- [Narrator] There's actually two different cycles occurring
during the menstrual cycle.
There's the ovarian cycle, which is dealing with
the FSH and LH hormones.
It's dealing with how the egg's,
the oocytes, are being matured.
But there's also the uterine cycle.
This is controlled by estrogen and progesterone.
This is the one that actually causes the menses
or causes the loss of tissue from inside the body.
So really, the menstrual cycle is a combination
of the ovarian and uterine cycles.
So we start off with the ovarian cycle.
You start with immature follicles, so immature eggs.
They're called primary oocytes.
Well, at this point, they can't do anything.
They're just sitting there.
So as they start to develop
they'll start to develop as FSH and LH increase.
Get a little more estrogen,
a little bit of progesterone in here.
But as they keep developing
they turn into a secondary oocyte.
Now, these secondary oocytes are more mature.
They're getting towards the point of ovulation.
Eventually, a surge of LH
is a cause
in ovulation to occur.
Once the ovulation occurs,
you have some scar tissue left over on the ovary.
That scar tissue is called the corpus luteum.
It's this corpus luteum that can actually produce
the estrogen and progesterone.
As these hormones, that will control the next week of two,
if there's no fertilization.
Or if there is fertilization, the corpus luteum
can keep going for up to three months
producing estrogen and progesterone.
So here's a blown up view of what an ovary looks like.
We'll start with the immature follicles.
Those immature follicles, really they're just sitting there.
The ones that are in stasis, they're waiting.
Then you get your primary follicle.
For the primary follicle, either the secondary follicle.
Secondary follicle, is when the follicle
finally can reach maturation.
Upon maturation, ovulation can occur.
And the remnant, or the scar tissue left over,
is the corpus luteum.
Now, eventually though, that corpus luteum
has to turn into an actual scar.
So once it's done producing hormones
it turns into the corpus luteum
down into the corpus albicans.
So it really just degrades into a actual scar at this point.
Now, the ovarian cycle does have that
little bit of a difference I mentioned
if fertilization does occur.
So what's gonna end up happening is
the tissue around the fertilized egg, or the chorion,
is gonna secrete hCG or human chorionic gonadotropin.
That particular hormone is very specific.
So specific that that's actually what
pregnancy tests are looking for.
So any of the pregnancy tests are looking to detect
levels of hCG in the body.
And that hCG, if it's present because of fertilization,
that's what keeps that corpus luteum, that remnants
after ovulation, going for up to three months time.
So it's really helping to produce more and more estrogen.
More and more progesterone.
All until the placenta can take over.
So that placenta eventually takes over
and then it will produce the progesterone and estrogen
for the rest of the pregnancy.
And the reason it's important
to have your higher levels of progesterone and estrogen
during pregnancy, is those high levels hormones
actually inhibits further ovulation.
So it prevents ovulation from occurring,
therefore preventing any possible fertilization occurring
while a female is already pregnant.
So one of the reasons why,
while a female is pregnant, she can not have
a second or third pregnancy start at the same time.
Now, that's different from having twins or triplets.
This is not having one that, maybe is three months along,
and having a new pregnancy.
That's not possible.
All because estrogen and progesterone are preventing
any possible ovulation.
So we shift into the uterine cycle now.
The uterine cycle is really meant to accept
the fertilized egg.
That's really all it's doing.
Every month, is prepare to try to receive a fertilized egg.
It starts off, the first couple days,
in the menstrual phase.
It's usually the first four or five days.
This varies from female to female.
Estrogen, progesterone, the hormones are dropping.
And this drop in hormone
actually causes the endometrial lining to break down.
That's that inside lining of the uterus
is breaking down now.
As it breaks down, the tissue and blood
at once being removed, so menstruation is occurring.
Then you have a proliferative phase,
this is about days five or six until about
midway point about day 14.
Estrogen and progesterone start to increase again now.
As they increase, instead of the endometrial lining,
that inside lining breaking down,
with more estrogen and more progesterone
the inside lining is starting to build back up.
It's getting thicker and thicker.
It's preparing for the possible implantation
of a fertilized egg.
So at day 14, ovulation.
Does that mean that every female ovulates on day 14, no.
Every female has their own unique cycle.
Some females might have a 27 or 26 day cycle,
some might have a 29 or 30 day cycle, 31 day cycle.
Ovulation might be on day 12, 13, 14, 15.
The thing is, 14 is just the average day
on an average 28 day cycle.
So the actual number is not important,
it's more the mid point throughout the cycle.
Once ovulation occurs
the rest of the cycle, days give or take 15 through 28,
is when that corpus luteum, that remnant
that's left over on the ovary after ovulation,
is what's producing the hormones progesterone and estrogen.
The endometrial lining just continues to build and build,
get thicker and thicker.
Basically what's happening, the uterus is getting ready
to nourish a fertilized egg.
But will there always be a fertilized egg?
No, for sure not.
So if it doesn't happen, if there is no fertilization,
if there's no implantation of an egg,
the cycle comes to an end and it starts all over again.
Right back to day one.
Now, this chart is on the larger side.
Don't worry, we're not trying to go into every little detail
but let's just look at the basic idea.
If you look at the top,
it's dealing with two hormones.
Notice right in the middle how it has that
big upsurge, that's the LH surge.
It's that surge that causes ovulation.
Well, after ovulation occurs
you can look down at the third line there,
you can see that progesterone and estrogen
are on the higher side.
As those hormones stay on the higher side
it shows that the uterine cycle, which is the bottom one,
is thicker at that time.
So really all it's trying to do
is have LH surge, create an ovulation,
increase progesterone and estrogen
to make the uterine lining thicker.
All in an attempt to nourish a fertilized egg.
Now as we start looking at cycles of hormones.
Throughout the entire menstrual cycle,
the ovarian phase and the uterine phase,
there's different hormones that are increasing,
decreasing at different times.
Well, one type of cycle is referred to
as a positive feedback.
Well in a case of a positive feedback
something good's going on.
If something go is happening
then you want it to continue to happen.
So, in an example here, is estrogen keeps increasing,
will that cause the surge in luteinizing hormone?
That surge causes ovulation.
So it's a positive feedback,
more estrogen, more and more and more,
eventually triggers LH which triggers ovulation.
Negative feedback,
well that means something wrong is going on.
Too much of something, too little of something,
something's happening that shouldn't.
A negative feedback is meant to stop a process.
Might be meant to reverse a process.
Basis, meant to undo whatever went wrong.
So in the secretory phase here,
if you keep pretty much steady levels
of estrogen and progesterone
you inhibit luteinizing hormone LH and FSH
follicle stimulating.
So by keeping those levels there
it prevents the other two from being released.
All negative feedback cycle.
So here you can see,
as you have the estrogen increasing, increasing, increasing,
eventually it hits a point where it stimulates
luteinizing hormone.
Well, if you look at the second half of the charts
you can estrogen and progesterone are higher levels
that keeps LH and FSH down.