Characterization of gonadotropic cells during continuous and seasonal spermatogenesis of two freshwater fish species: a histochemical and immunohistochemical study
Electron-microscopical observations were made on the pituitary gonadotropes of the precocious female chichibu-goby, Tridentiger obscurus, during the annual reproductive cycle. The gonadotropes underwent marked morphological changes during the cycle, and could be classified into five stages according to the extent and features of the rough endoplasmic reticulum, the abundance of the secretory granules and the shape of the nucleus. Stage-I cells first occurred in the late resting phase, followed by the appearance of Stage-II cells in the endogenous vitellogenic phase. Stage-IV cells appeared in the exogenous vitellogenic phase and predominated in the spawning phase, when Stage-III cells were also occasionally observed. In the following resting phase, Stage-V cells became predominant. The observations suggest that these five morphological stages may reflect different physiological conditions of the gonadotropes, and their functional significance is discussed.