Study of Menstrual Cycle (Reproductive physiology)

 Menstrual Cycle

The reproductive cycle in the 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, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. One ovum is released (ovulation) during the middle of each menstrual cycle

First of all, we should know the female reproductive system.  

Structure of the female reproductive system:

• The vagina: It is the canal between the uterus and the external reproductive organs.

• The uterus: It is a hallow pear-shaped organ where the fetus grows during pregnancy. It is separated from the vagina by a narrow canal called: the cervix.

• Fallopian tubes: They are narrow canals connected to the upper part of the uterus. Through them, the eggs travel from the ovaries to the uterus, and inside them, the egg meets the sperm (fertilization). Then, the fertilized egg travels through the tubes to the uterus where it can stick to the uterine wall and form an embryo.

• Ovaries: They are small oval-shaped glands located on the sides of the uterus. Eggs form inside the ovaries that also produce female hormones

First menstrual period:

The menstrual cycle often begins at puberty between the ages of 10 and 13  (average age of 12). It usually starts two years after breasts and pubic hair start to develop.

 Phases of the menstrual cycle:

There are four phases:

1.     Menstruation,

2.     Follicular Phase,

3.     Ovulatory phase

4.     The Luteal phase.

- Menstruation: (1^st to 5^th Day)

Menstruation is the elimination of the internal lining of the uterine wall along with other secretions from the body through the vagina. It usually lasts three to five days but this could change to more or less days from month to month and depending on health of  each woman. Menstruation only occurs if the released ovum is not fertilized. Lack of menstruation may be indicative of pregnancy The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina.

- The follicular phase: (6 to 13^th day)

The follicular phase starts on the last day of menstruation and ends with ovulation (14^th to 16^th Day). During this phase the pituitary gland releases a follicle (cyst) stimulating hormone. During this phase, the primary follicles in the ovary grow to become a fully mature Graffian follicle and simultaneously the endometrium of uterus regenerates through proliferation.

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

The developing follicles produce the hormone estrogen, which in turn thickens the lining of the endometrium in preparation to receive a fertilized egg. Often, only one follicle keeps growing and travels to the ovarian surface while the remaining follicles gradually fade and are absorbed back into the body.

Ovulatory phase  (13 to 17^th Day)

Ovulation occurs 14 to 16 days before the following period but not necessarily in the middle of the cycle. The term “ovulation” indicates the formation of a mature egg by one of the ovaries. The release of estrogen during the follicular phase causes the brain to produce a gonadotrophin-releasing hormone that prompts the pituitary gland to produce raised levels of luteinising hormone (luteotropic hormone). This release the mature egg from the  follicle (cyst) and transports it from the ovary to the fallopian tube.

If fertilization does not occur during this phase, the egg continues to the uterus and dissolves within 6 to 24 hours.

The luteal phase: 18^th to 28^th day )

During this phase, the follicle from which the mature egg burst out (now called the yellow body of the ovary or corpus luteum) released a large quantity of progesterone and some estrogen. The two hormones help thicken the lining of the endometrium and maintain its thickness.

If fertilization does not occur, the yellow body degrades and progesterone levels decrease, leaving the lining unstable. This causes disintegration of the endometrium leading to menstruation marking a new cycle.

In human beings, menstrual cycles ceases around 50 years of age; that is termed as menopause. Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause.

 

Summary of menstrual cycle

The reproductive cycle 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-

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

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

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

Follicular phase-

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

o    Endometrium regenerates through proliferation.

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

o    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.

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

o    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-

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

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

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

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

 In human beings, menstrual cycles ceases around 50 years of age and known as menopause.