Menstrual Cycle

The reproductive cycle in female primates, e.g., Monkeys, apes and human beings is called menstrual cycle.

The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days. The menstrual cycle stops around 50 years of age, in humans and this stage is termed as menopause.

The cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause. The cycle of events starting from one menstruation till the next one is called menstrual cycle.The menstrual cycle consists of following four phases

Menstrual Phase

The cycle starts with menstrual phase. It lasts for about 3-5 days and involves the following events

If fertilisation does not occur, the unfertilised egg undergoes autolysis.

Reduction of LH level causes regression of corpus luteum by autolysis and a consequent fall in the progesterone level in the blood.

The uterine lining dies due to he deficiency of progesterone and is sloughed off. Blood rupture, causing bleeding the process is called menstruation or menstrual flow.

Follicular (Proliferating) Phase

It lasts for about 14 days and involves the following events

The primary follicles in the ovary grow to become a fully mature Graafian follicle.

Simultaneously the endometrium of uterus regenerates through proliferation.

The changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.

The secretion of gonadotropins (LH and FSH) increases gradually Fertility during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.

Luteal Phase:

This phase lasts for about 10 days and involves the following events

The remaining part of Graafian follicle transform as the corpus luteum.

The peak of LH, together with prolactin hormone from anterior pituitary stimulates the follicular cells of the empty Graafian follicle to form corpus luteum.

The corpus luteum secretes large amount of progesterone essential to maintain endometrium.

The endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy, all events of the menstrual cycle stop and there is no menstruation. In the absence of fertilisation, the corpus luteum degenerate.

This causes disintegration of endometrium leading to menstruation and a new cycle starts.