Ovulation

Just prior to ovulation, most of the growing follicles have died off in a process known as atresia. This involves programmed cell death of all of the cells of the follicle, including the egg cell. No one knows exactly how this happens; however, elevated levels of FSH will prevent this programmed cell death. By the 13th day of the menstrual cycle, about 5 to 7 cells are competing for dominance, although only one cell reaches complete maturity by the 14th day of the menstrual cycle.

As the FSH level rises, antral follicles secrete both estrogen and inhibin, which exert a negative feedback loop reducing the number of FSH receptors on each cell. The cells with the least number of FSH receptors will cease growing and will die off. Only the dominant follicle with the most number of FSH receptors will continue to grow and will become the preovulatory follicle.

Interestingly, the mature follicle is not a spherical structure but becomes oval in shape. The oocyte places itself at an eccentric position on one side of the follicle because of greater cellular proliferation on just one side of the follicle. Other side of the follicle is fluid-filled and is called the follicular antrum. When a follicle reaches this size and shape, it is known as a vesicular follicle. The egg cell is protected by a cellular mound called the cumulus oophorus, which sticks out into the antrum. Final follicular development depends on LH as well as FSH.

Ovulation

The end of the follicular phase is marked by high estrogen levels. This results in a surge of both the LH and FSH level that lasts up to 36 hours and results in the eventual rupture of the primary ovarian follicle. The egg cell is released and enters the oviduct of the fallopian tube.

The process proceeds like this: proteolytic enzymes are secreted by the primary follicle that open up an area at one end of the follicle, causing the egg cell to be extruded from the follicle and into the oviduct. This is all under the influence of LH. The hole in the follicle is called the stigma. Through the stigma, the egg cell enters the peritoneum and is pushed along by cilia so that it preferentially enters the oviduct and begins its journey toward the uterus.

At this point in time, the egg cell has reached meiosis I and is actually two cells. The larger cell is called the secondary egg cell; it contains all the cytoplasmic material necessary for the egg cell survival. The smaller cell is called a polar body and contains very little cytoplasm. Following this, meiosis II occurs, but the process is arrested at metaphase until the time of fertilization. If no fertilization happens, the egg cell will degenerate at approximately 24-hour’s after ovulation. More than one egg cell is released in a single ovulatory cycle about 1 to 2% of the time. If this happens and both eggs are fertilized, this results in the formation of fraternal twins.

The ovulatory phase of the female reproductive cycle is the shortest phase. It occurs between the 12th and 16th day of the cycle and lasts only 1 to 2 days. Most women can tell that they have