“Menstrual Cycle” MHT-CET Class 12 Biology Crash Course| Chapter “Reproduction in Lower & Higher Animals”
In this session Jyoti A. Shete discussed the theory and MCQ’s of “Menstrual Cycle”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.
Menstrual Cycle:-
Hello everyone in previous article we have learnt in details about the female reproductive system and its different parts. We know that in case of female due to hormonal changes working of female reproductive system gets renewed every month. This resemblance to one of the most essential cycle, termed as menstrual cycle.
Shall we discuss the concept of menstrual cycle then? Let’s start it….!
Causes, affects of menstrual cycle..!
→ During the reproductive period of human females the ovaries and uterus undergoes series of cyclic changes which prepare females for fertilization and pregnancy and collectively form the menstrual cycle.
→ Most important feature of this cycle is periodic vaginal bleeding called as
→ The word mensum – a month the length of menstrual cycle varies widely but on an average month or 28 days.
Let’s understand the basic concepts/stage involved in menstrual cycle in detail…!
→ Puberty: It is the age at which the reproductive system becomes functional; sex organs begin to produce gametes and sex hormones. It occurs at the age of 12 – 15 years.
→ Menarche: It is the first menstrual cycle of a female which begins normally at about 12-15 years of age, the age of Menarche also varies.
→ Menopause: It is the cessation of menstrual cycle which generally happens at the age of 45 to 50 years.
→ Reproductive age: The period in between menarche and menopause is the reproductive age of the female and is approximately 32 years. In this time the female will be producing a maximum of about 416 eggs
(32 x 13 = 416 eggs)
Let’s learn in detail about Menstrual cycle:
We had discussed the basic stages involved in menstrual cycle. Now we are going to discuss the concept of menstrual cycle in details with the phases involved in it..!
Menstrual cycle is basically divided into four phases:
- Menstrual phase
- Proliferative phase
- Ovulatory phase
- Secretary phase or luteal phase
Let’s understand the menstrual phase…….!
→ It last for about 3 to 7 days and extends from 1 to 4th day of menstrual cycle.
→ If the egg remain unfertilized then Corpus luteum starts degenerate and results in the level of progesterone is declined due to low level of progesterone the thick endometrium starts to rupture.
→ Unfertilized egg along with ruptured uterine epithelium about 45-100 ml of blood and some mucus is discharged through a vaginal orifice and it is called as menstrual flow for menstruation.
→ Menstruation also called as “Funeral of unfertilised egg” or “Weeping of uterus…”because loss of egg.
→ Decrease in the level of progesterone and oestrogen in blood stimulate pituitary gland to release FSH which starts the follicular phase of menstrual cycle.
Let’s understand the Proliferative phase / Follicular phase / Post menstrual phase….!
→ This phase is the duration between the end of menstruation and release of ovum (ovulation).
→ Duration of this phase is more variable than other phases. Generally, it extends from 5th to 14th day of menstrual cycle.
→ FSH stimulates the change of primary follicle into a Graafian follicle.(Primary follicle Secondary follicle Tertiary follicle Graafian Follicle)
→ Follicular cells of Graafian follicle secrete estrogen.
Effects of oestrogen……………!
- It stimulates the growth, maintenance and functioning of secondary sex organs.
- Endometrial lining of the uterus becomes thick (2-3 mm), more vascular and more glandular.
- The uterine gland becomes cork-screw shaped and gets ready for implantation.
- The epithelium of fallopian tube becomes thick and densely ciliated to conduct the ovum and zygote.
- High level of Estrogens inhibits the secretion of FSH.
- High level of Estrogens also initiates the secretion of LH.
Let’s understand the Ovulatory phase…..!
- It is the shortest phase of menstrual cycle.
- It involves release of secondary oocyte or ovum from graafian follicle.
- Rupturing of the mature Graafian follicle and release of ovum (secondary oocyte) into the pelvic cavity, usually on 14th day or 15th day of menstrual cycle.
- Rapid secretion of LH by positive feedback mechanism causes the mature follicle to rupture.
- Ovulation may be accompanied by mild or severe pains in lower abdomen. That’s why menstrual cycle is painful…!
- LH also changes the empty graafian follicle into Corpus luteum.
- Ovum remains viable for two days.
- Not much change in uterine endometrium is seen during this phase.
Let’s understand the Secretory phase / Luteal phase
- It lasts for about 12 – 14 days and extends from 16th to 28th day of menstrual cycle.
- During this phase the following changes take place.
- Corpus leutum (yellow body) is formed from the empty Graafian follicle.
- It secretes progesterone which induces the following changes.
- High level of progesterone decreases the secretion of FSH and LH so inhibits maturation of new follicles and ovulation.
- Uterine and endometrium further proliferates (about 5 mm thick) and is ready for implantation.
- Uterine glands secrete uterine milk for the developing embryo It reduces the uterine movements.
- If fertilization takes place then blastula undergoes implantation and a chorionic placenta is formed which secretes hCG (human chorionic gonadotrophin) which maintains Corpus Luteum and progesterone secretion continues.
- Progesterone maintains pregnancy and prevents ovulation during gestation period.
- In absence of fertilization next menstrual cycle begins.
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