Question

Explain the ovarian cycle with its different phases.

Answer 1

The menstrual (ovarian) cycle involves a series of cyclic changes in the ovary and the female reproductive tract, mainly in the uterus.

It is divided into four phases:

1. Menstrual phase:
   The beginning of each cycle is taken as the first day when menses or loss of blood takes place. During this phase, about 45 - 100ml of blood is lost. This phase lasts for approximately five days (average 3-7 days). The blood in the menstrual discharge does not clot due to the presence of fibrinolysin. The menstrual phase occurs when an ovulated egg does not get fertilized and it is thereby shed out along with the menstruum. This process is also referred to as the ‘funeral of unfertilized egg’.
   Changes in the uterus: The endometrium of the uterus breaks down under the effect of prostaglandins released due to decreased levels of progesterone and estrogen. During menses, the blood, tissue fluid, mucus, endometrial lining, and the unfertilized oocyte are discharged through the vagina. Also, the endometrial lining becomes very thin i.e. about 1 mm.
   Changes in the ovary: During these five days, many primordial follicles develop into primary and a few of them into secondary follicles under the effect of FSH.
2. Proliferative phase/Follicular phase/Post-menstrual phase: This phase is the duration between the end of menstruation and the release of an ovum (ovulation). The duration of this phase is more variable than other phases. Generally, it extends from the 5th to the 13th day of the menstrual cycle.
3. Changes in the ovary: Generally, out of 6 to 12 secondary follicles that proceed to develop, only one develops into a Graafian follicle (mature follicle). while the rest of the follicles degenerate (atresia). The stimulation for the proliferation of new follicles is influenced by GnRH which stimulates the release of FSH. The developing secondary follicles secrete the hormone estrogen. Changes in the uterus: The endometrium begins to regenerate under the effect of gradually increasing the number of estrogens. Regeneration also involves the formation of endothelial cells, endometrial or uterine glands, and a network of blood vessels. The thickness of the endometrium reaches 3-5 mm.
4. Ovulatory phase: It is the shortest phase of the menstrual cycle. Changes in the ovary: It involves the rupturing of the mature Graafian follicle and the release of an ovum (secondary oocyte) into the pelvic cavity; usually on the 14th day of the menstrual cycle. Rapid secretion of LH by a positive feedback mechanism causes the mature follicle to rupture. Ovulation may be accompanied by mild or severe pains in the lower abdomen.
5. Secretory phase/Luteal phase: It is the phase between ovulation and the beginning of the next menses. This phase is the longest phase. It lasts for 14 days i.e., from the 15th to the 28th day of the cycle.
6. Changes in the ovary: After the release of the secondary oocyte, the remaining tissue of the Graafian follicle transforms into the corpus luteum under the effect of LH. The corpus luteum releases progesterone, a small amount of estrogen, and inhibin. The ovulated egg may get fertilized within 24 hours. In the absence of fertilization: the Corpus luteum can survive for only two weeks and then degenerates into a white scar called corpus Albicans. In the case of fertilization: The embryo is implanted, there is the secretion of human chorionic gonadotropin (hCG), which extends the life of the corpus luteum and stimulates its secretory activity. The presence of hCG in maternal blood and urine is an indicator of pregnancy. In absence of fertilization, the next menstrual cycle begins.
7. Changes in the uterus: Under the influence of progesterone and estrogen, the endometrial glands grow, become coiled, and start uterine secretions. The endometrium becomes more vascularized and thickens up to 8-10 mm. Inhibin stops the secretion of FSH. These changes are necessary for fertilization and subsequent implantation.