00:01
Let me just outline the
general structure of the eye. On this
diagram, on the right-hand side, it
outlines most of the structures that I
am going to describe histologically, but I
want to just remind you that on the
left-hand side the embryologist will explain
to you in embryology lectures that the
eye is actually derived from three
different sources. It is developed from
those components of lists that are on the
left-hand side. So we are now going to look at
each of those structures in turn.
00:38
I am going to start with the cornea, the front
of the eye, the anterior part of the eye and go
through the structures in sequence from
the anterior structure of the cornea and
then to the retina.
00:51
First of all, let me just outline the
structures that are actually produced by
the three different sources. On the left-hand
side, the outer layer or tunic
consists of the sclera and the cornea. The
cornea is the clear transparent
anterior portion of the eye. The sclera
is the white portion. It is the very fibrous
tuft, fibrous, outside of the eye that
the extraocular muscles insert on.
01:27
The sclera in young children is quite
thin and has a bluish tinge to it. As we age,
it has a yellow appearance to it and it
thickens. The next inner layer
consists of the choroid, the stroma of
the ciliary body and the iris.
01:48
This is termed the uvia. The choroid is
the vascular supply to
all these structures that I am going to describe.
The choroids is very important because it
provides support and blood supply of
nutrients to the retina, which is a
vascular. And then the most internal layer,
the inner tunic is the retina. And the
retina consists of two parts.
02:20
The neuro part which extends towards the
front of the eye and sees us to be the
neuro part at the ora serrata and then
it extends to cover the ciliary body,
ciliary processes and the iris as a
double epithelial layer which I will
describe later on. The ganglion cells
project their axons from the retina into
the optic nerve and notice in the diagram
on the right-hand side that the
optic nerve is actually covered by meninges,
the dura, arachnoid and pia.
02:53
So we should not call it really an optic nerve,
we should call it the optic tract because
it is actually brain tissue.
03:06
Okay, let me start off as I said, at the front
of the eye, at the cornea. The cornea
is shown in low magnification on the left-hand
side and then increased in magnification
in the middle and also on the right-hand side.
The cornea consists of two cell layers
and three other noncellular layers and
the thickest component is the stroma
labelled here. That corneal stroma is the
transparent part of our eye, the cornea.
03:43
It provides transparency and it does so
because when you look at the fine
structure of the stroma, it consists of
about 60 layers of collagen fibrils
and those layers are arranged in 19
group perpendicular to each other,
layer, on layer, on layer and they're held together
by glycoproteins and collagen type V and
those glycoproteins, those protein glycones
rather and collagen fibre actually hold all
these bundles of collagen fibrils in their
arrays and create this transparency.
04:19
The corneal epithelium the surface
epithelium is the stratified
squamous epithelium as you see in this
slide. The very surface cells, the
squamous cells you see at the surface
have very fine microvillus projections
and those projections help to maintain
tears on the surface of the eye and those
tears protect the eye and they also give
some ultraviolet radiation protection
from those cells within the cornea. Those
epithelial cells do not have melamin
like you see on the skin surface to
protect from excessive ultraviolet
radiation,
but the nuclei have in them ferritin which gives
that protection to the
nuclei from excessive ultraviolet radiation.
The corneal epithelium can
replace itself very rapidly, but it does
not replace itself from cells that
are immediately at the base of the epithelium
layer. They replace themselves by a stem
cell niche or population of cells in the
periphery right next to the junction
of the cornea with the conjunctiva and that
is to protect the cornea from conjunctival
epithelium migrating its way across
and being part of the cornea. The epithelium,
the outer epithelium is supported by Bowman's membrane.
These membranes are very
thick membranes, it is a big thick basement membrane
and once it is damaged it cannot regenerate. On the
other side, the inner aspect of the cornea
facing the anterior chamber is
another basement membrane. It is a thinner basement
membrane. This can repair itself on a regular
basis if required and then the surface cells
the inner surface of the
cornea is lined by a cuboidal epithelium, the
corneal endothelium it is called,
unusual name endothelium really
refers to the lining
of the blood vessel. But here is the special
exception for the cornea. Those cells are very
very metabolically active. They provide
nutrition and support for all
the fibres that make up the cornea. The
cornea is avascular. It is separated from
the body, the vascular system of the body
and therefore it can be used as a
donation in corneal transplants.
07:06
It does not suffer from any immunological
attack and that is why it can be easily
donated and transferred from
one person to another
for a person who is in need of corneal
transplant. The cornea then becomes
continuous with the sclera of the eye
and the junction of the epithelium with
the conjunctiva. I am going to
come back to this image
later on because I am going to describe
the canal of Schlemm, which is a very small
network canal you can see at the very
corner of this image, but I won't
bring it to your attention yet.