Question

(a) Explain the menstrual phase in a human female. State the levels of ovarian and pituitary hormones during this phase.
(b) Why is the follicular phase in the menstrual cycle also referred as a proliferative phase? Explain.
(c) Explain the events that occur in a Graafian follicle at the time of ovulation and thereafter.
(d) Draw a Graafian follicle and label antrum and secondary oocyte.

Answer 1

a) The reproductive organs of female primates undergo rhythmic changes called as the menstrual cycle. The average cycle in human females is about 28-29 days. The start of menstrual cycle at puberty is known as menarche. It involves four phases: Menstrual, follicular, ovulatory and luteal.

Menstrual phase: It forms the first phase of menstrual cycle which occurs for 3-5 days. The endometrial lining of the uterus breaks down during this phase. The unfertilized ovum, as well as the blood vessels, are discharged from the vagina. It leads to the abdominal cramps due to the contraction of the uterus and abdominal muscles. Estrogen and progesterone levels are low during day 1 of menstrual phase which stimulates the pituitary gland to secrete FSH for follicular maturation.

b) The follicular phase is known as the proliferative phase as in this phase the production and growth of follicles take place. During this phase, one of the follicles matures to become Graafian follicle and uterus endometrium lining undergoes regeneration through proliferation.

c) During mid-cycle, there is rapid secretion of LH hormone which causes LH surge leading to the rupture of Graafian follicle thus releasing the ovum by a process called ovulation. During ovulation, only the egg comes out of the follicle while the remaining ruptured follicle remains on the surface of the ovary. Within two weeks the follicle gets transformed into a structure known as corpus luteum, It releases progesterone and small amounts of oestrogen, both of which maintain the thickening of the walls of the uterus for implantation. If pregnancy takes place, the high levels of progesterone are maintained and if pregnancy does not occur then the corpus luteum withers away on day 22. There is a drop in progesterone level and wall of the uterus lining falls off leading to menstruation.

d) An image of Graafian follicle showing antrum and secondary oocyte is given above.