Hey y'all. Browns back, and this time we're gonna be learning about ovaries. In
this video we're not going to be learning so much about the general
histology of the ovaries, although that's very good, mostly what we need to know
here is that the ovary, which you can see here on the left, is divided largely into
a cortex and a medulla like many spherical organs are, and the
germinal epithelium is out here in the cortex and then the medulla is a highly
vascularized loose connective tissue that's the primary trophic center. That's
about all we're gonna say about the general histology of the ovary. What
we're really gonna focus on is the development of the ovarian follicles.
Females are born with between 700,000 and two million egg cells, and those egg
cells are all surrounded by a layer of epithelium and together the egg cell and
the epithelium make up a follicle. By the time a girl reaches menarche, or the
onset of puberty, her ovaries contain combined about 300,000 what we call
primordial follicles, and these things have been frozen in this state with an
oocyte or egg cell. So let's just draw that. So we've got an oocyte here. So
there's the egg cell. Here's its nucleus. There its chromatin in there for good
measure, and in a primordial follicle, that is these these kind of frozen
follicles in stasis almost, they're gonna be surrounded by a single layer of
squamous epithelial cells, or squamous, however you prefer to say that, I learned
squamous, that's how I say it. So we've got this simple squamous epithelium.
There's our nuclei like so. Probably be more than four, but that's how many I
managed to fit given the size that I drew it. So primordial follicle is going
to have an egg cell or oocyte, C-Y-T-E, and it's gonna be surrounded by
a simple squamous epithelium, and if you're wondering why I'm drawing on the
left, it's because I have photos to show you. So for each of these follicle types
I'll draw a sketch of it first to kind of help orient you to what you're gonna
see, and then we'll see the real deal in micrographs over here. Here is a
primordial follicle. You can see the oocyte or egg cell here, and it's
surrounded by these simple squamous epithelial cell nuclei, and you can
recognize them as squamous, because the nuclei are kind of dashes right they're
compressed. Okay. So these are out here primordial follicles and the primordial
follicles you can see live in little clusters way out in the cortex of the
ovary. So this is the the border of the ovary here, and these are called egg nests.
Now when a little girl child, a little girl infant, is born, as I said she's got
between 700,000 and 2 million of these, and over the course of her early years
some of these are going to degrade in a process known as atresia. So that by the
time menarche or the onset of puberty occurs she'll have about 300,000 of
these, and then every time once her monthly cycle becomes established, once a
month, on average of the...obviously these things vary from woman to woman, but on
average about once a month, follicle stimulating hormone from the pituitary
gland will cause several dozen of these across both ovaries to begin developing,
and the first stage of this as they become a primary follicle. So the big
transition from primordial to primary that's notable...noticeable to a
histologies is the change in the epithelium. So we have our same egg cell
here, there is a nucleus going on, but instead of a squamous epithelium around a
primary follicle what you're going to find
is a cuboidal epithelium. Now all primary follicles start off with a single layer,
so a kind of a simple squamous epithelium, like so. Sorry I said squamous,
I meant cuboidal. The squamous epithelium becomes cuboidal, so this layer of
squamous epithelial cells that was surrounding the oocyte is going to...oh
this is gonna be fun, my pen thinks I'm trying to advance the slide when I tap,
but what I'm really doing is trying to draw nuclei in these cells. Ins't this fun?
Yayyyy technology! Okay. So roughly...there we go, and okay I just need to press more
gently. So when we have a single layer of cuboid epithelial cells this is called a
unilaminar primary follicle meaning that it has a single layer of
cuboidal epithelial cells. Now once this morphological development changes from
squamous cells to cuboidal cells, those cells will continue to proliferate,
they'll divide, and we'll end up with what's known as a multilaminar primary
follicle. So I'm not going to be quite as careful as drawing the layers here, but
we'll have one layer of cuboidal epithelial cells surrounded by another
layer of cuboidal epithelial cells, and yet another layer of cuboidal epithelial
cells. So that this follicle is now multilaminar. I should also tell you the name
of these epithelial cells. So I'm just gonna draw a few more cell...cell
membranes in here. Let's get stuff good. Okay. So this is a multilaminar primary
follicle, and these cells are called granulosa cells. This layer of granulosa
cells or granulosa layer is actually where estrogens come from.
