Fundulus heteroclitus gonadotropin(s). I. Homologous bioassay using oocyte maturation and steroid production by isolated ovarian follicles.

Isolated ovarian follicles from several species were cultured to develop an in vitro bioassay system for Fundulus heteroclitus gonadotropin. An extract of F. heteroclitus pituitaries, when tested in heterologous systems using follicles from Rana pipiens. Xenopus laevis, and Carassius auratus, was ineffective in provoking either germinal vesicle breakdown or steroid production. In a homologous system using F. heteroclitus follicles, F. heteroclitus pituitary extract was capable of inducing both germinal vesicle breakdown and steroid production in a dose-dependent fashion. Testosterone, estradiol-17 beta, and 17 alpha-hydroxy,20 beta-dihydroprogesterone were detected in both the culture media and the follicle extracts after F. heteroclitus pituitary extract stimulation. The steroidogenic responses resulting from the pituitary extract stimulation were dependent on the size and stage of follicular development. Only large vitellogenic follicles (1.2-1.4 mm diameter) were able to produce 17 alpha-hydroxy,20 beta-dihydroprogesterone and testosterone. Small vitellogenic follicles (less than 1.2 mm) were unresponsive to stimulation by F. heteroclitus pituitary extracts as scored by either germinal vesicle breakdown or production of 17 alpha-hydroxy, 20 beta-dihydroprogesterone and testosterone. However, estradiol-17 beta production was detected in follicles of a much wider size range: Follicles as small as 0.8 mm diameter were responsive to F. heteroclitus pituitary extract stimulation and produced a large quantity of estradiol-17 beta. There was a marked seasonal sensitivity of F. heteroclitus follicles to pituitary extract stimulation in vitro. Follicles obtained from fish outside of the breeding season (January) were less responsive to stimulation by pituitary extract or steroid. The same preparation of pituitary extract was capable of provoking germinal vesicle breakdown in follicles obtained in May. Pituitary extracts prepared during October through January were also less potent than those prepared during the breeding season (February through September). We conclude that F. heteroclitus gonadotropin(s) shows a noticeable species specificity and that F. heteroclitus follicles exhibit both a season- and a size-dependent responsiveness to gonadotropin(s). Hence, with a judicious use of the appropriate types of F. heteroclitus ovarian follicles, we have been able to demonstrate that in vitro oocyte maturation and steroid production are sensitive, homologous bioassays for F. heteroclitus gonadotropin(s).