The menstrual cycle is a combination of a series of events occurring between a part of the brain called the hypothalamus, the pituitary gland in the brain, the ovaries, and the interior of the uterus (endometrium). Normal cycle lengths vary from 21 to 35 days even though considerable variation in the length occurs in individual women over their life span. The menstrual cycle is divided into four phases: the follicular, ovulatory, luteal, and menstrual phases.
HORMONE PRODUCTION AND REGULATION
At puberty, a compound called gonadotropinreleasing hormone (GnRH) is synthesized in a part of the brain called the hypothalamus and travels to the pituitary gland. Here GnRH stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), two hormones important for the functioning of the ovary. These hormones are all secreted in a pulsatile fashion and pulse frequency varies with the phase of the menstrual cycle. Within the ovary, FSH helps to synthesize the “female hormone” estrogen. As estrogen levels become higher (range of 200-300 pg/ml) and are maintained above this level for more than 24 hr, positive feedback loops on the pituitary gland then trigger a surge of LH and FSH. This leads to release of the egg from the ovaries. The endocrine system then increases synthesis of another hormone, progesterone, which will act on the uterine wall (endometrium) to facilitate the changes necessary for successful egg implantation to occur. In the absence of fertilization, the portion of the uterus that has prepared for egg implantation and pregnancy disintegrates and menstruation occurs. This sloughing of the endometrium is known as menstruation.
The endometrium responds to these cyclical changes of ovarian hormones. Estrogen stimulates growth by increasing both the number and size of the endometrial cells. Progesterone receptors are induced on the membranes of these cells in response to increasing estrogen levels. Estrogen also increases the blood flow to the endometrium. This phase of the cycle is called the proliferative or follicular phase and occurs immediately following menstruation as FSH and thus estrogen levels start to rise.
As the estrogen levels rise and remain in the critical range for 24-48 hr, the positive feedback loop is activated in the pituitary gland resulting in the LH surge. This surge can be detected using ovulation kits.
The luteal phase of the cycle is characterized by progesterone dominance. This action on the endometrium is manifested by decrease in the cell growth (proliferative) effects of estrogen. Progesterone production causes a rise in basal body temperature and forms the basis of tests used to confirm that ovulation has occurred.
In the absence of fertilization, there is a decline in estrogen and progesterone levels. This decline leads to abrupt loss of hormonal support of the endometrium and subsequent decrease in blood flow, loss of the organized architecture, and resultant menstrual flow. This is called the menstrual phase. Normal blood volume during this menstrual phase varies from 20 to 80 cm3 over the entire menstrual period.
SEE ALSO: Menstrual cycle disorders, Pregnancy
- Speroff, L., Glass, R., & Kase, N. G. (1994). Regulation of the menstrual cycle. Clinical gynecologic endocrinology and infertility (5th ed., pp. 183-230). Philadelphia: Williams & Wilkins.
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