Now this isn't actually where the root hormones are produced. I will talk more
about that kind of towards the end when we see kind of a mature follicle, but
this is so fascinating. Remember that before we began this process of becoming
a unilaterally primary and then a multi laminar primary, our primordial follicles were
just that they were primordial they were non-functional. So when this occurs, when
we go from primordial to primary follicle, a new endocrine gland is being
created. This granulosa layer of the of the developing ovarian follicle, and a
concomitant layer that surrounded that we'll talk about in a bit...in a minute
are brand-new endocrine tissue that didn't exist before, and now they do. So
this woman before was not making any estrogen and now she is, and it's the
development of the of the follicles every menstrual cycle that causes the
development of and maintenance of feminine secondary sexual
characteristics. I think this is really fascinating. So let's look at a
micrograph over here, and we can see both some unilaminar and multilaminar
primary follicles. So right here there's a you know laminar primary. Notice that
the epithelium around the the egg cell, around the oocyte, has gone from squamous
to cuboidal, and then over here we can see a multi laminar primary. We've got
about three layers of granulosa cells surrounding the oocyte, and this will
continue with proliferation of the granulosa cells until you come...you get
to these multi, multilaminar primary follicles. Now this one is almost ready
to transition from being a primary follicle to being a secondary follicle,
because we're starting to see just the beginnings of the hint that some space
might be opening up between these cells. In a primary follicle the granulosa
cells are always pressed right up against each other. In a secondary
follicle however, we start to see some gaps opening up. Okay. So if I draw...this
is the whole follicle. Here is the oocyte, there's the nucleus of the oocyte, and
what we're going to start to see, if these are the granulosa cells, so
we're gonna start to see some gaps open up between them, and these gaps are gonna
fill with fluid. Okay. So the gap is called an antrum, and the fluid inside is
one of my favorite words in all of science called liquor follicular. And as
the secondary follicle develops those spaces will grow larger and larger, and
will eventually end up with a situation where our egg cell is, you know, it's kind
of surrounded. There'll be this very large conspicuous antrum all the way
around, and then we'll have granulosa cells all over here. But even when the
multiple antra, that would be the plural, merge into a single antrum. You can
tell that this thing is still a secondary follicle because the...the egg
cell is still located in the center of the follicle. If you were to draw cross
hairs through this follicle you would see that the egg cell is still in the
center. So here we see an early secondary follicle. So I'll outline the whole thing
for you. Here's the egg cell here, or oocyte, but here you can see these spaces
opening up in between the granulosa cells. This, that I'm outlining right now,
is an antrum. This is going to be filled with liquor folliculitis this is a
fairly early secondary follicle. If I erase my annotations and bring in a
later secondary follicle, once again we have the antra are starting to merge
together to form a single large antrum. So you can see here's our kind of main
antrum here, and then over here we have some smaller antra that will eventually
merge into this larger one, but notice that the wall of folic...or sorry the
over...I can't say words today. The egg cell is largely
central within the antrum. Now that we have an antral follicle, that is a
follicle with an antrum, we can also start to see this layer out here,
this thecal layer, T-H-E-C-A-L, thecal layer, and this is going to be working with the
granulosa cells to produce those estrogens. And like I said at the very
end we'll go into that in a little bit more detail, but you can finally see now
this thecal layer that surrounds the granulosa layer and kind of encases the
entire follicle. So finally our secondary follicle will eventually end up as a
tertiary or graafian follicle. Graafian follicles they're gonna be
large, and you can tell it's graafian, like my biggest indicator the thing that
I look for is where is the egg cell located, because in a mature follicle, one
that's ready to ovulate, the oocyte is gonna be pushed off near the edge.
There's its nucleus there, and it's gonna be surrounded by a layer of granulosa
cells, and then there will also be a layer of granulosa cells wrapping
completely around the follicle, like so. Okay. So here's our giant antrum our
large single antrum. Here's our granulosa layer out here, and around the egg cell
itself is a layer of special granulosa cells called the corona radiata. Corona
as you know probably is Latin for crown, and in addition to the corona radiata
around of the cell, we also have an area down here where the the the egg cell and
the corona radiata are attached to the granulosa cell, but only just. This layer
of tissue right here where the connection between the corona radiata
cells and the rest of the granulosa cells are very tenuous is called the
cumulus oophorus. Kind of an eggy cloud. Cumulus meaning cloud, oophorus meaning egg,
and it is this connection that actually breaks during ovulation. We look at a
micrograph, and I'll have to erase my sketch this go around,
but we're gonna label these structures on this actual micrograph, so we don't
need my sketch much anymore. So if we look. Now this one is not
slapped through the middle of this graafian follicle, this is up towards the
top, so we're starting to kind of see the top of the sphere, but it's still not
deep not bad. It's decent. So here's our egg cell. Here is
the corona radiata surrounding the egg cell. So this area here is the cumulus
oophorus. You can see our granulosa cells out here. So this is the granulosa
layer, and then out here you can see that theca as well, with our single large
antrum. When it's time for ovulation luteinizing hormone is going to be
released from the posterior pituitary...or from the anterior pituitary gland, and
it's going to travel down here and it's actually going to rupture the wall of
the follicle, and this antrum, which has been accumulating liquor follicle for the last two weeks
is going to rupture, and the force of the rupturing will actually sever the
cumulus oophorus connection to the granulosa cells of the corona radiata, and the egg
cell and the corona radiata will be released from the surface of the ovary
in the process of ovulation. Finally the last thing we want to talk about is the
theca follicular. If you have say an antrum, we'll have an antral layer here, and then
you're gonna have your granulosa cells. So your granulosa layer here and these
are you know nice cuboid epithelial cells hanging out doing their thing.
We're also gonna have a theca follicular. And the theca follicular actually itself contains two
layers, a theca interna and a theca externa, which the theca externa is
actually going to connect to the rest of the cells of the ovarian cortex. But to
make estrogens, what actually happens is the cells of the theca interna are
steroid producing cells, and they're going to produce the...the androgen
androstenedione,
which will then be taken up by the granulosa cells and converted into
things like estradiol. So the business of producing estrogen is happening out here
on the edge of a follicle, and you can see it best in an antral follicle,
because there's that well-developed granulosa layer and the well-developed
theca follicular. So, you now...you know how we go from a primordial follicle to a
primary follicle to a secondary follicle and finally to a tertian or graafian
follicle. So that's it for now. The videos starting to go long. Study hard.
We'll see you later.