Mechanisms involved in development and regression of ovarian follicles and corpora lutea in the bovine ovary

The objectives of the experiments presented in this dissertation were to evaluate: (1) dose effects of a synthetic progestin (melengestrol acetate; MGA) on secretion of luteinizing hormone (LH) and 17$\beta$-estradiol; (2) pattern of ovarian follicular development and concentrations of steroids and activity of insulin-like growth factor binding proteins (IGFBPs) in follicular fluid of normal and persistent ovarian follicles; and (3) involvement of local regulators in development and regression of ovarian follicles and corpora lutea (CL) in cattle. Synthetic progestins have been used to synchronize the stage of the estrous cycle in beef cattle, however, acceptable fertility at the synchronized estrus has not been achieved. An increase in pulsatile secretion of LH and greater circulating concentrations of 17$\beta$-estradiol were observed when estrous synchronization treatments were compared to midluteal phase of the estrous cycle. Greater doses of MGA than those typically used in estrous synchrony protocols did not suppress LH and 17$\beta$-estradiol to the same extent as progesterone from CL. Greater pulsatile secretion of LH resulting from use of estrous synchrony treatments induces development of persistent ovarian follicles which may compromise fertility in cattle. Persistent ovarian follicles have greater concentrations of 17$\beta$-estradiol and less activity of IGFBPs in follicular fluid than normally growing ovarian follicles. Therefore, greater LH pulse frequency and possibly less activity of IGFBPs in follicular fluid may facilitate the growth of persistent ovarian follicles and reduce fertility in cattle. Local regulators in the ovary may regulate development and regression of ovarian follicles and CL. Fas antigen (cell death inducer) mRNA was present in ovarian follicles and CL. Amounts of Fas antigen mRNA were greater in atretic ovarian follicles and regressing CL compared with healthy ovarian follicles and functional CL from cattle. Research presented herein provides basic information that enhances our understanding of mechanisms underlying ovarian follicular development and subsequent ovulation of a viable oocyte, and the formation of a functional corpus luteum and successful pregnancy in cattle