Medical Animation Encyclopedia

All of the immature egg cells (oocytes) a woman will ever produce are stored in the ovaries by the time she is born. The average age that girls begin to menstruate is 12 years old. Each menstrual cycle occurs approximately every 28 days. During each cycle, hormonal messages from the brain cause the ovaries to develop a single mature egg cell for potential fertilization, even as other hormones instruct the uterine lining to thicken in preparation for nourishing the fertilized egg cell.

The cycle starts when a follicle grows within one of the ovaries. A follicle is composed of the developing egg cell and the support cells that surround and nourish it. On day 1 of the cycle, a small structure in the brain, the pituitary gland, releases two hormones, FSH and LH, both of which cause the follicle to begin growing.

Over the next 13 days, the growing follicle releases estrogen, a hormone that prepares the lining of the uterus to receive a fertilized egg cell. Meanwhile, the estrogen in the bloodstream causes the brain to release a surge of LH. In response to the LH surge, the follicle enlarges rapidly. On day 14, it ruptures and releases the egg cell in a process known as ovulation.

The ruptured follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for a fertilized egg cell. The egg cell is swept into the fallopian tube entrance by its waving structures called fimbriae.

Once the egg cell is within the fallopian tube, it will either be fertilized by a sperm cell, or fertilization will fail to take place. If the egg cell is not fertilized within 24 hours after its release from the ovary, it will stop developing and will dissolve before reaching the uterus. The absence of a fertilized egg cell gradually causes a woman?s body to stop releasing the hormones that would otherwise prepare the uterus for the developing egg cell. In response, the uterus sheds its lining on days 24 through 28 during menstruation.

If a sperm does fertilize the egg cell, tiny hair-like cells called cilia will transport it towards the uterus. The fertilized egg now called a blastocyst, lodges in the uterine wall in a process called implantation to receive nourishment from the uterine lining. The remaining cells of the ruptured follicle in the ovary produce progesterone so that the uterine lining will stay rich in blood vessels, and the fertilized egg cell will survive.