Histophysiological and immunocytochemical study on the nature of the thyrotrops in the pituitary of immature rainbow trout, Salmo gairdneri.

Alcian blue- and periodic acid-Schiff-positive "granular basophils" with electron-dense granules +/- 160 nm in diameter, and weakly developed irregular cisternae of the granular endoplasmic reticulum (GER) are present in the rostral pars distalis (RPD) and the proximal pars distalis (PPD) of immature rainbow trout. They all react with antisalmon alpha beta-gonadotropin. However, only granular basophils in the caudal RPD and the rostro-dorsal PPD, often bordering on and sometimes surrounded by neurophypophysial tissue, react with anti-human beta-TSH. These cells, considered as the source of thyrotropin, show degranulation and dilatation of the GER-cisternae in fish treated with potassium perchlorate. The thyroids of the goitrogen-treated animals had relatively numerous small follicles with a high epithelium. The remaining granular basophils are gonadotrops.