*numbers displayed in figure does not correspond to the numbers below.

1. Hypothalamus releases GnRH.

2. GnRH stimulates anterior pituitary to secrete FSH & LH.
3. FSH stimulates follicle growth. LH stimulates follicles to produce oestrogen.
4. Cells of growing follicles (theca cells) start to produce oestrogen.
5. When oestrogen secretion by the growing follicles begin to rise steeply, FSH & LH levels increase markedly. (High concentration of oestrogen stimulates hypothalamus to produce more GnRH)
*note: LH increases more steeply than FSH because high concentration of oestrogen increases the GnRH sensitivity of LH-releasing cells in pituitary.
6. The maturing follicle, in response to a peak in LH level (LH surge), releases the secondary oocyte. (Ovulation)
7. LH also stimulates the follicular tissue left behind in the ovary to transform into corpus luteum.
8. Under continued stimulation by LH, corpus luteum secretes oestrogen and progesterone.
9. As oestrogen & progesterone levels rise, the combination of these hormones inhibits the secretion of GnRH by hypothalamus, hence reducing the secretion of FSH and LH to very low levels.
10. Low FSH & LH levels cause the corpus luteum to disintegrate, thus a sharp decline in oestrogen and progesterone levels too.
11. The decreasing levels of oestrogen & progesterone therefore stop inhibition on hypothalamus to secrete GnRH.
12. The pituitary can then begin to secrete enough FSH to stimulate growth of new follicles in the ovary, initiating the next ovarian cycle.
What if fertilisation happens?
Though step 9 occurred (which causes low FSH & LH levels), but HCG produced by blastocyst (which formed from zygote) will take over the role of LH to maintain corpus luteum & stimulate corpus luteum to continue the production of oestrogen and oestrogen.
Hormonal control during pregnancy will be discussed in detail in the next post.

Menstrual cycle? Ovarian cycle?
Menstrual cycle is the monthly preparation of uterus to receive an embryo. It is marked by a discharge of blood from disintegration of endometrial lining of uterus when no embryo is formed. The cycle can be divided into 3 phases: Menstrual (day 1~5); Proliferative (day 5~14, before ovulation); Secretory (day 14~28, after ovulation).
Ovarian cycle is a series of events in ovaries that occur during & after maturation of oocyte. It is marked by the development of Graafian follicle and secondary oocyte and also the formation of corpus luteum. The cycle can be divided into 3 phases: Follicular phase (before ovulation); Ovulation; Luteal phase (after ovulation).
Do other animals have menstruation too??
Most female mammals undergo ovarian cycle.
But only human and certain other primates have menstrual cycle.
Other mammals have oestrous cycle instead.
Human females may engage in sexual activities anytime during menstrual cycle, but other mammals only mate during ovulation of oestrous cycle.
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