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Children Onset of Menstrual Cycle

In normal circumstances, each month a woman's body performs a three-actentitled Preparing for a Baby. What you are about to read is a summary of,ssential characters and plot. (As with many other aspects of human physiol-there are thousands of other details that will not he spelled out here and-rands more yet to be discovered. The design of this process is indeed-:te.)

• The hypothalamus: a multifaceted structure at the base of the brain thatregulates basic bodily functions such as temperature and appetite. Tt alsoserves as the prime mover in the reproductive cycle. • The pituitary: a small, punching-bag-shaped structure that ap-:dangle from the brain directly below the hypothalamus. It has -:called the "master gland" because it gives orders to many otherBut it also takes important cues from the hypothalamus. oThe ovaries: a matched pair of organs in the female pelvis tha:

critical functions—releasing one or more eggs (or ova) each r.-. -secreting the hormones estrogen and progesterone. At birth :1-contain about 2 million eggs, a woman's lifetime supply.

hood, the vast majority of these gradually disappear, and by-reaches puberty only about 300,000 will be left. During her re77years, she will release between three hundred and five hundre.: _ _rest will die and disappear.

•The uterus: a pear-shaped organ consisting primarily of musc::containing a cavity where a baby grows during pregnancy. This :lined with delicate tissue called endometrium, which changes r, --ably in response to estrogen and progesterone produced by the _The uterus, also called the womb, is located at the top of the va; - _positioned in the middle of the pelvis between the bladder and - - -rectum.

oThe fallopian tubes: a pair of tubes, about four to five inches I -attached to the upper corners of the uterus and extend toward .ovary. Their job is to serve as a meeting place for egg and spec:---.then to transport a fertilized egg to the uterus.

Act I: Preparing an egg for launch (the follicular phase). The hypothalar:- a reproductive cycle by sending a message called gonadotrophin relea _-mone to the pituitary gland just a short distance away. The messagefeet, "Send out the hormone that prepares an egg to be released by thepituitary responds by secreting follicle stimulating hormone (or FS7bloodstream. Each egg within an ovary is covered with a thin sheet c:..the term follicle (which literally means "little bag") refers to the ent:7of egg and cells together. Under the influence of FSH, eight or ten to grow and "ripen." Usually only one becomes dominant and progre _maturity.

This follicular phase of the cycle lasts about two weeks, during - -dominant follicle fills with fluid and enlarges to about three-quarters -The egg contained within it will soon be released from the ovary. AItime, this follicle secretes increasing amounts of estrogen, which (an:1:lates the lining of the uterus to proliferate and thicken. This is the

•7- reparation of the uterus for the arrival of a fertilized egg. egg is released (ovulation). As in Act I, this part of the story also be-- .-.vpothalamus. In response to rising levels of estrogen, the hypotha--..lis the pituitary to release a brief but intense surge of luteinizingor LH) into the bloodstream. This hormone sets off a chain reaction The dominant follicle enlarges, its outer wall becomes thin, and finally releasing egg and fluid. This mini-eruption called ovulation takes minutes and occurs approximately thirty-eight hours after the peak surge. Sometimes a tiny amount of blood oozes from the ovary as.; may irritate the lining of the abdomen, producing a discomfort_ mittelschmerz (German for "middle pain," because it occurs about--.-ough the cycle).

-e voyage of the egg and the preparation of the uterus (the luteal phase).not left to its own devices once it is set free from the ovary. At the end •_Thpian tube are structures called fimbria (Latin for "fingers"), whose :ntacies move over the area of the ovary. As soon as ovulation takes r-imbria gently escort the egg into the tube, where it begins a journey uterus. The cells that line the fallopian tube have microscopic hairlike called cilia, which move in a synchronized pattern that sets up a one-- 7 mt through the tube. If sperm are present in the outer portion of the one of them is successful in penetrating the egg, fertilization takes place life begins. The fertilized egg will incubate in the tube for about three re arriving at its destination, the cavity of the uterus, where it floats for- :ee more days before implanting. On about the seventh day it "rests," so... implanting in the cavity of the uterus. If the egg is not fertilized, it will twelve to twenty-four hours and then disintegrate or pass through the.7. uterus into the vagina. (Since sperm live for forty-eight to seventy-two-Here are three or four days in each cycle during which intercourse couldconception.)

r=while, much activity takes place in the ovary after ovulation. The newly_ follicle has another job to do: preparing the uterus to accept and nour-.7:ilized egg should one arrive. The follicle turns into a gland called the— _s iuteum (literally, "yellow body" because cells lining the inside of the fol-.1,.-elop a yellowish color), which secretes estrogen and, more important,terone. This hormone, which dominates this luteal phase of the cycle, pro-growth and maturation of the uterine lining. This layer of tissue eventually doubles in thickness and becomes stocked with nutrients _-

only prepares the uterine "nursery" for a new arrival but also reiaxe-:-of the uterus, decreasing the chance of contractions that might ac2pel its guest. Progesterone also temporarily stops the preparation ,.--eggs within the ovaries.

If a fertilized egg successfully implants and continues its growthuterus, it secretes a hormone called human chorionic gonadotrophin ccwhich sendssends an important message to the corpus luteum: "Keep the :71=flowing!" The corpus luteum obliges and for nine or ten weeks contir.-_:e,, xvide the hormone support that allows the uterus to nourish the baby g _1.1E:side. After ten weeks, the placenta (the complex organ that connectsthe inner lining of the uterus) Lakes over the job of manufacturingand the corpus luteum retires from active duty.

If there is no fertilization, no pregnancy, and nothe corF_ generates. Progesterone and estrogen levels fall, resulting in spasrr. I :-vessels that supply the lining of the uterus. Deprived of the nutrien::survive, the lining dies and passes from the uterus, along with bloo,'_ - _ =-in what is called the menstrual flow (also referred to as the period c 7

While the menstrual period might seem to be the end of the -day of flow is actually counted as day 1 of a woman's reprodu.::-while the flow is taking place, the three-act play is starting over_of follicles begins to ripen in the ovaries. This "circle of life" will _ -continue month after month throughout the reproductivemenopause unless interrupted by pregnancy or a medical conditic: -feres with this cycle