The reproductive organs of the female include the vagina, uterus, fallopian tubes and ovaries. The menstrual cycle is a coordinated process involving hormones produced by the brain, specifically the hypothalamus and the pituitary, the ovaries, and the uterus. Women are born with more than one to two million eggs that are stored in their ovaries. These eggs are depleted over the woman’s reproductive years. In order for an egg to grow and mature, the hypothalamus produces a hormone called gonadotropin-releasing hormone (GnRH).
This hormone stimulates the release of two other hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), from the pituitary. It is these hormones that tell the woman’s ovary to grow a follicle. The follicle is a fluid filled structure in the ovary that houses an egg (oocyte). The cells around the egg produce estrogen.
Estrogen causes the uterine lining to thicken in order to prepare for implantation and it regulates the release of FSH and LH. Only one follicle, each housing an egg, is chosen from a group that are available that month and all the other follicles/eggs that are not chosen die off. When the follicle and egg mature and the follicle is ready to release the egg, the high estrogen levels cause the pituitary to release LH, producing a surge of this hormone that tells the follicle to release the egg. This process is known as ovulation.