Histological Changes in the Pituitary, Thyroid Gland and Gonads of the Fourspine Sculpin (Cottus kazika) during Downstream Migration.

The Fourspine sculpin (Cottus kazika) is a catadromous fish which is widely distributed in the rivers of Japan. The fish was used to examine the relationship between the migration behavior and hormonal control by studying the histological changes in the pituitary gland, thyroid gland and gonads during its downstream migration. By use of the immonocytochemical and histochemical techniques, 7 types of cells were identified in the pituitary gland namely; immunoreactive (ir)-PRL, GH, TSH, GTH, ACTH, MSH and SL cells. From among the first 4 types of the aforementioned cells, remarkable histological changes were observed in cells containing ir-GTH during the downstream migration. At this time also, the gonads were obsereved to be well developed, while the thyroid glands did not show clear changes morphologically. These results suggest that the gonadotropin regulates gonadal development in the Fourspine sculpin during downstream migration and possibly sex hormones synthesized by the gonads cause the downstream migration of this catadromous fish.