Menstrual Cycle

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From the course by Duke University

Introductory Human Physiology

1580 ratings

Duke University

Introductory Human Physiology

1580 ratings

In this course, students learn to recognize and to apply the basic concepts that govern integrated body function (as an intact organism) in the body's nine organ systems.

From the lesson

The Reproductive System

Welcome back! This module continues our discussion of the endocrine system and its control of homeostasis. In this series of videos, we consider how the endocrine system regulates the production of gametes (egg and sperm) in the female and male, respectively, as well as the production of the sex hormones. The things to do this week are to watch the 4 videos, to answer the in-video questions, to read the notes, and to complete the problem set. It will be most effective if you follow the sequence of the lectures on the reproductive system. Again please consult the notes for a more detailed summary of each topic. If you have not tried the problems sets, please do so. They will reinforce your understanding of the newly learned information by applying it. Since these problem sets are delivered in the same electronic format as the exams, by completing them, you will gain confidence in using the electronic test format.

Hypothalamus-Pituitary-Gonad Axis18:45

Menstrual Cycle16:35

Meet the Instructors

Jennifer Carbrey
Assistant Research Professor
Department of Cell Biology
Emma Jakoi
Associate Research Professor
Department of Cell Biology

Hello. We're going to finish the female

reproductive system.

We're almost there.

We now understand how the cycle of the ovary occurs, but now to understand

the full menstrual cycle we've got to understand what happens with the uterus.

So that's what we're gonna start to talk about.

First, we're gonna just talk about the ovarian cycle a little bit more.

Where we see the different phases of the follicle in the ovary.

As well as the different amounts of estrogen, progesterone, luteinizing

hormone, and follicle-stimulating hormone that occur throughout the cycle.

The menstrual cycle is going to start with menstruation,

the first day of menstruation,

because that's the only day when it's obvious where you are in the cycle.

So that's day one of the cycle.

At the beginning of the menstrual cycle,

have just a small increase in FSH and luteinizing hormone.

That is going to cause the follicular phase of the cycle,

which is when the follicle starts to mature or continues to mature.

Remember the first part is not dependent on GnRH or LH or

SFH, it's the second part of maturation.

So that's what's going to start because we have a little bit of

an increase in LH and FSH.

We're gonna have our follicles maturing, making more and more estrogen.

We're gonna have our chosen follicle making a lot of estrogen

enough to now have a small decrease in LH and

FSH production that's going to kill the other follicles.

And then, finally, we're gonna have the maturing follicle large enough that it

produces this large rapid increase in estrogen that acts on that

second group of kisspeptin neuron to cause the LH surge.

So that's what's shown in green right here.

This LH surge, which triggers ovulation,

the release of the egg, and causes corpus luteum formation,

where the granulosa cells and theca cells that remain

are gonna continue to make estrogen, as shown right here,

but then also increase dramatically the amount of progesterone made.

That will continue for a certain amount of time,

[COUGH] until eventually, the corpus luteum is gonna degenerate,

and when it does that, it's going to stop producing its hormones,

estrogen and progesterone will fall dramatically.

If they fall, or when they fall, decrease in levels,

then that allows a small increase in FSH and LH levels.

Which is what we said happened at the beginning of the cycle

to start another round of follicles to mature.

So that's what happens at the end, and that's why,

at the beginning of the cycle we have the small increase in FSH and

LH, which causes a whole another cohort of follicles to start to mature.

Because the corpus luteum just degenerated, which means estrogen and

progesterone levels fell.

GnRH went up.

Now let's add in the uterus.

So we're looking at the same figure,

it's just we've added the uterus to the bottom of the figure.

Where we said we're gonna have this drop in estrogen and

progesterone cuz the corpus luteum says, okay, we didn't get pregnant.

We'll have to try again next time.

We'll degenerate.

And as a result of that drop in estrogen and

progesterone, remember the ovary in controlling the uterus.

So it's that drop in estrogen and progesterone that is the signal, okay,

we didn't get pregnant.

Uterus, you need to start with a clean slate, almost literally,

and so menstruation happens when the lining of the uterus sloughs off.

So, that's what's happening for roughly the first week of the cycle is that

didn't get pregnant, the corpus luteum degenerates, estrogen and

progesterone fall, that's the signal to the uterus it's time for

menstruation, and so the lining of the uterus,

which is what's shown in this bottom figure, sloughs off during menstruation.

Then, during the follicular phase, or later during the follicular phase,

we've got this increase in estrogen coming from the chosen follicle.

So estrogen, by itself, will cause

the proliferative phase of the uterus.

Where now menstruation has occurred and

we need to build back up the lining of the uterus, and so the estrogen coming

from the chosen follicle is what causes that proliferative phase of the uterus.

So that it can build itself back up.

Then, in the second half, in the luteal phase when we've got the corpus luteum

active, we've got estrogen and progesterone.

That causes the secretory phase of the uterus

where it's still

getting taller, as you can see in the diagram.

But it's also starting to secrete substances.

So basically, the lining of the uterus is getting ready to

possibly receive an forming embryo.

So the estrogen, and very specifically, the progesterone

are gonna be needed to put the uterus into the secretory phase to begin

the final stages of preparing for the implantation of an embryo.

So, that will continue until if there is no pregnancy, the degeneration

of the corpus luteum and sloughing off and menstruation all over again.

So you can see how the hypothalamus and pituitary are controlling the ovary,

and the ovary is what is controlling the uterus and determining what phase it's in.

So what happens if we do have a pregnancy then the embryo that

is going to be starting to implant in the uterus that

is has been prepared because of the secretion of estrogen and progesterone,

the embryo will make a hormone called human chorionic gonadotropin,

or hCG, which is very similar to LH.

