Breifly describe the Menstrual Cycle?
Each month during a female's reproductive years, the hypothalamus secretes a hormone that stimulates the anterior lobe of the pituitary gland to releases another hormone. This is the follicle-stimulating hormone, or FSH. By stimulating the follicle, FSH sets off a series of events in preparation for fertilization. The hormonal changes and the effects they produce occur in cycles of about 28 days. This periodic sequence of events is called the menstrual cycle. The reproductive life of a woman begins at the first onset of menstruation, or menarche, at age 11 to 14. Cessation of menstruation, or menopause, occurs around ages 45 to 52. The pituitary, however, continues to secrete FSH following menopause.
FSH causes development of several follicles, sheaths of specialized cells surrounding developing oocytes within the ovary. Usually, only one follicle completes development each month. As the follicle enlarges, it secretes the hormone estrogen. Estrogen enters the bloodstream and stimulates the hypothalamus and the pituitary to produce more FSH and luteinizing hormone, or LH. The elevated levels of FSH and LH stimulates the oocytes to undergo division and causes rapid growth of the follicle. Estrogen also stimulates growth and thickening of the endometrium of the uterus. Thickening of the endometrium involves increased vascularization and the growth of gland cells that serve to support an implanted fertilized egg.
At about the middle of the menstrual cycle, in response to a surge of LH production from the pituitary, the mature egg is released from the follicle into the abdominal cavity in a process called ovulation. The ends of the oviducts are fringed with finger-like structures covered with cilia (fimbriae), which pull the egg into the Fallopian tube. The egg remains in the Fallopian tube for three to four days, during which fertilization may take place; then it is carried into the uterus.
Following ovulation, the remnants of the follicle that remain in the ovary are transformed into the corpus luteum, which, under the influence of LH, secretes more estrogen and another hormone, progesterone. Progesterone and estrogen stimulate the uterus to thicken and produce mucus. If an egg becomes fertilized, it will implant itself in this nutrient-rich lining for further embryonic development.
If the egg is not fertilized, the high levels of estrogens and progesterone inhibit the pituitary's production of the FSH and LH, and the corpus luteum degenerates. It is thought that the high levels of hormone act like a negative feedback loop for the pituitary. With decreased hormonal stimulation, the uterus contracts and the excess endometrial tissue is sloughed off and out, along with some blood and the unfertilized egg. This process is called menstruation (from the Latin mensis, meaning "month"). The menstrual flow exits the body through the cervix and vagina.
Following the end of the cycle, progesterone levels drop, so that the hypothalamus is again activated. In response to signals from the hypothalamus, the pituitary increases secretion of FSH, and the cycle begins anew.