Mentrual Cycle - Human Reproduction - Class 12 PDF Download

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.

Duration – 28 days Ideally (Range - 22 to 32 days)

Fig: Menstrual cycle

In this cycle the female body prepares itself for a possible pregnancy. It the pregnancy does not occur then the body aborts all preparation done and restarts the preparation for pregnancy again in a monthly cyclic manner.

Menstrual cycle has three main phases:
(i) Bleeding phase or menstruation phase.
(ii) Proliferative/preovulatory/follicular phase or oestrogenic phase.
(iii) Secrectory/post ovulatory/luteal phase or progesteronic phase.

(i) Menstrual Phase: The cycle starts with bleeding phase in its first four to five days. During this bleeding the part of the layer of endometrium gets shed off. An average of 40 to 80 ml blood/cycle is lost. This blood can not clot.
(ii) Preovulatory/Proliferative phase: After first four of five days this phase begins. During this phase, Due to release of some GnRH, Pituitary secretes some FSH to stimulate the ovarian follicle. The ovarian follicle now begins to develop. Developing follicle now starts secreting an increasing amount of estrogen.
The rising level of estrogen causes the endometrium to proliferate and thicken. Rising level of oestrogen also activates the hypothalamus. Due to this, the hypothalamus releases more of GnRH. This GnRH induces the pituitary to release more of FSH. The rising FSH levels now cause

• Further growth and development ovarian follicle to form Graafian follicle
• Even further release of estrogen from the theca interna of this developing follicle.

As the estrogen level goes on rising, by the end of 10 day the extreme levels of estrogen (which have by then caused maturation of Graafian follicle and growth of endometrium) now give a positive feed back of high concentration of estrogen causing a rise in LH secretion but due to release of inhibin by graffian follicle, FSH falls but the LH secretion from the pituitary goes on rising. This abrupt rise (on 11th to 13th day) in LH concentration in blood is called as LH surge.
This LH now causes the Graafian follicle to rupture release of egg (secondary oocyte) which occurs around 14 day is called as ovulation.
(iii) Post ovulatory/secretory phase - After ovulation the ruptured Graafian follicle transforms into corpus luteum. The granulosa and theca cells of the ruptured Graafian follicle (which is now called as corpus luteum) is found only in mammals and contain a yellow lutein or carotene pigment.
In case of absence of pregnancy this corpus luteum will get degenerated after 14 days of its formation. The degenerated corpus luteum is called corpus albicans (white body).

FUNCTION OF CORPUS LUTEUM 
Stimulated by the rising levels of LH, the corpus luteum secretes progesterone hormone. The progesterone facilitates the preparation of endometrium for receiving the embryo and its implantation. Progesterone inhibits the contractions of uterus so that the pregnancy could be maintained.
Progesterone also inhibits development of next new ovarian follicle. If pregnancy occurs then the corpus luteum persists and secretes progesterone. Progesterone is important to maintain the pregnancy and it is thus called as the pregnancy hormone. By the fourth month of pregnancy, the placenta has developed completely. This placenta now takes over the job of further progesterone secretion. Ovary also secretes some amount of relaxin at the time of parturition.
If pregnancy does not occur after ovulation, then as the progesterone level rise, its rising levels inhibits the release of GnRH from hypothalamus. Due to this FSH, LH secretion by pituitary falls and thereby progesterone secretion by the corpus luteum (which was due to influence of LH) also now falls.
As the progesterone level drops, the corpus luteum begins to degenerate and transform in corpus  albicans (which can not secrete progesterone). Due to the lack of progesterone

• The overgrown endometrium now begin to break and separate from the inner uterine wall causing mentrual  bleeding.
• The uterine contraction (which was till now inhibited due to presence of progesterone) now start.

The period between ovulation and next menstrual bleeding (post ovulatory period) is always constant (i.e. 14 days). However, the ovulation date may vary (causing a change in pre-ovulatory period).
After ovulation the ovum is viable only for two days, while sperms introduced into the vagina can survive for a maximum of four days.
On basis of the above data, safe period method for family planning is calculated. Normally it is considered to be day 1 to day 8 and then from day 20 to day 28.

Fig: Schematic representation of Menstrual cycle

Menstrual cycle remain suspended during pregnancy
Menopause (or female climacteric)
The menstruation stops at 45 - 50 years of age, women loses reproductive ability. Urine of a menopausal women contain large amount of FSH because estrogen production by ovaries stops and pituitary gets relieved of inhibitory effect of estrogen.

• Menarche: The first menstruation which marks the onset of puberty.
• Dysmenorrhoea: Abnormal or irregular menstruation with cramps and profuse bleeding due to locally produced prostaglandins.
• Amenorrhoea: Menstruation does not occur due to many physiological/hormonal or mental defects this is the sign of sterility.