Question

• Explain the different stages of the ovarian and menstrual cycles

Answer 1

Ovarian cycle :
Cyclical changes occurring in the Ovary of the female under the influence of the gonadotropin hormone of the pituitary gland occurring during her reproductive life are called ovarian cycle.
It involves the following phases:

1) Follicular phase:
The follicular phase starts on the first day of menstruation and ends with ovulation. It is regulated by the hypothalamus, the pituitary gland releases follicle-stimulating hormone (FSH). This hormone stimulates the ovary to produce around five to 20 follicles (tiny nodules or cysts), which bead on the surface.

Each follicle has an immature egg. Usually, only one follicle will mature into an egg, while the others die. This can occur around day 10 of a 28-day cycle. The growth of the follicles stimulates the lining of the uterus to thicken in preparation for a possible pregnancy.

2) Ovulation:
Ovulation is the release of a mature egg from the surface of the ovary. This usually happens at mid-cycle, around two weeks or so before menstruation starts.

During the follicular phase, the developing follicle causes a rise in the level of estrogen. The hypothalamus in the brain recognizes these rising levels and releases a chemical called gonadotrophin-releasing hormone (GnRH). This hormone prompts the pituitary gland to produce raised levels of luteinizing hormone (LH) and FSH.

Within two days, ovulation is triggered by high levels of LH. The egg is propelled into the fallopian tube and toward the uterus by waves of small, hair-like projections. The life span of the typical egg is only around 24 hours. Unless it meets a sperm during this time, it will die.

3) Luteal phase:
During ovulation, the egg bursts from its follicle, but the ruptured follicle stays on the surface of the ovary. For the next two weeks or so, the follicle transforms into a structure known as the corpus luteum. This structure starts releasing progesterone, along with small amounts of estrogen. This combination of hormones maintains the thickened lining of the uterus, waiting for a fertilized egg to stick (implant).

If a fertilized egg implants in the lining of the uterus, it produces the hormones that are necessary to maintain the corpus luteum. This includes human chorionic gonadotrophin (HCG), the hormone that is detected in a urine test for pregnancy. The corpus luteum keeps producing the raised levels of progesterone that are needed to maintain the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum withers and dies, usually around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to fall away. This is known as menstruation. The cycle then repeats.

Menstrual Cycle:

It is the cyclical changes occurring in the uterus due to hormonal influence of the ovarian hormone

It consists of following phases:

1. Menstruation: It marks the beginning of the menstrual cycle. it is day when bleeding starts. During this phase, the prepared endometrium of the uterus gets sloughed off along with the unfertilized ovum, blood ad clots, uterine secretions.

It lasts for 3-5 days.

2. Preparatory phase: During this phase, uterine endometrium grows under the influence of the estrogen. It lasts till the ovulation occurs.

3. Secretory phase : It occurs when ovary starts forming progesterone which in turn stimulate the uterine glands to secrete the uterine secretions. It lasts untill the next menstrual cycle.