Now, let's talk about the ovarian cycle.
There are actually three phases within the ovarian cycle.
And they are follicular, ovulatory, and secretory.
What's really great about these phases,
if you think about the words,
it actually tells you what's going on.
So now let's talk about what's going on in the ovary
very specifically,
because it's a very intricate process.
Now the whole goal of the ovary is to release a mature egg.
That's how we get a baby.
But we need to think about,
what happens to make that process go?
Okay, let's break down what's going on in the ovarian cycle.
Do you see that word follicular phase?
We want to focus on the follicle,
and what's going on inside the follicle.
Now the follicle is actually a collection of fluid
and it's a space.
And inside that space, the ovum or the egg
is going to reconstitute, or grow, or mature.
And inside that follicle is where we have the production of estrogen.
Remember that feedback loop with the anterior pituitary
and also with the hypothalamus?
That happens because of what is going on inside the follicle.
Now, FSH is going to predominate in the follicular phase
because we have to make sure the follicle is ready
and we have to make sure the ovum is matured.
But about 24 to 36 hours prior to ovulation,
then we need a message to say, "Pop, the egg is ready."
And that's the role of LH or luteinizing hormone,
which is also produced by the anterior pituitary.
So remember that 24 to 36 hours prior to ovulation,
we're going to experience a surge in luteinizing hormone,
which is going to be the trigger for ovulation.
So now let's talk about the last half of the ovarian cycle.
The luteal phase.
Luteal comes from the word lutein, which means yellow.
So guess what color it is.
It is yellow.
And it makes a small mass of cells that are left over
after the egg erupts from the follicle.
So as the cells collapse, the corpus luteum is formed.
And its major function is to produce progesterone.
And progesterone has an incredible responsibility,
because it is the hormone that stabilizes the endometrium.
Estrogen makes the lining of the uterus fluffy...
for a baby.
And progesterone helps to keep the lining in place.
And it will do that all the way up until about
8 to 10 weeks in pregnancy
when the placenta takes over the function of producing progesterone.
So that's the role of the luteum, the corpus luteum, the yellow body.
Now, if there's a pregnancy,
there is going to be the production of hCG
Human chorionic gonadotropin.
And that hCG will send a message to the corpus luteum that,
"Hey, I need you to keep producing progesterone
until the placenta is ready to take over."
However, in 12 to 14 days,
if there's no production of HCG, and no baby,
then the corpus luteum will degenerate
and the progesterone levels will drop.
And that drop in progesterone sends a message back to the hypothalamus
that I need you to start producing GnRH
and then begin this cycle all over again.
That is the ovarian cycle.
Now, let's look again, at those two specific neurotransmitter hormones,
LH and FSH.
And I want you to really see how those levels change
over the course of the menstrual cycle.
So on this graphic, you'll see days
1 through 28 of a menstrual cycle.
And you'll see that at the beginning of the cycle,
the LH and the FSH levels are low.
But as the ovum and the follicle mature,
you start to see those levels go up.
Now remember, what happens 24 to 36 hours prior to ovulation?
We get a surge of...?
LH. Exactly. So now you see it.
That's a trigger for ovulation.
And after ovulation.
Well, we don't need any more LH
and we certainly don't need to stimulate the ovary anymore.
So those levels are going to drop.
And they continue to drop until we get to the end of the cycle.
And if you can see sort of at the end,
the FSH level is starting to go back up again,
that comes from the drop in progesterone.
So this is how that fits together in the ovarian cycle.
Now, let's take a look at progesterone.
So, you can see again, in the follicular phase,
the progesterone levels are low,
because who is responsible for producing progesterone?
The corpus luteum.
And it's not there until after ovulation.
So you see around day 12 to 14 ovulation occurs,
and then progesterone levels begin to rise.
And if there's no pregnancy, and there's no message
from the conceptus back to the corpus luteum,
those progesterone levels will then begin to fall.
And you can actually see it in this graphic.
And that decrease in progesterone is the signal back to the hypothalamus
that it's time to release GnRH again.
Let's look at the final hormone, estradiol.
And it's not the least important because we talked about it last.
So don't get it confused.
So, estradiol, again, is a type of estrogen.
And I want you to look at where it is on the graph
and how it changes over the course of the cycle.
So, at the beginning of the follicular phase,
then we see that estradiol levels are low.
As we get towards ovulation,
we want to make sure that the endometrium is nice and fluffy.
Estrogen and estradiol make things fluffy,
so the levels are going to begin to go up.
And then after ovulation, they're going to drop down a little bit,
and that's because of that feedback loop.
We don't want to produce any more mature eggs.
So that level is going to drop.
And then we're going to find that estrogen levels begin to go up
and then drop again.
So, if we have to think about the hormones that predominate
in which part of the cycle,
what do you think predominates in the first half
when we talk about estrogen and progesterone?
Estrogen, exactly.
What about the second half, which hormone is the highest?
Progesterone.
Perfect.