AIPMT Biology Aspirants, read out the next AIPMT Biology Study material/ Notes of Menstrual Cycle, important for AIPMT Biology.Free online notes for AIPMT. Female Reproductive cycle.
- Reproductive cycle in female primates (e.g. monkeys, apes and human beings) is called as Menstrual cycle.
- First menstruation begins at puberty and is called Menarche. In human females, menstruation
is repeated at an average interval of about 28-32 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. Here, we will discuss for 28 days cycle.
- Each cycle involves Oogenesis and preparation of the uterus to receive a fertilized ovum.
- Hormones secreted by the hypothalamus, anterior pituitary, and ovaries control the main events.
- Menstrual cycle involves series of changes in the Endometrium of the uterus to prepare it for the arrival of a fertilized ovum that will develop there until birth.
- Menstrual cycle can be divided into four phases: (1) Menstrual phase, (2) Pre-ovulatory phase, (3) Ovulation, and (4) Post-ovulatory phase
- Menstrual Phase: Menstrual cycle starts with menstrual phase which lasts for 3-5 days. During this phase, menstrual flow occurs. Day 1 of the menstrual phase is counted as day 1 of the new cycle. Also called as Menstruation or menses.
- Menstrual flow contains about 50–150 mL of blood, tissue fluid, mucus, and epithelial cells shedded from the endometrium of the uterus.
- Menstrual flow results due to breakdown of stratum functionalis of endometrium of the uterus and its blood vessels which forms liquid that comes out through vagina.
- Endometrium is divided into two layers: Stratum functionalis and Stratum basalis. Stratum functionalis is the outer layer lining the uterine cavity and it shed away during each menstruation while stratum basalis is inner to statum functionalis and is permanent layer and it gives rise to a new stratum functionalis after each menstruation.
- Declining levels of progesterone and estrogen hormone are responsible for shedding of stratum functionalis and menstrual flow.
- Menstruation only occurs if the released ouvm is not fertilised. Lack of menstruation may be indicative of pregnancy, but may also be caused due to some other underlying causes like stress, poor health etc.
2.Pre-ovulatory phase: This phase starts at the end of menstruation and continues till ovulation.
- It lasts from day 6 to 13 of the cycle.
- This phase is also called as Follicular phase because follicles in the ovary are growing and developing.
- During this phase, in the ovary, primary follicles begin to grow, but only single secondary follicle outgrows all other and becomes dominant follicle. This single dominant secondary follicle finally grows to become a fully mature Graafian follicle. This mature follicle continues to increase the production of estrogen.
- In uterus, increasing level of estrogen (by mature graafian follicle), stimulates stratum basalis of endometrium to produce new stratum functionalis of endometrium. Due to formation of new layer in endometrium, its thickness increases.
- This phase is also called as Proliferative phase because the endometrium is proliferating.
- These changes in the ovary and the uterus are induced by changes in the levels of anterior pituitary and ovarian hormones. The secretion of gonadotropins (Luteinizing hormone, LH and Follicle-stimulating hormone, FSH) by anterior pituitary increases gradually during this phase, which stimulates development of follicles, 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).
3.Ovulation/ Ovulatory phase: During the mid-cycle (day 14 of the cycle) LH leads to its maximum level called as LH surge which induces rupture of Graafian follicle and thereby the release of ovum (ovum is still in its secondary oocyte stage).
Ovum only gets released if there is chance of fertilization. Surge of LH is necessary for ovulation.
- During ovulation, secondary oocyte remains surrounded by zona pellucida and corona radiata.
- Only one ovum is released (ovulation) during each cycle.
4. Post-ovulatory phase/ Luteal phase/Secretory phase: Ovulatory phase is followed by the luteal phase during which remaining parts of the Graafian follicle transform as Corpus Luteum.
- It is the time between ovulation and onset of the next menstrual cycle. It lasts from day 15 to day 28 of 28 days cycle.
- As the mature follicle or Graafian follicle gets ruptured during ovulation, granulosa cells and theca interna of follicle also breaks down which leads to minor bleeding and formation of blood clots and ruptured follicle now called as Corpus hemorrhagicum.
- Under the influence of LH, granulosa cells get mix with theca interna cells and they all change rapidly into lutein cells. Follicle is now called as Corpus luteum. This process is called as Luteinization. Lutein cells give corpus luteum a yellowish appearance.
- Corpus luteum secretes mainly progesterone. Other hormones include estrogens, relaxin, and inhibin.
- Progesterone is essential for maintenance of the endometrium, which is necessary for implantation of the fertilized ovum and other events of pregnancy.
- In the absence of fertilization, or if the oocyte is not fertilized, corpus luteum has a life span of only 2 weeks. Then, its secretory activity declines, and corpus luteum degenerates into Corpus albicans. This causes disintegration of the endometrium. No secretion of progesterone and estrogens leads to menstruation, marking a new cycle.
- If fertilization oocurs, corpus luteum does not degenerate. Another hormone called as Human chorionic gonadotropin (hCG) hormone, produced by chorion layer of developing embryo prevent degeneration of corpus luteum.
- Presence of hCG in maternal blood or urine is an indicator of pregnancy and is the hormone detected by home pregnancy tests.
- During pregnanacy all events of the menstrual cycle stop and there is no menstruation.
In human beings, menstrual cycle ceases around 50 years of age which is termed as Menopause. Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause.