A human female is born with a million or so immature eggs, or oocytes, within each of her ovaries. These oocytes are arrested at an early stage of meiosis. Most of these oocytes will die before they ever have a chance to mature, and by the time a woman reaches sexual maturity, each ovary contains about 200,000 oocytes. Every month, a dozen of these oocytes re-initiate their development, but usually only one of them makes it to full maturity.
The cycle in which an oocyte matures, erupts from the ovary, and then travels down the oviduct to the uterus is called the ovarian cycle. The cycle varies in length, lasting on average 28 days. The ovarian cycle is tightly coordinated with the uterine cycle, in which the lining of the uterus grows and prepares for an embryo to implant. If, by the end of the cycle, the oocyte has not been fertilized and an embryo has not implanted, then the lining of the uterus sloughs off in a process called menses, or menstruation.
An ovarian cycle lasts about 28 days, beginning at the first day of menstruation, or menses. During the first seven days of the cycle, six to twelve primary oocytes begin to mature. As the follicles develop, the follicle cells communicate with oocytes and pass them nutrients through pores called gap junctions.
Each oocyte grows larger and the surrounding follicle cells divide, proliferating to produce thousands of follicle cells in a single follicle.
By day 7, all but one of the developing follicles begins to degenerate. The remaining follicle continues to develop, and its follicle cells continue to pump it with nourishment and also supply it with proteins and informational molecules needed for early stages of development.
The maturing primary oocyte completes meiosis I and divides into two haploid cells. Each of these cells receives half the chromosomes. However, one cell, called a polar body, receives very little cytoplasm. The other, now a secondary oocyte, enters meiosis II and arrests there until fertilization.
At day 14, ovulation occurs, and the secondary oocyte erupts from the ovary. The oviduct contains microscopic cilia that beat and draw in the released oocytes. This immature egg enters an oviduct, where it may become fertilized by a sperm cell and complete meiosis.
The follicle cells that are left behind develop into a small mass of endocrine tissue called the corpus luteum. The corpus luteum remains in the ovary for two weeks, secreting the hormones estrogen and progesterone. At the end of the ovarian cycle, if the woman is not pregnant, the corpus luteum disintegrates.
Each month, the ovarian cycle is tightly coordinated with the uterine cycle. In the uterine cycle, the lining of the uterus builds up and then sloughs off. The cycle begins with the sloughing of the uterine lining. This is the first day of menses, also called menstruation.
After menstruation, the uterine lining starts to grow again and to prepare for implantation of an embryo. During this phase of the uterine cycle, up until ovulation, the uterine lining proliferates. Capillary beds supply this tissue with nutrients.
Just before menstruation occurs again, the capillary beds degenerate and no longer deliver the surrounding tissue with nutrients. During menstruation, this tissue dies and sloughs off through the vagina to the outside of the body.
FSH and LH stimulate ovarian follicles to grow. As the follicles grow, they begin to secrete estrogen. During this phase of the cycle, the increasing levels of estrogen feed back on the pituitary to inhibit the release of additional FSH and LH. During the next week, the levels of FSH and LH drop.
Beginning around day 12, the increasing levels of estrogen suddenly have the opposite effect on the pituitary gland. Instead of exerting a negative feedback on the pituitary, these hormones now exert a positive feedback, stimulating the pituitary to release FSH and large amounts of LH.
LH reaches a peak at day 14 of the ovarian cycle. This LH surge triggers the mature follicle to rupture and release the egg—the process of ovulation. LH then triggers the remaining follicle cells to differentiate into the corpus luteum, which secretes estrogen and progesterone.
The corpus luteum remains in the ovary, secreting estrogen and progesterone, for the last two weeks of the cycle. At this point in the cycle, these hormones again inhibit the release of FSH and LH. A decline in FSH and LH restricts follicles from beginning to develop during the second half of the cycle.
LH (or a hormone produced by an implanted embryo) is required to maintain the life of the corpus luteum. At the end of the cycle, if an embryo is not implanted, the corpus luteum degenerates. When the corpus luteum degenerates, it no longer releases estrogen and progesterone.
About a week into the ovarian cycle, the developing follicle increases its secretion of estrogen, and estrogen levels in the body begin to rise. This hormone triggers the cells of the uterine lining to proliferate, and the lining becomes thicker.
Just before ovulation, the level of estrogen in the body has reached its peak. Afterward, the follicle cells remaining in the ovary develop into the corpus luteum, a structure that releases estrogen and progesterone. The hormones maintain the uterine lining at a peak thickness and preparedness for embryo implantation.
At the end of the cycle, if the egg is not fertilized or has not implanted, the corpus luteum breaks down and stops releasing estrogen and progesterone. Without these hormones, the uterine lining also breaks down, initiating menses.
The chart on the left summarizes the roles of hormones in controlling the ovarian and uterine cycles. In addition to luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and progesterone, there is yet another hormone that plays an important role in these cycles. Called gonadotropin-releasing hormone (GnRH), this hormone is released from the hypothalamus. GnRH triggers the anterior pituitary to release LH and FSH.
Most of these hormones have clear effects:
The effect of estrogen and progesterone on the brain is not as straightforward, however. During the few days prior to ovulation, these ovarian hormones have a stimulatory effect. The rest of the time, these hormones inhibit the hypothalamus and anterior pituitary from releasing GnRH, LH, and FSH.
A pregnancy puts the 28-day cycle on hold. When a multicelled embryo, called a blastocyst, implants in the uterine lining, a new hormone controls the cycle. This one, called human chorionic gonadotropin (hCG), is secreted by a layer of cells surrounding the blastocyst. hCG is similar in structure to LH and, like LH, it maintains the function of the corpus luteum in the ovary. The corpus luteum continues to release estrogen and progesterone, which, in turn, continue to inhibit the brain from releasing GnRH, LH, and FSH. Without LH and FSH, the pregnant woman will not ovulate.