question_answer 65)

    What role does pituitary gonadotropins play during follicular and ovulatory phases of menstrual cycle? Explain the shifts in steroidal secretions.

Answer:

                  Menstrual Phase (1-5 days) Endrometrium breaks down the cell of endometrium secretions unfertilized ovum constitute menstrual flow. Progesterone production is reduced Follicular Phase (6-13 days) Endometrium rebuilds, FSH and oestrogen secretion is increased. Ovulatory Phase (14-16 days) Both LH and FSH attain peak level. Estrogen level is also high. It leads to ovulation. Luteal Phase (16-28 days) In absence of fertilization corpus luteum secretes progesterone. Endometrium Thickens and uterine glands become secretary. The menstrual cycle starts with the menstrual phase, when menstrual flow occurs and itlasts tor 3-5 days. It results due to breakdown of endometrial lining of the uterus and its blood vessels. Follicular Phase The menstrual phase is followed by the follicular phase. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.   The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium.Such an endometrium is necessary for implantation of the fertilized ovum and other eventsof pregnancy. In the absence of fertilization, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation.