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MENSTRUAL CYCLE
The period of regularly recurring physiologic changes in the endometrium that culminates in its shedding (menstruation).
The female is fertile only during few days of the months and the pattern of hormone secretion is intricately related to the cyclical release of a secondary oocyte from the ovary.
The cyclical production of the hormones controls the development of a secondary oocyte n a follicle. Gonadotropin-releasing hormone (GnRH) from the hypothalamus acts on the anterior pituitary gland, which releases follicle-stimulating hormone (FSH) and luteinzing hormone (LH) to bring about the oocyte's maturation and release from the ovary.

These hormones regulate the menstrual cycle, which is the cyclic preparation of the uterus to receive a fertilized egg, and the ovarian cycle, during which the oocyte matures and ovulation occurs. This monthly preparation of the uterine lining for the fertilized egg normally begins at puberty. When a female reaches 45 to 55 years of age, the ovaries lose their sensitivity to FSH and LH, they stop making normal amounts of progesterone and estrogen, and the monthly menstrual cycle ceases in what is called the menopause.
One way to understand the hormonal pattern in the normal monthly cycle is to follow the development of the oocyte and the physical events in the menstrual cycle. On average, it takes 28 days to complete one menstrual cycle, although the range may be from 22 to 45 days. During this time, the following events take place:
1. The controlling center for ovulation and menstruation is the hypothalamus. It releases, on a regular cycle, GnRH, which stimulates the anterior pituitary to secrete FSH and LH.
2. FSH promotes the development of the oocyte in one of the immature ovarian follicles.
3. The follicles produce estrogen, causing a buildup and proliferation of the endometrium, as well as the inhibition of FSH production.
4. The elevated estrogen level about midway in the cycle tigers the anterior pituitary to release LH, which causes the mature follicle to enlarge rapidly and release the secondary oocyte. LH also causes the collapsed follicle to become another endocrine tissue, the corpus luteum.
5. The corpus luteum secretes estrogen and progesterone, which act to complete the development of the endometrium and maintain it for 10 to 14 days.
6. If the oocyte is not fertilized, the corpus luteum disintegrates into a corpus albicans, and estrogen and progesterone secretion cease.
7. Without estrogen and progesterone, the endometrium breaks down, and menstruation occurs. The menstrual flow is composed mainly of sloughed-off endometrial cells, mucus and blood.
8. as progesterone and estrogen levels decrease further, the pituitary renews active secretion of FSH, which stimulates the development of another follicle, and the monthly cycle begins again.

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