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A study into the regulation of ovarian follicular dynamics by progestins in the bovine Taylor, Christopher C.


Ovarian follicles grow and regress in a wave-like pattern during the luteal phase of the estrous cycle in the bovine. Each wave of follicular growth gives rise to a dominant follicle. What causes non ovulatory dominant follicles to stop growing, become atretic, and regress is not understood. The synthetic progestin, norgestomet, has been shown to induce the maintenance of dominant follicles in the absence of a corpus luteum in the bovine. This provides an excellent model for studying the regulation of dominant follicle growth, maintenance and regression. A series of in vivo experiments using norgestomet and progesterone (P₄) were undertaken to determine how progestins modulate dominant follicle growth, maintenance, and regression. In addition, in vitro experiments were conducted to determine the effects of P₄ on steroidogenesis at the granulosa cell level. The results indicate that in the absence of a corpus luteum norgestomet induces the maintenance of dominant follicles and new follicular growth is arrested. This is the result of persistent high-frequency luteinizing hormone (LH) pulses from the pituitary gland. When the circulating concentration of progestin is increased, either with norgestomet or P₄, circulating LH decreases, the maintained dominant follicle regresses and new follicular growth is restored. Administration of exogenous P₄ early in the estrous cycle, a period normally characterized by low plasma P₄ concentrations and high-frequency LH pulses, induces premature regression of the first wave dominant follicle. Thus, high circulating P₄ and low circulating LII is permissive to normal follicle turnover. The in vitro results suggest that P₄ may enhance bovine granulosa cell estradiol-17ß and P₄ production. Progesterone had no suppressive effects on granulose cell steroidogenesis, even at very high concentrations (10.6 M). Taken as a whole, the results suggest that progestins modulate dominant follicle growth, maintenance and regression by regulating pituitary LH release. High-frequency LH pulses induce the maintenance of dominant follicles. A normal pattern of follicular growth and regression is restored with a decrease in LH pulse frequency and decreased circulating LH. These results may have important implications for improving estrus synchronization and superovulatory protocols and lead to a better understanding of some reproductive disorders in cattle such as cystic ovary condition. The norgestomet induced maintained dominant follicle model may also provide a valuable tool for conducting research into the establishment, maintenance and loss of follicular dominance.

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