Question

Describe in brief the structural and hormonal changes during the ovarian cycle.

Answer 1

1. Menstrual phase:
   1. The begining of each cycle is taken as the first day where menses or loss of blood (45-100 ml) takes place and it lasts for approximately five days (average 3-7 days).
   2. Endometrium of uterus breaks down under the effect of prostaglandins released due to decreased levels of progesteron and estrogen.
   3. 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.
   4. The menstrual discharge continues for an average of 5 days, however, this blood does not clot due to presence of fibrinolysin.
   5. 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’.
   6. During these five days, many primordial follicles develop into primary and few of them into secondary follicles under the effect of FSH.
2. Proliferative phase/Follicular phase/Post-menstrual phase:
   1. 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.
   2. 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.
   3. Developing secondary follicles secrete the hormone estrogen. The stimulation for proliferation of new follicles is influenced by GnRH which stimulates release of FSH.
   4. Endometrium begins to regenerate under the effect of gradually increasing quantity of estrogens. Regeneration also involves formation of endothelial cells, endometrial or uterine glands and network of blood vessels. Thickness of endometrium reaches 3-5 mm.
3. Ovulatary phase:
   1. It is the shortest phase of menstrual cycle.
   2. 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.
   3. Rapid secretion of LH by positive feedback mechanism causes the mature follicle to rupture.
   4. Ovulation may be accompanied by mild or severe pains in lower abdomen.
4. Secretory phase / Luteal phase:
   1. 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.
   2. After release of secondary oocyte, remaining tissue of graafian follicle transforms into corpus luteum under the effect of LH.
   3. 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.
   4. Under the influence of these hormones, the endometrial glands grow, become coiled and start uterine secretions.
   5. Endometrium becomes more vascularized and thickens up to 8-10 mm.
   6. Inhibin stops secretion of FSH. These changes are necessary for fertilization and subsequent implantation.
   7. However, if the ovulated egg gets fertilized and the embryo is implanted, there is secretion of human chorionic gonadotropin (hCG), which extends the life of corpus luteum and stimulates it’s secretory activity.
   8. Presence of hCG in maternal blood and urine is an indicator of pregnancy. In absence of fertilization, next menstrual cycle begins.