02 Reproduction in Lower and Higher Animals - part 05 - Menstrual cycle

02 Reproduction in Lower and Higher Animals - part 05 - Menstrual cycle


Menstrual cycle (Ovarian cycle): 
  • Menstrual cycle is the characteristic feature of primates including human. 
  • It involves a series of cyclic changes in the ovary and the female reproductive tract, mainly in the uterus. 
  • These changes take place under the effect of gonadotropins and the ovarian hormones respectively. 
  • The cycles are repeated with a periodicity of approximately 28 days. 
  • The middle of each cycle is characterised by the release of an egg.This egg in every cycle comes alternately from one of the two ovaries. 
  • The cycle is divided into four phases - 
  1. Menstrual phase
  2. Proliferative phase / Follicular phase / Post menstrual phase
  3. Ovulatary phase
  4. Secretory phase / Luteal phase

 a. Menstrual phase: 

  • The begining of each cycle is taken as the first day where menses or loss of blood (45-100ml) takes place.
  • It lasts for approximately five days (average 3-7 days). 
  • Endometrium of uterus breaks down under the effect of prostaglandins released due to decreased levels of progesteron and estrogen. 
  • Due to this blood, tissue fluid, mucus, endometrial lining and the unfertilized oocyte is discharged through vagina. 
  • The endometrial lining becomes very thin i.e. about 1 mm. 
  • The menstrual discharge continues for an average of 5 days, however this blood does not clot due to presence of fibrinolysin. 
  • Menstrual phase occurs when an ovulated egg does not get fertilized and it is thereby shed out along with the menstrum. It is thus called `funeral of unfertilized egg'. 
  • During these five days, many primordial follicles develope into primary and few of them into secondary follicles under the effect of FSH.


 b. 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 13th day of menstrual cycle. 
  • A few (6 to 12) secondary follicles proceed to develop but usually one of them develops into a graafian follicle (mature follicle). 
  • The other secondary follicles degenerate. This process of degeneration is called atresia. 
  • Developing secondary follicles secrete the hormone estrogen. 
  • The stimulation for proliferation of new follicles is influenced by GnRH which stimulates release of FSH. 
  • Endometrium begins to regenerate under the effect of gradually increasing quantity of estrogens. 
  • Regeneration also involves  - 
  1. formation of endothelial cells,
  2. endometrial or uterine glands and 
  3. network of blood vessels. 
  • Thickness of endometrium reaches 3-5 mm.


c. Ovulatary phase: 

  • It is the shortest phase of menstrual cycle. 
  • It involves rupturing of the mature graafian follicle and release of ovum (secondary oocyte) into the pelvic cavity; usually on 14th 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.

d. Secretory phase / Luteal phase: 

  • Duration of this phase is between the ovulation and beginning of the next menses. 
  • This phase is the longest phase. 
  • It lasts for 14 days; from 15th to 28th day of the cycle. 
  • After release of secondary oocyte, remaining tissue of graafian follicle transforms into corpus luteum under the effect of LH. 
  • Corpus luteum begins to secrete progesteron and estrogens. 
  • The ovulated egg may get fertilized within 24 hours. 
  • However, in the absence of fertilization, corpus luteum can survive for only two weeks and then degenerate into a white scar called corpus albicans. 
  • The corpus luteum releases progesteron, small amount of estrogens and inhibin. 
  • Under the influence of these hormones, the endometrial glands grow, become coiled and start uterine secretions. 
  • Endometrium becomes more vascularized and thickens up to 8-10 mm. 
  • Inhibin stops secretion of FSH. 
  • These changes are necessary for fertilization and subsequent implantation
  • However, if the ovulated egg gets fertilized and the embryo is implanted, there is secretion of human chorionic gonadotropin (hCG).
  • hCG extends the life of corpus luteum and stimulates it's secretory activity. 
  • Presence of hCG in maternal blood and urine is an indicator of pregnancy. 
  • In absence of fertilization, next menstrual cycle begins.


Always Remember
 Hygiene practices during menstruation : 

  1. Keep the pubic area clean. 
  2. Change the sanitary napkin every 4-5 hours. 
  3. Maintaining personal hygiene during menstruation is important to reduce the risk of infection. 
  4. Dispose used sanitary napkin properly. 
  5. Using damp and dirty clothes or using a sanitary napkin for a longer time can act as a perfect environment for growth and multiplication of harmful bacteria and lead to infections.

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