Menstrual cycle in human female


Human female do not undergo a seasonal oestrous cycle as lower mammals do, instead one egg is released from an ovary about once every 28 days. This is often called ovarian cycle. This correlated with certain uterine changes which occur to prepare it for a possible pregnancy; this often is called uterine cycle. This monthly egg maturation and uterine preparation is collectively menstrual cycle (often called menses, month). Menstruation begins around the age of thirteen and stops menopause around the age of fifty. Menstrual cycle is the preparation of uterus for a possible pregnancy. The menstrual cycle is controlled by hormones and completes every 28 days. It occurs in four distinct phases (i) Menstruation or M phase (ii) follicle or f phase (iii) ovulation or O phase (iv) Corpus luteum or L phase.

Menstruation:
Menstruation is the onset of bleeding that is discharge of blood and discarded tissue of the uterus through vagina. Menstruation takes place when the body becomes aware chemically that no fertilization or pregnancy has occurred following the last ovulation. Progesterone secretion is stopped by corpus luteum and as a result the soft spongy vascular internal lining of uterus called endometrium breaks off and starts flowing along with blood out of vagina in the form of menstrual flow. The first day of menstrual flow is taken as the beginning of menstrual cycle. The stage lasts about five days and extends from day to day 5.

Follicle phase: This phase starts just after menstruation and ends with the release of ovum. It lasts about 7 days (day 6 to 12). This stage stimulates the development of many follicles in the ovary. Only one follicle matures to produce an egg. This egg producing follicle is sometimes called graffian follicle. The rest of the follicles do not develop and ultimately degenerate. This process is initiated by FSH (follicle stimulating hormone) secreted in blood from pituitary gland. FSH stimulates the ovary to produce a hormone of its own called oestrogen. Oestrogen affects pituitary on one hand to inhibit the secretion of FSH and on the other hand initiates the thickening of uterine wall. Low FSH level and high oestrogen level in the blood initiate the secretion of another hormone LH (leutinizing hormone) from pituitary gland.

Ovulation: This last phase lasts not more than three days ovary ruptures and the mature ovum is released. This mature egg enters the oviduct. This stage is initiated by the increase of LH level.

Corpus luteum phase: This phase is the longest period of menstrual cycle and lasts about 12-14 days (day 16-28). It is so called because LH causes the ruptured follicle to change into a yellowish body, the corpus luteum, which starts producing another hormone the progesterone. Progesterone maintains and enhances the growth of mucous lining (endonetrium) of the uterus. If the mature ovum, now is oviduct, is not fertilized the corups luteum gradually degenerates, progesterone secretion stops and the internal lining of uterus disintegrates and sloughs off with blood, menstruation begins and the cycle starts again.

Malmitrition, ill health, mental strain and physical strains as in gymnasts and athletes cause harmonic imbalance which disturbs or even stops their menstrual cycle resulting in temporary or permanent infertility on part of these women.

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