Studies of isolated cell types from sheep follicles revealed several functional changes which occur during follicular maturation. Cyclic AMP production by granulosa cells from the smallest follicles studied (1-3 mm diameter) was stimulated by FSH but not by hCG, suggesting functional FSH receptors at this early stage of differentiation. Medium-sized follicles (4-6 mm) responded to both FSH and hCG. Granulosa cells were unable to synthesize androgens, but readily converted exogenous testosterone to oestradio1-1713. This conversion occurred to a limited extent in the cells from the smallest follicles, but was much greater in medium and large (> 6 mm) follicles. Oestradiol production by theca preparations from small follicles was barely detectable, but increased markedly with increasing follicle size. Androgen (androstenedione and testosterone) production by theca preparations was stimulated by hCG. This stimulation was short-lived, and levels declined to below control value's after 6 h of culture. This decline could not be prevented by addition of cyclic AMP. The presence of granulosa cells with thecal preparations (i.e. follicle wall tissue) enhanced production of androgen by the theca, the effect being more marked for testosterone than for androstenedione. In-vivo studies in which granulosa cells and follicular fluid were removed during the preovulatory period suggested that granulosa cells and/or follicular fluid contributed to the oestradiol secreted into the ovarian vein during this period, but did not exclude a significant contribution by the theca as well.

This study was undertaken to examine the possibility that the prolonged anovulatory period frequently experienced by the post-partum cow is due to a disruption of function at the ovarian level promoted by the high, suckling-induced, blood prolactin concentrations. Fifteen cows, less than 35 days post partum, were allocated to three groups (1, 3 and 5) and given no hormonal treatment, prostaglandin plus pregnant mare serum gonadotrophin (PMSG) treatment or injected with 2-bromo-ex-ergocryptine to reduce circulating prolactin levels. Ten synchronized cyclic cows were allocated to two groups (2 and 4) and given prostaglandin or prostaglandin plus PMSG treatment. All cows were ovariectomized 1 or 2 days after treatment and Graafian follicles less than 9 mm in diameter were selected after dissection from the ovaries. The follicles were cultured for 18 h with or without prolactin (1 fLgfml) and steroid accumulation in the culture medium estimated. The follicles were then separated into theca and granulosa which were incubated for 40 min with LH (1 fLgfml) or FSH (5 fLgfml). Cyclic AMP concentrations were estimated as an indication of tissue responsiveness to gonadotrophins.The secretion of oestradiol-17 p, progesterone, testosterone or androstenedione during 18 h culture did not differ between follicles isolated from post-partum or cyclic cows. The presence of prolactin in the culture medium had no overall effect on steroid secretion although some specific effects within each group were noticed. Incubation with LH increased cyclic AMP levels in the theca but the granulosa did not respond. Likewise FSH increased cyclic AMP levels in granulosa preparations but not in theca. There were no differences in response between post-partum and cyclic cows, but exposure of the follicles to prolactin in vitro did significantly reduce the LH-induced increase in cyclic AMP levels in isolated theca.We have concluded that endogenous prolactin may modify but does not inhibit the resumption of ovarian function following parturition in the beef cow.