Each month, the female reproductive system prepares and releases an ovum in a series of events called the ovarian cycle. During this time, an egg matures and enters a fallopian tube, where it is able to fuse with a sperm. If the egg does not fuse with a sperm, the egg degenerates. The ovarian cycle has three stages: the follicular phase, ovulation, and the luteal phase. These stages are regulated by hormones secreted by the endocrine system. While the ovarian cycle occurs, the menstrual (MEN-struhl) cycle prepares the uterus for a possible pregnancy. For most women, the ovarian and men-stral cycles last about 28 days. Figure 51-8 summarizes the stages of the ovarian and menstrual cycles.
An immature egg cell completes its first meiotic division during the follicular (fuh-LIK-yoo-luhr) phase. This phase begins when the hypothalamus secretes a releasing hormone that stimulates the anterior pituitary to secrete follicle-stimulating hormone (FSH). FSH stimulates cell division in a follicle, a layer of cells that surrounds an immature egg. Follicle cells supply nutrients to the egg. They also secrete estrogen, which stimulates mitotic divisions of cells in the lining of the uterus, causing the lining to thicken. The follicular phase lasts approximately 14 days. During this time, the estrogen level in the blood continues to rise until it reaches a peak and the egg moves to the surface of the ovary. The elevated estrogen level acts as a positive feedback mechanism by stimulating the anterior pituitary to secrete luteinizing hormone (LH), which initiates the next stage of the menstrual cycle.
The sharp rise in the LH level that occurs midway through the ovarian cycle causes the follicle to rupture and release its egg. The release of an egg from a ruptured follicle is called ovulation (AHV-yoo-LAY-shuhn). Following ovulation, an egg is swept into a fallopian tube, where it awaits fertilization as it travels through the tube toward the uterus. The egg has enough stored nutrients to survive about 24 hours.
The cells of the ruptured follicle grow larger and fill the cavity of the follicle, forming a new structure called a corpus luteum (KAWR-puhs LOOT-ee-uhm). Thus, this stage of the ovarian cycle is called the luteal (LOOT-ee-uhl) phase. The corpus luteum begins to secrete large amounts of progesterone and estrogen. Progesterone stimulates growth of blood vessels and storage of fluids and nutrients in the lining of the uterus during the menstrual cycle. This stimulation causes the uterine lining to become thicker. In addition, increased levels of estrogen and progesterone acts as a negative feedback mechanism by causing the pituitary gland to stop secreting LH and FSH. The luteal phase lasts about 14 days. During this time, estrogen and progesterone levels in the blood rise, while the FSH and LH levels drop.
If an egg is fertilized, the resulting zygote attaches to the lining of the uterus, where it will develop for the next nine months. A hormone that is produced early in pregnancy stimulates the corpus luteum to continue producing estrogen and progesterone, and the thickened lining of the uterus is maintained. If the egg is not fertilized, the corpus luteum stops producing sex hormones, which marks the end of the ovarian cycle. Without estrogen and progesterone to maintain the thickened uterine lining, the lining begins to slough off. In this stage of the menstrual cycle, called menstruation (MEN-STRAY-shuhn), the lining of the uterus and blood from ruptured blood vessels are discharged through the vagina. Menstruation lasts the first five to seven days of the follicular phase.
Menstruation continues in most women until about age 50. At this time, a woman no longer ovulates. Most of a woman's follicles have either matured and ruptured or degenerated. Without follicles, the ovaries cannot secrete enough estrogen and progesterone to continue the menstrual cycle, and menstruation ceases. This stage is called menopause (MEN-uh-PAWZ).
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