It's going to bind LH receptors, and

remember LH is what formed the corpus luteum, and so

hCG will bind those receptors and maintain the corpus luteum.

It will remain to produce estrogen and progesterone.

And if estrogen and progesterone remain elevated then the uterus is gonna

remain in the secretory state, we're not gonna have menstruation and

GnRH levels are gonna remain low as well.

So we will not start a new cycle as long as we've got hCG being produced,

which maintains the corpus luteum, maintains estrogen and

progesterone so we don't have menstruation and GnRH remains low.

HCG is what will spill out into the urine and

is what is measured in an at home urine pregnancy test, or

it can also obviously be measured in the blood.

And we'll see in this slide, how hCG levels rise

dramatically at the beginning stages of pregnancy, so

these are hormone levels at different months of pregnancy,

and you can see 0 is gonna be the first day of menstruation

So about a month when finally the embryo is implanting and

now really making lots of hCG you can see how dramatically it increases.

And then it also dramatically decreases at about the end

of the first trimester at three months pregnancy.

And as a result of hCG no longer being produced,

there corpus luteum will now start to degenerate, so

instead of degenerating at the end of the menstrual cycle,

is now degenerating at the end of the first trimester, so that the estrogen and

progesterone will no longer be coming from the corpus luteum.

That's okay.

You can see estrogen and progesterone levels continue to rise throughout

pregnancy, because now out here, it's starting to be made by the placenta.

And so, at the end of that first trimester,

there's a shift from estrogen and progesterone being made

by the corpus luteum to it being made by the placenta, until at delivery,

when we're gonna have a rapid decrease in both hormones with delivery of the baby.

Let's go back to thinking about some of our estrogen and

progesterone response of tissues like the uterus,

and the characteristics of estrogen and progesterone.

So remember, we said that estrogen alone is going to make

the uterus be in the proliferative phase, where it needs to build back up.

That's going to be in the first half of the menstrual cycle.

And when this happens and

we have estrogen by itself, it acts to increase its own receptor.

So again, it's gonna be this positive feedback action.

Where you have estrogen, it's gonna cause an increase in the number of estrogen

receptors which means the tissue is more sensitive to estrogen.

It's also, estrogen is also going to increase the number of

progesterone receptors which we call a process called priming.

Because at the time we don't have progesterone around, but

soon, once the corpus luteum forms, we will.

And so, it's getting the tissue ready so that it will be nice and

sensitive to progesterone once progesterone is made.

And so, in this way, estrogen acts to cause proliferation

rapidly, by making the tissue more and more sensitive to it.

And then it also prepares it for progesterone,

which by causing expression of progesterone receptors.

Now, you need to stop this somehow,

because you don't wanna keep proliferating.

And so progesterone is gonna be important, because it's going to act to reduce

the number of estrogen receptors, to stop that process, since we have this

positive feedback of estrogen increasing the number of estrogen receptors.

Progesterone's gonna stop that by decreasing the number of estrogen

receptors.

So, this shows why giving estrogen only can be possibly

cause something like cancer of the uterus, of the lining of the uterus,

if it's without opposition from progesterone.

If you have estrogen by itself, that can be a dangerous things in terms of cancer

of certain tissues like the uterus, which are estrogen responsive.

You need to have some progesterone around to decrease the number of

estrogen receptors.

Let's move now to talking about oral contraceptives, where we are going to

reduce the amount of GnRH in order to prevent pregnancy.

The first birth controls were widely produced in the early 1960s.

The first one was called Enovid and

it was composed mostly of progesterone with a little bit of estrogen.

The estrogen originally was there as a contaminant, but

once they realized that and then took it out, they realized that to have a little

bit of estrogen there was good, in terms of some of the side effects it prevented.

So the pills were mostly progesterone with a little estrogen.

But the progesterone will reduce the amount of GnRH

produced and will prevent the LH surge.

In the 1980s, there was a big change with birth control pills because they started

to be made with synthetic hormones which allowed them to be

much smaller doses of hormones because they had a much higher half life.

So these pills in the 80s had 10 to 20 fold

lower levels of hormones than the earlier pills,

and caused fewer side effects as a result.

So, birth control pills, as I said are going to act to keep GnRH low,

it's gonna act to prevent ovulation by preventing the LH surge, but

they also have other effects that are very important as well in preventing pregnancy.

One is to alter the mucus in the cervix which is the first part of the uterus

that makes the mucus sticky so that sperm can't enter the uterus.

So that's another very important effect of the birth control pills.

And then there are also other changes in the uterus and

oviduct that make pregnancy less likely as well.

So it's just an important illustration of, basically,

the negative feedback nature of this system.

Just like we've seen with so

many of these hormonal regulation or regulatory systems in the body.

Where if you add extra hormone you're gonna depress the axis.

So we've talked about now, we've got the follicular phase where we're

gonna have the uterus in the proliferative phase because we're producing estrogen.

And then in the luteal phase when we've got the corpus luteum making estrogen and

progesterone, then the uterus is gonna be in the secretory phase so

that it's ready for implantation of an embryo.

If we do have implantation, then we're gonna have hCG from the embryo,

which is gonna act very similarly to LH, to maintain the corpus luteum.

That's gonna last for the first three months, but

then after the first trimester, the corpus luteum is going to degenerate.

And because hCG levels are gonna fall, that's okay because

the placenta is gonna take over producing estrogen and progesterone.

And then we finished up talking about how oral contraceptives

are going to reduce LH and FSH release by repressing the H-P axis.

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