The objective of this proposal is to identify factors and conditions that regulate estradiol synthesis in ovarian follicles. Estradiol is the dominant steroid product of developing preovulatory follicles and continued estradiol secretion is essential for follicles to reach ovulatory status. Recent investigations in several species have led to formulation of a two cell, two gonadotropin model of the regulation of estradiol production. According to this model, LH stimulates theca cells to produce androgen and FSH stimulates granulosa cells to aromatize androgen to estradiol. This proposal is designed to investigate further interactions between gonadotropins and follicle cells and interactions between theca and granulosa cells that promote follicular androgen and estrogen production. Specifically, we will determine when and why LH and FSH are essential for developing follicles by a combination of experiments in vitro and in vivo. Theca and granulosa cells isolated at different stages of follicular development will be tested in vitro for steroidogenic capabilities and responses to LH and/or FSH and bovine follicles will be deprived of LH or FSH stimulation in vivo at specific times of the estrous cycle. Since it is believed that in vivo gonadotropins are released from the pituitary in pulses, a second and related question that will be addressed is the effect of pulses of LH on steroidogenesis by isolated bovine theca and granulosa cells and by whole bovine ovaries perfused in vitro. Third, we will test the hypothesis that the androgen precursor for estradiol synthesis is synthesized via the Delta5 pathway. Theca cells from rats and cows will be cultured with exogenous androgen precursors and with inhibitors of steroidogenic enzymes and the effects on androgen production will be assessed. Finally, studies on the steroidogenic capabilities of bovine cystic follicles will be continued through experiments in which cysts will be induced experimentally and their steroidogenic capabilities assessed in vitro. The proposed experiments will provide a better understanding of the hormonal control of follicular development and thus, may eventually lead to the development of better methods of inducing ovulation and treating infertility.