Regulation of the menstrual cycle

Hypothalamus secretes GnRH which travels through the hypothalamic pituitary portal system to act on the gonadotrophes in the pituitary which secrete both LH and FSH which then enter the peripheral circulation and act on the ovaries to make estrogen and or progesterone depending on the stage of the menstrual cycle. Estrogen and progesterone then feedback at the leve of the hypothalamus and pituitary to regulate the production go GnRH and LH/FSH.

the closed loop negative feedback system

• At the beginning of the menstrual cycle the LH and the FSH levels increase
• The increase in FSH causes multiple early antral follicles to grow and these follicles secrete estrogen and inhibin. 
• as a result, levels of estrogen in the circulation increase
• from the cohort of growing follicles, one follicle with become selected and become the dominant follicle. 
• the dominant follicle secretes large amounts of estrogen and inhibin and as a result these increase dramatically. 
• These high levels of estrogen and inhibin feed back tot he hypothalamus and the pituitary to decrease the circulation FSH levels. 
• at the end of the follicular phase, when the estrogen levels have reached their peak, feedback in the hypothalamus switches from negative feedback to a positive feedback 
• the positive feedback of estrogen results in hypothalamic GnRH surge which triggers an LH surge from the pituitary. 
• The high circulation levels of LH causes that the oocyte completes meiosis 1, ovulation occurs, and the granulosa and theca cells because luteinized to form the CL which secretes a large amount of progesterone
• the increased levels of P, E and Inhibin increases during the early luteal phase and remain elevated for about 10 days
• FSH and LH levels are inhibited due to negative feedback of P,E and inhibin secreted by the CL
• Near the end of the luteal phase the CL begins to regress which decreases the amount of P, E and inhibin which results in the loss of negative feedback at the hypothalamus and pituitary and circulation levels of LH and FSH increase and the cycle ends or begins again.

So this is is the time at which the endometrium is shed and usually lasts a couple of days.

It is called menstrual flow and it consists of a mixture of venous blood and endometrial tissue.

During the early follicular phase, FSH stimulates early antral follicles to accelerate their growth and begin producing greater amounts of estrogen and the estrogen from these follicles stimulates the uterine endometrium to proliferate and the thickness of the endometrium increases substantially during this phase. As the dominant follicle is selected, the estrogen levels continue to rise and the endometrial proliferation continues until the midcyle LH surge and ovulation.

• LH surge causes the resumption of meiosis 1 in the pre-ovulatory follicle
• it also causes ovulation of the preovulatory follicle
• corpus luteum formation from the ganulocyte and theca cells.

So in the actual mestrual phase they are rather thin but in the follicular phase the endometrial glands are straight and narrow due to the estrogen and during the luteal phase due to the progesterone the glans are sacculated and tortuous.

• Follicular: thin and watery due to the estrogen
• Luteal: Thick and viscous due to the progesterone

Well the newly formed corpus luteum produces progesterone which transforms the proliferative endometrium into the secretory endometrium in preparation to the implantation of the embryo. The appropriate secretions of the secretory endometroium is important for the normal implant ion of the embryo. In the non-fertile enstrual cycle, the CL produces progesterone for 2 weeks and then regresses. As it undergoes luetolysis its production of progesterone declines and the circulating levels of progesterone along with estrogen and inhibin (the other hormones that the corpus luteum secretes) decline. The loss of progesterone support to the endometrium causes destabilization of th elands and stroma and rsults in sloughing off go the endometrium during menstruation. The decrease int he progesterone, estrogen, inhibin also removes the negative feedback at the levels of the hypothalamus and pituitary and circulating FSH levels increase with the start of the next cycle.