oryzias latipes (Temminck & Schlegel). Histologically, the mass was mainly composed of thyroid follicles measuring up to 292 mu m in diameter. The follicular epithelium consisted of cuboidal or s quamous cells. The follicles lacked colloidal material in the lumen. T he thyroid tissue infiltrated into the kidney. Electron microscopicall y, the tumour cells were characterized by dilation and vesiculation of rough endoplasmic reticulum, which extended throughout the cytoplasm, and an irregular outline of the nucleus. Mitotic figures and partly d egenerate cells were frequently observed. Based on the above features, the tumour was diagnosed as a follicular carcinoma of the thyroid, th e first reported case in the medaka.