Physiology of the Estrous Cycle 1

Current and emerging concepts of follicle growth and ovulation, hypothalamic-pituitaryovarian interactions, corpus luteum function and estrous cycle regulation in cattle, sheep and swine are reviewed. Follicles grow, regress and are replaced by other large follicles continuously throughout the cycle; intra-ovarian factors appear to play important roles in this process. Estradiol concentrations rise in preovulatory follicles until the preovulatory LH surge and then decline sharply. A two ceil-two gonadotropin model for control of follicular steroidogenesis has emerged and the idea has been advanced that follicular androgens are synthesized via the A s pathway, with high concentrations of intrafoUicular estradiol suppressing progesterone production prior to the LH surge. The production of nonsteroidal inhibitors of FSH secretion and hypothalamic hormones by ovarian tissues has been suggested. Our present concepts are probably only partial explanations of the events that occur during corpus luteum regression. Evidence is summarized to suggest that prostacyclin and PGF2 have luteotropic roles during the growth phase of the corpora lutea of cows and ewes, respectively. Several recent studies suggest that oxytocin may have a physiological function in luteal regression in the cow and the ewe. Phospholipid methylation appears to be an important regulatory step in the mechanism by which LH stimulates luteal cell adenylate cyclase and subsequent progesterone production. Gonadotropins are secreted in distinct episodic patterns that vary with stage