Question

• Explain the different stages of the ovarian and menstrual cycles

Answer 1

STAGES OF OVARIAN CYCLE -

Premenarche ovary-

From birth to menarche, ovarian weight increases steadily due to increased volume of developing follicles and an increase in stroma.

Postmenarchal ovary -

1)At the menarche, hypothalamic maturation leads to onset of cyclic ovulation (i.e release of ovum from the ovary at periodic interval)

2)During each cycle, of approximately a month time, some 10-15 follicles enlarge to become secondary follicles, under the influence of follicle stimulating hormone.

Fluid accumulation occurred in these follicles but only one out of 15 enlarged follicles proceeds to the stage of ovulation, probably it is due to its ability to secrete more of oestrogen than the others.

3) At the time of ovulation, antral fluid escapes and the follicle wall collapses, leading to hemorrhage into the theca interna and this forms corpus hemorrhagicum.

4)After ovulation capillaries from the theca interna invade the rapidly dividing granulosa layer and the clotted blood is replaced with yellowish, lipid rich luteal cells, forming the corpus luteum.

a) if fertilization has not occured, the corpus luteum regressed and eventually becomes corpus albicans.

b)if pregnancy occurs, the corpus luteum continues to grow for several months under the influence of vascular endothelial growth factor and begins to degenerate at approximately the 6th month.

6)ovulation- It is the release of ovum from the ovary at fixed fairly intervals.

STAGES OF MENSTRUAL CYCLE -

Menstruation - It is recurrent monthly discharge of blood from the female genital tract.

It is normally 28 days of cycle.

There is 4 stages-

1)menstrual stage (days 0-4)

If the ovum shed at ovulation is not fertilized, menstruation occurs, which on average lasts for about 3-5 days. During this stage, progesterone and oestrogen secretions fall rapidly secondary to degeneration of corpus luteum.

There is bleeding and shedding of superficial endometrium which occurs sequentially in different parts of the endometrium due to spasm of the spiral arteries for several hours leading to endometrial necrosis.

When the vessels relax, shedding of necrotic endometrium, leakage of blood and release of mucus make up the debris lost during menstruation.

2)proliferative phase or oestrogenal phase or follicular phase(days 0-14)-

It represents the restoration of the epithelium from the preceding menstruation.

The oestrogen secreted from the developing ovarian follicles under the influence of follicle stimulating hormone is responsible for this phase.

The primary follicles in the ovary grow to become a fully mature graafian follicle and simultaneously the endometrium of the uterus regenerates through proliferation.

3)Ovulatory phase (day 14)-

Both luteinising and follicle stimulating hormone attain a peak level in the middle of cycle (about 14th day).

Rapid secretion of luteinising hormone(LH) leading to its maximum level during the mid cycle is called LH surge.

It induces rupture of graafian follicle and thereby the release of ovum causing ovulation.

4)Luteal phase(days 14-28)-

During luteal phase, the remaining parts of the graafian follicle transforms as corpus luteum.

The corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the uterine endometrium. Such an endometrium is necessary for implantation of the blastocyst stage and other events of pregnancy.

In the absence of fertilization, the corpus luteum degenerates into a whitish body called corpus albicans. This causes disintegration of the endometrium leading to menstruation.