Q.Write an essay on Menstrual Cycle.

The menstrual cycle is a ~28-day hormone-controlled cycle with four phases — menstrual, follicular (proliferative), ovulatory and luteal (secretory) — that prepares the uterus for pregnancy each month.

The menstrual cycle is the reproductive cycle in human females (and other higher primates), beginning at puberty (menarche) and continuing until menopause. Each cycle lasts about 28 days on average and prepares the female body for a possible pregnancy. It is controlled by the coordinated action of hormones from the pituitary gland (FSH and LH) and the ovary (oestrogen and progesterone). The cycle is described in four phases:

1. Menstrual phase (days 1–5):

• If the ovum released in the previous cycle is not fertilised, the corpus luteum degenerates and the levels of oestrogen and progesterone fall.

• The thickened lining of the uterus (endometrium) can no longer be maintained; it breaks down and is shed along with blood and mucus through the vagina. This bleeding is called menstruation (menstrual flow), lasting about 3–5 days.

2. Follicular / proliferative phase (days 6–13):

• The pituitary secretes Follicle Stimulating Hormone (FSH), which stimulates a few ovarian follicles to grow; usually one becomes the dominant Graafian follicle.

• The growing follicle secretes oestrogen. Under oestrogen, the endometrium of the uterus repairs and thickens (proliferates), and its blood supply and glands increase, preparing it to receive an embryo.

3. Ovulatory phase / ovulation (around day 14):

• The rising oestrogen triggers a sharp surge of Luteinising Hormone (LH) (and a smaller FSH surge) from the pituitary — the 'LH surge'.

• This LH surge causes the mature Graafian follicle to rupture and release its ovum (secondary oocyte) into the fallopian tube — this release is ovulation. This is the fertile period, when fertilisation is most likely.

4. Luteal / secretory phase (days 15–28):

• After ovulation, the ruptured follicle in the ovary transforms into a yellow endocrine body, the corpus luteum.
• The corpus luteum secretes large amounts of progesterone (and some oestrogen). Progesterone maintains and further develops the endometrium (making it richly secretory and vascular) so it is ready for implantation of an embryo.
• If fertilisation occurs, the embryo implants, the corpus luteum is maintained (by hCG from the developing placenta), progesterone continues, and menstruation is stopped for the duration of pregnancy. …