The female menstrual cycle made ridiculously simple
by Michael Turner
Quick facts and figures
- day of ovulation? → day 14
- what causes ovulation? → the LH surge
- what causes the LH surge?
1. a burst of estrogen production from the follicle on day 12, AND
2. the switch of estrogen from NEG to POS feedback on LH productn
- when is progesterone highest? → Day 21-- from productn by corpus luteum
- when is estrogen highest? → Day 12— a burst secreted by the follicle causes the LH surge
- estrogen: builds up the uterine lining (“growns the lawn”)
- progesterone : maintains the uterine lining (“waters the lawn”)
Overview: what's the point of it?
The basic idea is that every month the female body does 2 things:
1) Offers an opportunity to reproduce by releasing a mature egg into the fallopian tube
2) Prepares itself for a possible pregnancy by creating a environment where a fertilized egg could develop.
- This “environment” is a thick layer of blood and nutrients in the uterus.
Releasing a mature egg, and building up a thick uterine lining ( "endometrium") in which it could develop, are not tasks
which are accomplished quickly and easily. Rather, the body must work over a period of time to accomplish these goals, and
this work is initiated and carried out by the action of certain hormones.
Overview: the 2 keys to understanding the cycle
1) Estrogen and progesterone have two effects in the menstrual cycle:
A)They act on the uterus to cause the building up and maintanence of the endometrium (the blood and nutrient lining)
- The endometrium will never build up on its own. The blood and vasculature only
stick to the wall and build up when estrogen and progesterone act on the cells of the uterus.
B) They inhibit the production of GnrH and FSH/LH ("negative feedback")
- The effect of this inhibition is that follicular growth and development is severly limited.
2) The central, and mysterious, event of the whole cycle is the switch of estrogen from NEGATIVE to POSITIVE
feedback on the release of LH and FSH (days 12-14)
-the surge in LH levels is what causes the follicle to rupture and release the mature egg (this release is called
"ovulation"). Thus, it is said that “the LH surge causes ovulation”.
- this is one of the only examples of positive feedback in the entire body. (FYI- another one is the relationship
between stimulation and sexual arousal. More on that later….(makes you want to read ahead, eh!?)
Description of phase 1: the follicular development stage (days 1-12)
It all begins when the brain decides it's time to start maturing some eggs so that they will be ready to get released (and
hopefully fertilized!) soon. So the hypothalmus sends a signal (GnRH) to the anterior pituitary that says "Release the
Follicular Stimulating H'one and the Leutinizing H'one so that we can start developing an egg!"
The Anterior pituitary replies "Yes sir!" and it releases the FSH and LH just as it was commanded. The FSH and LH then
travel to the ovaries and cause the follicles to start maturing.
As the follicles start maturing, they begin to release estrogen. One of estrogen's jobs is to go to the uterus and to build up the
endometrium…..Why does it make sense that maturing follicles would produce estrogen? B/c by releasing estrogen (and
thus causing the endometrium to build up), the maturing follicles are preparing, in advance, a “home” for themselves in the
event that one of them gets fertilized and needs a place to implant.
So, under the influence of estrogen produced by maturing follicles, the endometrium begins to build up.
One week into this process of follicular maturation, one follicle is annointed as "The Chosen Follicle". All the other follicles
that had been developing under the influence of FSH and LH wither away, leaving just “The Chosen One” to luxuriate in the
bath of hormones. Ahhh…..
As the Chosen Follicle matures, it produces more and more estrogen. Now we see in action the other role of estrogen.
Estrogen, in addition to building up the endometrium, acts on the hypothalmus to inhibit the release of Gnrh, and on the
anterior pituitary to inhibit the release of FSH and LH
- What’s the point of inhibiting FSH and LH?
short answer: B/c you don't want a whole new round of follicular maturation to start.
explanation: Remember--FSH and LH stimulate a bunch of follicles to start maturing…. But you have already
done this! You have ALREADY matured a bunch of follicles, and you have ALREADY selected the one “Chosen Follicle”
for the month. So why keep stimulating more to develop? It’s a waste of energy and follicles. ….So it’s time to “turn down”
the FSH and LH levels. You allow only a trickle--just enough to keep "The Chosen Follicle" developing, but not enough to
start a whole new round of follicle maturation.
Phase 2: the release of a mature egg from the follicle (aka "ovulation") (days 12-14)
As the Chosen Follicle matures, the egg within nears the point of being released into the deep, dark world of the fallopian
tube. Just before the moment of full maturity arrives, and it is time to be released into the fallopian tube, something
mysterious and amazing occurs.…da-da-da-dum…..
Suddenly, the estrogen being released from the maturing follicle switches from Negative to Positive feedback on GnRH and
FSH/ LH!! Uh oh!! Estrogen and FSH/ LH become locked in a wonderful, sensual, POSITIVE feedback loop—each
stimulating the other toward higher levels of arousal (and you thought this topic was boring!)
The almost fully mature follicle releases estrogen, which now stimulates the release of FSH/LH, which stimulates further
maturation of the follicle, which causes it to release even more estrogen.
They are locked in a spiral of mutual stimulation. (As Austin Powers would say: YEAH BABY!!) FSH, LH, and estrogen
levels all surge together. (gggrrrrrrrrrr…)
The climax, I mean the culmination, occurs when the action of extremely high levels of LH (the "LH surge") finally causes
the follicle to rupture and release the mature egg. Thus, it is said that "the LH surge causes ovulation". The LH surge ALSO
causes the remaining ruptured follicle cells to organize into the corpus luteum.
Phase 3: the action of the corpus luteum to maintain the endometrium (aka "The Luteal Phase")
(days 15-23)
Once the egg has been released, it begins its slow journey down one of the fallopian tubes (either R or L) towards the uterus.
