Responses of two different types of pituitary gonadotrophs of the loach, Misgurnus anguillicaudatus, to gonadectomy and to exogenous sex steroids

Abstract

In the proximal pars distalis of the pituitary gland of the loach, Misgurnus anguillicaudatus, two basophilic cell types with different localities, staining properties, and cytological characteristics are discernible other than thyrotrophs and somatotrophs. Ultrastructural changes of these two types of basophils, termed the globular and the vesicular cells according to their ultrastructural features, were examined in loaches of both sexes which had been subjected to bilateral gonadectomy and to treatment with sex hormones during the late prespawning period. Complete gonadectomy stimulated the globular cells to show dilation of the rough endoplasmic reticulum and a decrease in the amount of cytoplasmic granules. The changes were less intense in males than in females 60 days after gonadectomy. The vesicular cells did not show evident alterations following ovariectomy in females but did respond to castration in males with an enlargement of the cytoplasmic vesicles characteristic of the cell type. After six injections of estradiol (50 μg/fish) to females during a month, the globular cells became atrophic with accumulation of cytoplasmic granules, while the vesicular cells exhibited rather slight changes of their activity including an activation of the Golgi apparatus. Similar administrations of testosterone (100 μ/fish) to males elicited regressive changes of both the globular and the vesicular cells, and shrinkage to a lamellar form of cytoplasmic vesicles in the latter cells is particularly notable. The results were discussed in terms of the possible existence of two distinct types of pituitary gonadotrophs in the loach, each of which may play a role at different stages of the sexual cycle of the fish of each sex.