Estradiol-17β production in amago salmon (Oncorhynchus rhodurus) ovarian follicles: Role of the thecal and granulosa cells

Separation of the follicular thecal cell and granulosa cell layers of oocytes of the amago salmon (Oncorhynchus rhodurus) facilitated assessment of their roles in estradiol-17β production by an in vitro incubation method. Four different follicular preparations, intact follicles (oocytes with complete follicle layers), thecal layers contaminated with less than 10% granulosa cells, pure granulosa layers and zona radiata, and thecal layer-granulosa layer co-cultures, were incubated in the presence or absence of partially purified chinook salmon gonadotropin (SG-G100). Estradiol-17β and testosterone levels in the medium were measured by specific radioimmunoassay. SG-G100 (0.1, 1 μg/ml) stimulated estradiol-17β production by intact follicles and co-culture preparations, but not by the isolated thecal or granulosa layers, indicating that both layers are necessary for gonadotropin-stimulated estradiol-17β production. In contrast, SG-G100 greatly stimulated testosterone production by thecal layers (up to 80 times basal control values) but only slightly stimulated testosterone production by the other follicular preparations. Incubation of granulosa layers with exogenous testosterone (0.01, 1 μg/ml) or with testosterone and SG-G100 (1 μg/ml) together resulted in elevated testosterone levels. No significant difference was found between the two treatments, suggesting that gonadotropin did not stimulate the conversion of testosterone to estradiol-17β. Isolated thecal layers incubated with testosterone produced relatively small amounts of estradiol-17β, 7–8 times less than granulosa layers under the same conditions. These results suggest a two-cell-type model for the production of follicular estrogens, the thecal layer possibly contributing to estradiol-17β production by synthesizing androgens which are transferred to the granulosa layer and aromatized to estradiol-17β.