Menstrual cycle.
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<Li> The hypothalamus releases GnRH (Gonadotropin-Releasing Hormone).
<Li> The pituitary secretes FSH and LH.
<Li> The FSH and LH stimulate follicle growth.
<Li> The  follicles start to make estradiol. Several follicles begin to
grow with each cycle, but usually only one matures.
<Li> Around day 12 estradiol levels rise steeply.
<Li> Rising estradiol stimulates a LH surge by positive feedback.
<Li> The  LH surge triggers ovulation: the follicle ruptures, releasing
the secondary oocyte.
<Li> The  follicle  left  in  the  ovary forms the corpus luteum, which
secretes progesterone and estradiol.
<Li> Rising  progesterone  and estradiol levels stimulate thickening of
the uterine wall, or endometrium.
   10. If   pregnancy  occurs,  the  corpus  luteum  continues  secreting
progesterone  and estradiol to maintain the endometrium; otherwise
it disintegrates and hormone levels drop, resulting in the loss of
endometrial tissue as menstrual flow.
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