Mrer the appearance of LH receptors, the preovulatory 10 to 12 hours before ovulation and stimulatcs resumption of granulosa cells begin to secrete small quantities of progesterone. meiosis in the ovum and release of the irst polar body. Studies The preovulatory progesterone secrecion, although somewhat suggest that in response to LH, greater progesterone and proslimited, is believed to exert positive feedback on the estrogentaglandin production by the cumulus cells, as well as GDF9 primed pituitary to either calise or augment LH release. In addiand BMP-15 by the oocyte, activates expression of genes critition, during the late follicular phase, LH stimulates thecal cell cal to formarion of a hyaluronan-rich extracellular matrix by production of androgens, particularly androstenedione, which the cumulus complex (Richards, 2007). As seen in are then transferred to the adjacent follicles where they are aroduring synthesis of this matrix, cumulus cells lose contact with matized to esnadiol (see Fig. 5-2). During the early follicular one another and move outward from the oocyte along the hyalphase, granulosa cells also produce in hi bin 3, which can feed uronan polymer-this process is called expansion. This results back on the pituitary to inhibit FSH release (Groome, 1996). in a 20-fold augmentation of the cumulus complex volume and As the dominant foHicle begins to grow, estradiol and inhibin coincides with an LH-induced remodeling of the ovarian extraproduction rises and results in a decline of follicular-phase cellular matrix. hese allow release of the mature oocyte and FSH. This drop in FSH levels is responsible for the failure of its surrounding cumulus cells through the surface epirhelium. other follicles to reach preovulatory status-the Graaian folActivation of proteases likely plays a pivotal role in weakening licle stage-during any one cycle. Thus, 95 percent of plasma the follicular basement membrane and ovulation (Curry, 2006; estradiol produced at this time is secreted by the dominant Ny, 2002). follicle-thc one destined to ovulate. Concurrently, the con tralateral ovary is relatively inactive.
Following ovulation, rhe corpus luteum develops from the remains of the Graaian follicle in a process referred to as The onset of rhe gonadotropin surge resulting from increasluteiniation. The basement membrane separating the granuing estrogen secretion by preovulatory follicles is a relatively losa-lutein and theca-lutein cells breaks down, and by day 2
FIGURE 5-3 An ovulated cumulus-oocyte complex. An oocyte is at the center of the complex. Cumulus cells are widely separated from each other by the hyaluronan-rich extracellular matrix. (Used with permission from Dr. Kevin J. Doody.) postovulation, blood vessels and capillaries invade the granulosa cell layer. The rapid neo�ascularization of the once-avascular granulosa may be due to angiogenic factors that include vas by theca-lutein and granulosa-lutein cells in response to LH (Albrecht, 2003; Fraser, 2001). During luteinization, these cells undergo hypertrophy and increase their capacity to synthesize hormones.
LH is the primary luteotropic factor responsible for corpus luteum maintenance (Vande Wiele, 1970). Indeed, LH injec tions can extend the corpus luteum life span in normal women by 2 weeks (Segalof, 1951).