Ovarian follicular and luteal physiology

Abstract

Follicular maturation and development is a complex process of interrelated intra- and extraovarian events that ultimately lead to ovulation of a mature oocyte and transformation of the ruptured follicle into a corpus luteum. The primordial follicle consists of an immature oocyte arrested in the dictyate stage of meiosis, surrounded by a single layer of relatively undifferentiated granulosa cells. The oocyte remains in the immature state because of many factors, one of which is the oocyte maturation inhibitor (OMI) secreted by granulosa cells. The oocyte subsequently increases in size, and as the antrum forms it becomes surrounded by cumulus cells. The cumulus cells may be intimately involved in the action of O,I to arrest the oocyte in the immature state within the follicle, as well as the resumption of meiosis during the LH surge. The compartments of the follicle that change most dramatically during follicular maturation are the cells lining the follicle--the granulosa and thecal cells. Under the influence of estrogen and FSH, the granulosa cells proliferate and also acquire FSH receptors. At this time, the thecal compartment differentiates and surrounds the granulosa cells, but remains separated from them by a basement membrane. Steroid secretion by the antral follicle involves the interplay of androgens, estrogens, and progestins. Both the granulosa and thecal cell compartments contribute to follicular fluid and serum levels of steroids; the interaction of both cell types may be necessary for estrogen and progesterone secretion in some species. As a consequence of the presence of an elevated number of FSH receptors, the granulosa cells of the small antral follicle are able to respond to FSH in many ways, including increased cyclic AMP accumulation, activation of the aromatase system, and induction of LH receptors, which permits the granulosa cells to later respond to LH. The mechanism by which thecal cells acquire their LH receptors is presently unknown. The granulosa cells of the follicle may indirectly control their own maturation and the number of follicles maturing through the secretion of follicular inhibin, which decreases the pituitary output of FSH. Even though the granulosa cells have acquired large numbers of LH receptors, they are prevented from luteinizing prematurely by factors in follicular fluid, including estrogen and a luteinizing inhibitor (LI). As serum LH levels increase during the preovulatory LH surge, a number of events occur: resumption of oocyte meiosis, transformation of the steroid enzyme complex from estrogen to progesterone secretion, follicular rupture, and formation of the corpus luteum. Granulosa cells form the bulk of the corpus luteum, which secretes elevated amounts of progesterone for a fixed time period depending on the species. Before ovulation the preovulatory follicle must be exposed to and respond to adequate LH and FSH levels in order for the eventual corpus luteum to secrete elevated amounts of progesterone for its normal lifespan...