A combination of hormonal changes and enzymatic effects is responsible for the actual release of the secondary oocyte in the middle of the menstrual cycle (i.e., on the 14th day of a 28-day cycle). These factors include:  Increase in the volume and pressure of the follicular fluid  Enzymatic proteolysis of the follicular wall by activated plasminogen Hormonally directed deposition of glycosaminoglycans between the oocyte–cumulus complex and the stratum granulosum Contraction of the smooth muscle fibers in the theca externa layer, triggered by prostaglandins
Just before ovulation, blood flow stops in a small area of the ovarian surface overlying the bulging follicle. This area of the germinal epithelium, known as the macula pellucida or follicular stigma, becomes elevated and then ruptures (Fig. 23.8a). The oocyte, surrounded by the corona radiata and cells of the cumulus oophorus, is released from the ruptured follicle. At the time of ovulation, the fimbriae of the uterine tube become closely apposed to the surface of the ovary, and the cumulus mass containing oocyte is then gently swept by the fimbriae into the abdominal ostium of the uterine tube. The cumulus mass firmly adheres to the fimbriae and is actively transported by the ciliated cells lining the uterine tube, preventing its passage into the peritoneal cavity. Recently, nonsurgical ultrasound technology is being used to monitor ovarian follicle development. Transvaginal ultrasound examination can provide detailed information about
FIGURE 23.8 • Endoscopic and ultrasound examination of the ovary. a. This photograph shows a view of the human ovary during the endoscopic oocyte harvest surgery. The ovary is in stage just before ovulation. Note the area of bulging follicle with follicular stigma that is clearly visible. The germinal epithelium covering the tunica albuginea is ruptured in the area of imminent ovulation.
b. Development of nonsurgical techniques such as ultrasound imaging helps in monitoring of follicular growth and is useful as a method for determining the time for harvesting preovulatory oocytes. (Courtesy Dr. Charles C. Coddington, III. Mayo Clinic.) the number and size of developing follicles (Fig 23.8b).
After ovulation, the secondary oocyte remains viable for approximately 24 hours. If fertilization does not occur during this period, the secondary oocyte degenerates as it passes through the uterine tube.
Oocytes that fail to enter the uterine tube usually degenerate in the peritoneal cavity. Occasionally, however, one may be fertilized and implant in the peritoneal cavity on the surface of the ovary or intestine or inside the rectouterine (Douglas) pouch. Such ectopic implantations usually do not develop beyond early fetal stages but may have to be removed surgically. Ectopic pregnancy that develops at any site other than the endometrium of the uterine cavity continues to be the most common cause of death in the first half of pregnancy.
Normally, only one follicle completes maturation in each cycle and ruptures to release its secondary oocyte. Rarely, oocytes are released from other follicles that have reached full maturity during the same cycle, leading to the possibility of multiple zygotes. Drugs such as clomiphene citrate (Serophene) or human menopausal gonadotropins that stimulate ovarian activity greatly increase the possibility of multiple births by causing simultaneous maturation of several follicles.
The primary oocyte is arrested for 12 to 50 years in the diplotene stage of prophase of the first meiotic division.