(A) The menstrual cycle is a series of cyclic physiological changes that occur in the female reproductive tract of primates. The cycle typically lasts for $28$ days. The end of the cycle is marked by the breakdown of the uterine endometrium,which is released as blood and mucus through the vagina; this process is called menses.
The hormones that regulate the menstrual cycle include Follicle Stimulating Hormone $(FSH)$,Luteinizing Hormone $(LH)$,estrogen,and progesterone.
$1$. During the follicular phase,the levels of $FSH$ and $LH$ secreted by the anterior pituitary gland increase. $FSH$,stimulated by the Releasing Hormone $(RH)$ from the hypothalamus,promotes the development of primary follicles into Graafian follicles.
$2$. The gradual increase in $LH$ levels promotes follicular growth and estrogen secretion. Estrogen inhibits $FSH$ secretion,stimulates $LH$ secretion,and causes the thickening of the uterine endometrium.
$3$. $A$ surge in $LH$ levels (the $LH$ surge) causes the rupture of the Graafian follicle,releasing the ovum into the fallopian tube (ovulation).
$4$. The ruptured Graafian follicle transforms into the corpus luteum,which secretes progesterone during the luteal phase. Progesterone is essential for maintaining the endometrium for embryo implantation. High levels of progesterone inhibit the secretion of $LH$ and $FSH$,preventing further ovulation.