Now, we see a very large mature follicle,
a secondary follicle on the left-hand side.
You can see the oocyte within it. And on the far
right-hand side, you can see a huge follicle.
These are taken at different magnifications.
And the mature follicle or the Graafian follicle,
as it’s called that is about to ovulate,
is very very large. It can be up to 10 or
even 20 microns in size. It’s a huge structure.
And because it’s so huge, you don’t often
see section through the oocyte as you do
on the left-hand side. It’s just a chance
that when you section some of these large
follicles, that you actually see the egg as
well, or the oocyte. Now on the left-hand side,
the secondary or the large mature follicle,
it’s gone from a secondary perhaps to the large
follicular stage, you can see its accumulated
fluid that’s in that space. It’s called
the antrum space. And the antrum space actually
pushes all the granulosa cells add into the
periphery. Look at the oocyte. You can see
the clear stained zona pellucida around it.
The oocyte is bright pink stained in this
section. The cluster of granulosa cells around
the oocyte are a group of cells we term the
corona radiata, the crown around the oocyte.
This group of granulosa cells are going to
persist even after ovulation. And the very
inner layer of these corona radiata cells
actually penetrates microvillus projections
through the zona pellucida and contact the
microvilli of the oocyte. So there is communication
between the two. There's interactions and
control mechanisms going on. And then notice
that the oocyte and the corona radiata are
attached to the other cells around the periphery
of the follicle by another mass of cells.
That’s called the cumulus oophorus. So, here’s
the antrum. There’s the cumulus oophorus
that I’ve referred to, and the corona radiata
is shown around the oocyte. The Graafian follicle
is shown here as I pointed out, but I just
want to mention a little change that’s occurring
just prior to ovulation. If we can imagine,
we could see the oocyte inside this Graafian
follicle, I want to just describe a few changes.
Firstly, during the follicular phase, as I
said before, around the sixth day, waves of
follicles are stimulated to go through this
growth phase. But only one eventually is going
to eventually ovulate. The rest, as I said
before, undergo degeneration or atresia. Something
happens by chemically inside these follicles.
And I just want to summarize a very important
concept. Have a look at the membrana granulosa
in this large secondary follicle on the left-hand
side. Recall that those follicular cells,
those granulosa cells are secreting estrogens
under the influence of FSH. They’re secreting
those estrogens because they get androgens
from the theca interna under the influence
of LH. Well, what happens just prior to ovulation?
One of these follicles, or perhaps a couple
of them, actually starts to get receptors
for LH in the granulosa cells. Those granulosa
cells up until now have only had receptors
for FSH, which controls their production
of estrogens. Suddenly, about 12 hours before
ovulation, or maybe even a little bit earlier,
they acquire, or some acquire the ability
to have LH receptors on their cell membranes
as well. And that’s extremely important for
two reasons. One is one of these follicles
is going to get more of these receptors than
the others. And therefore, they’re in a position
to respond first or best to the LH surge that
we see at the time of ovulation that induces
ovulation of one of these follicles. So the
dominant follicle, if you like, the one that
has the more numbers of LH receptors, can respond
to that LH surge, and therefore, ovulate.
But another thing also happens. Those LH receptors
are also very important for the eventual destination
of this Graafian follicle once it ovulates.
Those LH receptors are going to be extremely
important on these cells for those cells to
convert to produce progesterone after ovulation.
And therefore, we enter the progesterone or the
luteal phase of the ovarian cycle. And some
of that progesterone gets produced by these
cells just prior to ovulation. So those two very
important concepts are very important to understand
with regard to responding to the LH surge
ovulation, and then finally, being out to
produce progesterone in the corpus luteum,
which is what this follicle will develop into
after ovulation. Again, you can see the theca
layers here. They’re becoming more engorged with
blood which assists in also the ovulatory process.
But also, at the time of ovulation, these
theca layers, the theca interna anyway, is
going to penetrate into the ruptured follicle.
Up until now, follicular cells do not have
a direct blood supply. They receive all their
nutrients by diffusion from the theca interna
into all the cells of the granulosa. But after
ovulation, as we’ll see this theca interna
layer, the theca interna, the very vascular
layer you can see here quite prominently,
will invade the developing corpus luteum, and
convert the corpus luteum into an endocrine
organ. As I mentioned earlier, a lot of the
follicles undergo atresia degeneration.
And you see certain signs of atretic follicles
in the ovary. The one on the left, you can
see that the cells just don’t look as pretty
as they do in a normal mature or developing
follicle. There's a lot of cells that are
lost and moving into the antral space.
If you look on the right-hand side on the very
right bottom corner, you see these little
sharp pink structures, shiny pink structures
found in the ovary. These represent the zona
pellucidas that have remained after degeneration
of follicles of certain sizes. And again, on the
right-hand side of the right-hand section, you
can see a rather elongated pink structure.
That’s called the glassy membrane because one
thing that happens during atresia is that
the basement membrane separating the theca layers
from the granulosa layers of the developing
follicle hypertrophies. And during atresia, it
gets thicker and thicker, and rather amorphous
in staining, and we refer to it as being
a glassy membrane. These are all evidence
of follicular atresia.