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Answer 1

MENSTRUAL CYCLE

The reproductive cycle in  starting from the one menstruation till the next one in the female primates is called menstrual cycle. The first menstruation which begins at puberty and is called menarche . The cycle is repeated at an interval of 28-29 days. Menstrual cycle involve three phases- menstrual phase, follicular phase and luteal phase.

Menstrual phase-

Menstrual flow occurs and lasts for about 3-5 days.

The endometrial lining of the uterus breaks along with the blood vessels which forms a red fluid and results in menstrual flow.

If the ovum is fertilized by a sperm menstrual flow does not occur and hence indicates pregnancy.

Follicular phase-

In this phase, the primary follicles in the ovary grow to become a fully matured graafian follicle.

Endometrium regenerates through proliferation.

Changes in Pituitary hormone and ovarian hormones induce the formation of graafian follicle and regeneration of endometrium.

The secretion of gonadotropins like luteinizing hormone and follicular stimulating hormone increases gradually during this phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.

Both LH and FSH attain a peak level in the middle of cycle about 14th day.

Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum known as ovulation

Luteal phase-

In this phase, the ruptured part of Graafian follicle transforms into yellow body called Corpus luteum.

The corpus luteum secretes large amounts of progesterone hormone which maintains the endometrium for implantation of the fertilized ovum.

During pregnancy all events of the menstrual cycle stop and there is no menstruation.

In the absence of fertilization, the corpus luteum degenerates hence causes disintegration of the endometrium leading to menstruation and a new cycle begins.