Interrelationships between ovarian follicles grown in culture and possible mediators

Dominance or cooperation between ovarian follicles can determine the number of ovulations and fecundity, but interrelationships between follicles in mono- and poly-ovulatory species and their mechanisms are poorly understood. The goals of this work were to determine the existence and compare the character of mutual influence of cultured ovarian follicles from a mono-ovulatory species (cow) with established follicular dominance with those from a poly-ovulatory species (pig), in which interrelationship between follicles remain unknown, and to examine the role of ovarian cell proliferation, the insulin-like growth factor I (IGF-I)- oxytocin (OT) system, and steroid hormones in mediating interrelationships among ovarian follicles. Bovine and porcine ovarian follicles were isolated and cultured alone and in pairs, and the percentage of growing follicles was calculated. Porcine follicles were cultured alone and in pairs after addition of exogenous OT and IGF-I (100ngmL^-1) or inactivation of endogenous OT and IGF-I by antisera against these hormones (1%). Proliferation of porcine follicular cells was assessed by SDS PAGE-Western immunoblotting, the release of IGF-I, progesterone, androstenedione and estradiol by cultured porcine ovarian follicles was analyzed by RIA/EIA. Overall, our observations suggest (1) competition/dominance (mutual suppression of growth) in bovine ovarian follicles, (2) cooperation (mutual support of growth) in porcine ovarian follicles, (3) that this mutual growth of porcine ovarian follicles was caused by the promotion of cell proliferation, (4) that this mechanism was probably not involved in bovine follicular dominance, (5) that communication between both porcine and bovine follicles affects their secretory activity, and (6) that both follicular dominance in cows and cooperation of follicles in pigs can be mediated by either down- or up-regulation of the IGF-I-OT system, which in turn affects follicular steroidogenesis and promotes follicular cell proliferation and follicular growth.