00:01
Let us now look and see how epithelium is
classified or named. Well, the first thing to do
when you look at an epithelium is decide how
many cell layers make up the epithelium. If
there is only cell layer as you see in these
diagrams, we call the epithelium a simple
epithelium. And the second thing we do, is
we look at the shape of the cells. If the
cells are a lot wider than they are in height
or they are flattened, we called it a squamous
epithelium, a simple squamous epithelium.
If the height, the width and the depth of
the cell is about the same, we call it a simple
cuboidal epithelium. And if the height is far
greater than both the width and the depth,
then we call it a simple columnar epithelium.
01:02
Sometimes the epithelial cells or the epithelial
layers may have many more cells, not just
the one cell. In that case, we call the epithelium
stratified, and again we further extend that
classification depending on the shape of the
surface cells. They could be squamous at the
surface, so we call it a squamous nonkeratinized
epithelium or stratified squamous nonkeratinized
epithelium. The surface cells may be cuboidal,
so we call it a stratified cuboidal epithelium.
01:43
Or as we see in skin, the surface cells
may be keratinized, so we call it a stratified
keratinized epithelium. Rarely we may find
an epithelial surface that is stratified and
the surface cells are columnar. In that case,
it is called a stratified columnar epithelium.
02:07
There are a few unusual epithelia such as
the one shown here. If you look at the pseudostratified
epithelium in this diagram, the nuclei of
the epithelium appear to be at different levels
or different heights and it gives you the
impression that that epithelium is stratified.
02:28
But in actual fact, if you look at sections
through this epithelium using electron microscope,
you will see that all the cells sit on the
basement membrane. So although it appears
to be stratified, isn't. It's simple,
so we call this a pseudostratified epithelium.
02:49
And sometimes on the surface of the cells,
there are special lycations that I will talk
about later on. It could be cilia or microvilli.
Sometimes we might call this a pseudostratified
ciliated epithelium. The other two diagrams
show a section through or representation of
the epithelium in the bladder or parts of
the urinary passages. We call this a transitional
epithelium because when the bladder is relaxed
or emptied, the cells adopt a rather stratified
cuboidal appearance. But when the bladder distends
and fills with urine, then the surface cells
then tend to flatten out. In other words,
the epithelium goes through a transition from
one appearance to another depending on the
state of the bladder, hence the name transitional
epithelium. Now here is an example of a simple
cuboidal epithelium. You have seen this picture
before. The cells are both the same height,
width and depth, hence the cuboidal classification.
04:03
Now epithelia really carry out four major
functions. And if you are aware of those four major
functions, sometimes it is easy to assign
those functions to the particular epithelia.
04:17
We saw earlier with the simple squamous epithelium,
the function was very effective transcellular
transport, very thin. Other epithelia such
as the one shown here, the simple cuboidal
epithelium are designed for very efficient
absorption and also secretion of material.
04:43
This happens to be epithelium of the collecting
duct as I pointed out earlier, and these collecting
ducts are very busily both absorbing and also
secreting materials. Here is a simple columnar
epithelium, again specialized for absorption
and secretion. Often when the cell is very
very busy they get very very tall and the
nucleus packs down to the basal area of the
cell. Now if you look at the shape of these
nuclei, they are elongated towards this luminal
surface. When you see elongated nuclei, it
is a fairly good indication that the cells
are columnar, whereas, in the previous slide,
you might have noticed that the cell nuclei
were nice and rounded. When you see rounded
nuclei, you can be pretty sure that the epithelium
is a cuboidal type of epithelium. Well, here is
a stratified epithelium designed to be a barrier
to protect. It's a wear and tear type of
epithelium. It's found in places like the
oral cavity, the vagina, the esophagus, places
where there is significant wear and tear and
the cells are lost as they move towards the
surface. They are rapidly produced in the
basal part of the epithelium and those cells
move to the surface and as they move to the
surface, they change their shape and are lost
to the surface as a result of wear and tear.
06:16
That's the fourth major function of an epithelium
and that is why this epithelium is structured
in this way. Remember the four functions,
transcellular transport, absorption or secretion,
or both, and here wear and tear. Sometimes
you have an epithelial surface such as the one
shown here of pseudostratified epithelium.
And remember that although the nuclei here
seem to be at different heights, all the cells
are sitting on the basement membrane. Well,
this epithelium also has surface specializations
on it that I will explain later on. The surface
specializations are cilia and they can transport
secretions along the surface, foreign bodies
and also cells and certain organs. They are
mainly associated with pseudostratified epithelium
as you see here. You can also see pale secreting
cells in this epithelium. They are secreting
material onto the surface, whereas the darker
stain cells are absorbing. So sometimes in
an epithelium, you can have a number of cells
performing the functions of that epithelium.
07:41
Here is a section through the urinary tract,
the bladder, and on the left-hand side you
can see the cells have a cuboidal type of
appearance. It's very thick epithelium, but
they are flattened on the right-hand image
when the bladder is distended. And at the apex
of the cells, particularly those on the surface,
you can see rather an eosinophilic or a pink or
reddish stain. They are special plaques inside
the epithelial cells that prevent water and
also salt from passing across the epithelial
surface. And that is very important in the
urinary tract and in the bladder because the
kidney goes through a lot of work, a lot of
functions to make sure that we get rid of the
excessive electrolytes from our body such
as salt. We do not want that absorbing back
into the body through the bladder that would
defeat the purpose of our kidneys. So it is
important that in this epithelium they have
the special role of resisting the transport
of water and salt. Often this epithelium is also
called urothelium. That has an example of stratified
squamous keratinized epithelium skin and in
the very top part of the image, you can see
some purple stain material that's keratin.
09:08
It's very thick skin such as we have on the
palms of our hands and the soles of our feet,
wear and tear as I said earlier. I will talk
about skin in a later lecture.
09:21
Well, epithelia in other parts of the body, or in
some parts of the body at least, are given
special names. I am not going through all
the names here. You can read through these
names, but the main important point is that
sometimes you'll come across terms like endocardium
or endothelium or respiratory tract epithelium
or mesothelium or olfactory epithelium. They
relate the very special names we give to epithelia
in certain parts of the body and we will come
across these special names in later lactures.