Menstrual Cycle

The female menstrual cycle typically lasts about 26-28 days in a pre-menopausal, post-pubertal woman.

  • Day 1-5, estrogen falls, FSH rises. Menstrual bleeding begins on day 1 of the cycle and lasts about 5 to 7 days.  This bleeding is due to the signals that the endometrium of the uterus receives prior to day 1 (falling estrogen and progesterone levels) to shed its layer.
  • During the menstrual phase, there is a temporal offset as estrogen levels fall; this allows FSH and LH (Follicle stimulating hormone and Luetinizing hormone) to rise because of the negative feedback that estrogen has on the two  hormones.
  • Into the start of the profliferative phase, LH and FSH are targeting the luteal and granulosa cells within the follicles in the female ovaries.  These hormones are precursors to estrogen production.  One follicle responds more readily and creates larger estrogen production, called the graafian follicle, this follicle induces ovulation.
  • The rise in estrogen production caused by the mature graafian follicle accomplishes two main things:
  1. It stimulates the thickening of the endometrial lining of the uterus to be able to receive the fertilized egg if necessary.
  2. It suppresses further secretion of FSH, and after a certain level of estrogen is produced it will create a positive feedback of LH production known as the LH surge
  • This rise in LH is a sign that ovulation is about to occur and the uterus is going to enter the secretory phase.  The rise in estrogen is also effecting the endometirum by allowing blood vessels to dilate to transport proper nutrients for endomedtrium development of the uterus.
  • Once ovulation of the graffarian follicle has occurred, it becomes a luteum and there is no longer an increased production of estrogen.  Now, the corpus luteum’s main job is to secrete high amounts of progesterone.  The role of progesterone is to keep the uterus “quiet” by not allowing the smooth muscles of the endometirum to disturb the replenishment of endometrium cells.
  • The decreased level of estrogen in the secretory phase creates a sharp decrease in LH production just enough to stimulate the corpus luteum to produce the progesterone (and some estrogen) that is needed for the secretory phase.  Declining levels of LH below the threshold value, ultimately causes thee corpus luteum to degenerate.  This degeneration of the luteum causes the termination of progesterone and estrogen production at midpoint of the secretory phase, which causes the blood vessels to decrease dilation and blood flow.  This decreased amount of blood supply to the endometrium is the ultimate sloughing that is known as menstruation, which marks the beginning of the menstrual cycle again.

Menopause is the period when a woman has exhausted the follicle stock in her ovaries during the late 40s or early 50s.  This signals the end of the fertile phase of a woman’s life.  The underlying cause of menopause is the low estrogen production that remains steady after a woman has her last ovulation.

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