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Mammalian preovulatory follicles produce primarily oestradiol and androgens before the LH and FSH surges. The production of these steroids by the follicle requires both theca and granulosa cells, since theca cells synthesize androgens from progestins but lack aromatizing capacity, whereas granulosa cells aromatize exogenous androgens to oestrogens but lack the ability to convert progestins to androgens (Fortune and Quirk, 1988). In ruminants, the gonadotrophin surge triggers a rapid decrease in oestradiol and androgen production and a marked increase in progesterone and oxytocin biosynthesis (reviewed in Fortune, 1994). It is unclear how changes in gonadotrophin concentrations in vivo regulate this follicular to luteal phase shift in hormone production. Theca and granulosa cells obtained from bovine preovulatory follicles before the gonadotrophin surge 'spontaneously luteinize', as evidenced by increased production of progesterone and oxytocin; high concentrations of gonadotrophins accentuate these changes (Aladin Chandrasekher and Fortune, 1990; Voss and Fortune, 1991). In contrast, similar concentrations of gonadotrophins do not stimulate or maintain thecal androgen production or granulosa oestradiol secretion in vitro (Tsonis et al., 1984; Fortune and Quirk, 1988; Saumande, 1991). The failure of gonadotrophins to sustain androgen or oestradiol secretion by follicle cells from domestic species in vitro has hampered studies on the regulation of hormone production by developing preovulatory follicles in these species. Oestradiol secretion by granulosa cells from pregnant mares' serum gonadotrophin (PMSG)-treated immature calves is stimulated by very low doses of FSH (Saumande, 1991). The objective of this study was to test the hypothesis that low doses of gonadotrophins maintain follicular secretion of androgens and oestradiol by cultured theca and granulosa cells, whereas higher doses induce luteinization (i.e., progesterone and oxytocin secretion). Therefore, theca interna and granulosa cells were isolated from bovine preovulatory follicles before the gonadotrophin surge and cultured with var ous doses of LH or FSH.

Effects of gonadotrophin concentration on hormone production by theca interna and granulosa cells from bovine preovulatory follicles