Changes in Ovarian Activity and Fine Structure of Pituitary Gonadotrophs during Spawning Cycle of the Chichibu-goby Tridentiger obscurus

Abstract

Repeated spawnings of the chichibu-goby Tridentiger obscurus and changes in ovarian activity and fine structure of the pituitary gonadotrophs during the spawning cycle were studied.
The goby repeated spawning from 2 to 10 times in the experimental tanks. Although the spawning intervals were variable, they spawned mostly at intervals of about 10 days. Just after spawning (Day 0) GSI decreased to 2.0-3.5% and began to increase by Day 3. Afterward the oocytes grew rapidly with a sharp increase in GSI, while plasma E2 level kept high.
In the pituitary gland three stages (II, III and IV) of gonadotrophs were observed during the cycle. Stage II cells contained numerous secretory granules, and Stage IV was characterized by the presence of cisternal rough endoplasmic reticulum. Stage II was considered to be a transient, intermediate stage between Stage II and IV. Our observations suggest that the preovulatory GTH surge results mainly from the transformation of the gonadotrophs from Stage II to IV via Stage III.