Effect of thyroid-stimulating hormone on the physiology and morphology of the thyroid gland in coho salmon,Oncorhynchus kisutch.

Activity of the thyroid gland of the coho salmon,Oncorhynchus kisutch, was assessed by physiological, histological and ultrastructural criteria after treatment with graded doses of bovine thyrotropin (bTSH) in January and March. Average plasma thyroxine (T4) levels increased from about 0.8 ng/ml in saline-injected controls to about 15 ng/ml in fish treated with four intraperitoneal injections of 0.8 lU bTSH. Light-microscope observations of one μm-thick sections stained with methylene blue and azure II, showed that bTSH treatment increased epithelial height in both presmolts and smolts. Ultrastructural manifestations of increased activity owing to bTSH treatment were also seen, along with evidence of follicle proliferation. Cytoplasmic organelles and secretory granules increased in numbers with increased dosage of bTSH.