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Physiology of the Estrous Cycle¹

Cornell University, Ithaca, NY and Merck Sharp & Dohme Research Laboratories, Rahway, NJ

Abstract

Current and emerging concepts of follicle growth and ovulation, hypothalamic pituitary ovarian 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 cell two gonado tropin model for control of follicular steroido genesis has emerged and the idea has been advanced that follicular androgens are synthesized via the ⁵ pathway, with high concentrations of intrafollicular 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 of cycle and reproductive status. Data cited are consistent with the view that consecutive exposures to pulsatile releases of GnRH, under estrogen dominance, progressively increase the magnitude of pulses of LH release a seriatim, thereby creating the preovulatory LH surge. Efficient methods for regulating the estrous cycles of cows and sheep have been developed so that groups of animals may be inseminated at preset times without checking for estrus. The most efficient of these methods are based on combining single PGF₂ injections with short term progesterone treatments. The most promising method for controlling estrus and ovulation in swine is by the use of the orally active synthetic progestational compound, L, allyl trenbolone. It may be possible to develop even simpler methods for synchronization of cycles in cattle, based on single injections of long acting luteotropic agents to prolong the functional life of the corpora lutea, followed six days later by luteolytic doses of PGF₂.

Key Words: Estrous Cycle • Follicle Growth • Ovulation • Corpus Luteum • Hypothalamic Pituitary-Ovarian Interrelationships

¹ The authors gratefully acknowledge the assistance of R. A. Milvae and J. J. Ireland in reviewing the literature and preparing the manuscript, and thank Joanne Fortune for permission to use figure 1.

² Dept. of Physiol., NY State College of Vet. Med., Cornell Univ.

³ Merck Sharp & Dohme Research Laboratories, Rahway, NJ.