Steroidogenic activities of follicle-stimulating hormone in the ovary of Japanese eel, Anguilla japonica

To clarify the physiological functions of follicle-stimulating hormone (FSH) during oogenesis in Japanese eel, Anguilla japonica, the steroidogenic activities of recombinant Japanese eel FSH (rjeFSH) were assessed in the eel ovary. Female eel were injected with salmon pituitary homogenate to enhance the ovarian development, and the ovaries at different developmental stages were subjected to steroidogenic bioassay. These ovaries could be classified into three types according to oocyte growth and development of ovarian follicular cells. The type-A ovary possessed poorly developed follicular cells around pre- or early vitellogenic oocytes, and rjeFSH did not induce sex steroid secretion. Testosterone (T) secretion was stimulated by rjeFSH in the type-B ovary with developed theca cells and undeveloped granulosa cells around early to mid-vitellogenic oocytes, whereas estradiol-17β (E2) secretion was not enhanced. The rjeFSH stimulated both T and E2 secretion in a dose-dependent manner from the type-C ovary with fully developed theca and granulosa cells around mid-vitellogenic oocytes. Salmon GTH fraction (sGTH) and a membrane permeable cAMP analogue, 8-bromo-cAMP (8-Br-cAMP) also enhanced T and E2 secretion from the type-C ovary. Human chorionic gonadotropin (hCG) similarly enhanced T secretion, but failed to stimulate E2 secretion from the type-C ovary, suggesting different effects on steroidogenic activities between eel FSH and hCG in eel ovary. There was a positive correlation between the oocyte diameter and E2 secretion from eel ovaries stimulated by rjeFSH. These results suggest that aromatase activity is accelerated by eel FSH in the granulosa cells, which develop following theca cell development in this species.