Ovulation is a hormone-mediated process resulting in the release of the secondary oocyte

Ovulation is the process by which a secondary oocyte is released from the Graafian follicle. The follicle destined to ovulate in any menstrual cycle is recruited from a cohort of several primary follicles in the first few days of the cycle. During ovulation, the oocyte traverses the entire follicular wall, including the germinal epithelium.

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 stigma, becomes elevated and then ruptures. The oocyte, surrounded by the corona radiata and cells of the cumulus oophorus, is forcefully expelled from the ruptured follicle (Fig. 22.8). The oocyte is then transported into the abdominal ostium of the uterine tube. At the time of ovulation, the fimbriae of the uterine tube be-

0 0

Post a comment