Out of the whole month, this is the only window of time in which a woman can become pregnant!!
As the egg makes its journey, it is imperative that the uterine lining contiue to be maintained (since a conception could
happen at any time). But how can this be done, now that the follicle has ruptured and stopped producing estrogen?….
The answer is the action of the corpus luteum!
Remember that the LH surge also causes the leftover ruptured follicle cells to organize into the corpus luteum (or C.L.) Once
the C.L. is formed, it starts pumping out estrogen, and especially progesterone (a new player on the stage). Both of these act
to maintain the uterine lining.
Now that ovulation has ocurred, estrogen switches back to its normal state of inhibitng GnRH and FSH/LH release (so as
NOT to trigger a new round of follicular maturation… Who needs new follicles? We just released one and we’re still waiting
to see what happens to it). Progesterone joins the fray and also acts w/ neg feedback on GnRh and FSH, LH.
So now the whole body waits expectantly-- will the egg be fertilized??!! Will the little spermie survive the rigors of the
vaginal canal and find its way to the mothership??!!
If the egg is fertilized (Yipee!)---> Implantation and maintanence of endometrium
If the egg is fertilized, the newly formed thingy is called a "zygote". The zygote immediately starts dividing, and when it
reaches the uterus, it implants in the endometrium. Now the thingy has enough cells that it gets a new name-- a "blastocyst".
Cells on the outer layer of the blastocyst secrete a H'one called "human chorionic gonadotropin” (HCG), which acts to keep
the corpus luteum functional. Who cares about keeping the C.L. functional? A functional C.L. secretes estrogen and
progesterone. Estrogen and progesterone keep the endometrium from sloughing. Why do we care about keeping the
endometrium from sloughing? Well, now that our little blastocyst is snuggled safely into the endometrium, the last thing
we would want to happen is for the endometrium to slough off and carry him away. That would be terrible! (FYI—this is
how RU 486 causes an abortion).
High levels of estrogen and progesterone also act to inhibit productn of GnRH and LSH/FH. This makes sense; since you are
already pregnant, you don't want to waste energy or risk another pregnancy by maturing a new round of follicles.
Over time, the corpus luteum slowly begiuns to die out, and its estrogen and progesterone secreting function is taken over by
the outer layer of cells of the blastocyst.
As the embryo grows and developes in the uterus, some part of it must continually produce estrogen and progesterone so that
the uterine lining neccessary for growth is maintained. (remember—if estrogen and progesterone levels ever drop, the
endometrium will immediately start sloughing!)
If the egg is NOT fertilized ----> sloughing off of the endometrium ("Menstruation")
Here’s the flow: If the egg is not fertilized, it doesn't implant, it doesn't develope into a blastocyst, and it doesn't secrete
HCG. In the absence of HCG, the corpus luteum soon deteriorates.
As the corpus luteum deteriorates, estrogen and progesterone levels fall off. Falling estrogen and progesterone levels cause 2
things:
1) the endometrium is no longer maintained and therefore it begins to slough off.
- This sloughing off occurs over a period of a few days, during which time the blood and cells of the lining
flow out of the vagina. This “sloughing-off” period of time is called "menstruation"
2) the negative feedback on GnrH and FSH/LH production subsides.
- This, by definition, means that the levels of GnRH and FSH/LH begin to rise. As they do, they start to
stimulate the maturation of a whole new round of follicles. Thus, the cycle begins again. The last egg failed to
be fertilized; so it is time to mature another batch of follicles, pick a “Chosen One”, and see if there isn't better
luck with the next egg.
THE END
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note: Although the stages discussed above always occur in the order given, the time frame over which the cycle is spread
varies from woman to woman. Some have a relatively contracted cycle is which each stage is reached quicker than normal;
while for others the reverse is true.
Thought Questions and scenarios
-a common pregnancy variation: the zygote implants in the wrong place (called an "ectopic pregnancy"). The wrong place
is usually the wall of the fallopian tube ( a "tubal pregnancy"). (Basically the poor little zygote got stuck on the wall of the
fallopian tube before it had a chance to get all the way down to the uterus). Ectopic pregnancies usually end in spontaneous
abortions.
-How does a home pregnancy test work?
Home pregnancy tests look for the presence of human chorionic gonadotropin (HCG) If HCG is present, that means that a
blastocyst implanted in the uterine wall is secreting it. I other words, that means that you are pregnant!
-How does the birth control pill work?
The birth control pill is simply a very high dose of synthetic H'ones that resemble the structure of estrogen and progesterone.
Thus, when you take the pill, you body feels high levels of estrogen and progesterone.
High levels of estrogen and progesterone do 2 things, only one of which is the desired effect of the pill user:
1) the uterine lining is maintained at high levels (as if you were in the luteal phase, or in the phase of blastocyst
implantation).
(since the uterine lining is being maintained, instead of sloughing off, this is why you don’t have your
period when you’re on the pill)
*** 2) the production of GnRH and FSH/LH is inhibited **** (b/c estrogen has neg feedback on these)
- this is the desired effect of the pill user!
-The job of FSH and LH is to stimulate the maturation of a group of follicles. Thus, if no FSH and LH
are present, no follicles mature, no mature egg is released into the fallopian tube, and no pregnancy
can ocurr.
-How does natural planning as a birth control method work?
Natural planning works by mapping a particular woman's cycle, and then avoiding sex for the window of time just before
ovulation (in case it comes early that month), and just after. (As we learned, the period of time just after ovulation is the only
time when pregnancy can occur).
We also learned something else cool. If you do not want to get pregnant, when is the safest time to have sex? ………
answer: right after menstruation, b/c this is when a whole new round of follicles is just starting to develop, and thus
ovulation is at the farthest point in the future.