May 06 2008
Adolescent Health-education: What goes on during the menstrual cycle? continue…
Act II: The egg is released (ovulation). As in Act I, this part of the story also begins in the hypothalamus. In response to rising levels of estrogen, the hypothalamus signals the pituitary to release a brief but intense surge of luteinizing hormone (or LH) into the bloodstream. This hormone sets off a chain reaction in the ovary. The dominant follicle enlarges, its outer wall becomes thin, and finally it ruptures, releasing egg and fluid. This mini-eruption called ovulation takes only a few minutes and occurs approximately thirty-eight hours after the peak of the LH surge. Sometimes a tiny amount of blood oozes from the ovary as well. This may irritate the lining of the abdomen, producing a discomfort known as mittelschmerz (German for “middle pain,” because it occurs about halfway through the cycle).
Act III: The voyage of the egg and the preparation of the uterus (the luteal phase).
The egg is not left to its own devices once it is set free from the ovary. At the end of each fallopian tube are structures called fimbria (Latin for “fingers”), whose delicate tentacles move over the area of the ovary. As soon as ovulation takes place, the fimbria gently escort the egg into the tube, where it begins a journey toward the uterus. The cells that line the fallopian tube have microscopic hairlike projections called cilia, which move in a synchronized pattern that sets up a one- way current through the tube. If sperm are present in the outer portion of the tube, and one of them is successful in penetrating the egg, fertilization takes place and a new life begins. The fertilized egg will incubate in the tube for about three days before arriving at its destination, the cavity of the uterus, where it floats for about three more days before implanting. On about the seventh day it “rests,” so to speak, implanting in the cavity of the uterus. If the egg is not fertilized, it will live only twelve to twenty-four hours and then disintegrate or pass through the tube and uterus into the vagina. (Since sperm live for forty-eight to seventy-two hours, there are three or four days in each cycle during which intercourse could lead to conception.)
Meanwhile, much activity takes place in the ovary after ovulation. The newly vacated follicle has another job to do: preparing the uterus to accept and nourish a fertilized egg should one arrive. The follicle turns into a gland called the corpus luteum (literally, “yellow body” because cells lining the inside of the follicle develop a yellowish color), which secretes estrogen and, more important, progesterone. This hormone, which dominates this luteal phase of the cycle, promotes growth and maturation of the uterine lining. This layer of tissue eventually doubles in thickness and becomes stocked with nutrients. Progesterone not only prepares the uterine “nursery” for a new arrival but also relaxes the muscles of the uterus, decreasing the chance of contractions that might accidentally expel its guest. Progesterone also temporarily stops the preparation of any other eggs within the ovaries.
If a fertilized egg successfully implants and continues its growth within the uterus, it secretes a hormone called human chorionic gonadotrophin (or HCG), which sends an important message to the corpus luteum: “Keep the hormones flowing!” The corpus luteum obliges and for nine or ten weeks continues to provide the hormone support that allows the uterus to nourish the baby growing inside. After ten weeks, the placenta (the complex organ that connects the baby to the inner lining of the uterus) takes over the job of manufacturing progesterone, and the corpus luteum retires from active duty.
If there is no fertilization, no pregnancy, and no HCG, the corpus luteum degenerates. Progesterone and estrogen levels fall, resulting in spasm of the blood vessels that supply the lining of the uterus. Deprived of the nutrients it needs to survive, the lining dies and passes from the uterus, along with blood and mucus, in what is called the menstrual flow (also referred to as the period or menses).
While the menstrual period might seem to be the end of the story, the first day of flow is actually counted as day 1 of a woman’s reproductive cycle. For while the flow is taking place, the three-act play is starting over again as a new set of follicles begins to ripen in the ovaries. This “circle of life” will thus normally continue month after month throughout the reproductive years until menopause unless interrupted by pregnancy or a medical condition that interferes with this cycle.
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