Ovulation occurs because of surge of -

Ans. is 'a' i.e., LH * Ovulation occurs because of luteinizing hormone (LH) surgeHormonal regulation of the menstrual cycle* Menstrual cycle involves the interplay of ovarian, pituitary and hypothalamic hormones.* At the beginning of the cycle, the estrogen level is very low and progesterone level is negligible.* Therefore, the pituitary and hypothalamus are free from the negative feedback effect of estrogen and progesterone.* Hence the hypothalamus secretes GnRH and the anterior pituitary secretes FSH and some LH.* FSH stimulates follicular development.* Ovarian follicles secrete estrogen.* Estrogen tends to suppress FSH secretion, but progressive follicular development ensures a rise in estrogen level.* Around day 11 of the cycle, the estrogen level crosses a certain threshold.* Above this threshold level, estrogen has a positive feedback effect on LH secretion, and to some extent also on FSH secretion.* This leads to LH and FSH surge.* The LH surge is essential for ovulation.* After ovulation has taken place on day 14, the corpus luteum starts developing under the influence of LH.* Corpus luteum secretes estrogen, progesterone and inhibin.* The combined effect of these hormones is to suppress the secretion of FSH and LH by negative feedback effect on the anterior pituitary, and to a lesser extent also on the hypothalamic GnRH secretion.* Absence of support from FSH and LH leads to regression of the corpus luteum.* Regression of the corpus luteum leads to a sharp decline in estrogen and progesterone levels.* Withdrawal of support from estrogen and progesterone leads to menstrual bleeding.* Thus one cycle ends and the next cycle begins.