Re: How is the human menstrual cycle contrilled.

Billy:

The human menstrual cycle is controlled by hormones.  There is a region in 
the brain called the hypothalamus.  The hypothalamus produces a small 
hormone called "gonadotropin releasing hormone" (GnRH).  GnRH travels to 
the pituitary gland (very close to the hypothalamus) in a special series 
of blood vessels called the hypothalamo-pituitary portal veins.  Once GnRH 
arrives at the pituitary, it stimulates special cells to release two more 
hormones; 1) Luteinizing hormone (LH) and 2) Follicle-stimulating hormone 
(FSH).  These pituitary hormones are released into the blood stream and 
are carried to the ovaries.  Each hormone has specific target cells in the 
ovaries and together will cause the follicles containing the eggs to grow 
rapidly and to release the steroid hormone estradiol.  This is called the 
follicular phase of the reproductive cycle. Once each month, LH is 
released from the pituitary gland at a much higher rate than any other 
time.  Usually only one ovarian follicle will be ready to accept this 
higher amount of LH and will be caused to rupture or ovulate.  The egg 
will be released from the follicle and will move into the Fallopian tubes 
of the uterus where it can be fertilized if spermatozoa are present.  The 
cells of the follicle that stayed attached to the ovary after ovulation 
(called the corpus luteum) will be changed and will respond to LH 
stimulation by releasing the steroid hormone, progesterone. This is called 
the luteal phase of the reproductive cycle.  The estradiol and 
progesterone will "feedback" on the hypothalamus and pituitary to ensure 
that LH and FSH are released in the proper amounts.

The lining of the uterus (endometrium) will respond to the estradiol 
released during the follicular phase.  There will be a thickening of the 
endometrium due to increases in blood vessels and in glands found there.  
This thickening prepares the uterus to accept the embryo if fertilization 
occurred and will be the site where the embryo grows and develops into a 
baby.  Following ovulation, the levels of estradiol will decrease slightly 
and progesterone levels will dramatically increase.  Progesterone will 
stimulate the endometrial glands to secrete and will also keep the uterine 
muscles from contracting.  This will protect the developing baby during 
the early stages of pregnancy. If no fertilization had occurred and no 
embryo is present the menstrual cycle will continue because the corpus 
luteum will eventually stop producing progesterone. This will allow the 
pituitary to release more LH and FSH to begin the cycle again.  The rapid 
drop in progesterone will also cause the endometrium to be sloughed 
off,leading to the menses or menstrual period, marking the end of one 
menstrual cycle and the beginning of the next menstrual cycle.  

I have included a link for a web site with diagrams and charts showing the 
levels of hormones for the entire cycle.
 http://www.endote
xt.org/female/female3/female3.htm

I hope this helps...

Dr. David S. Mallory
Professor of Biological Sciences
Marshall University
Huntington, WV