00:01
I mentioned earlier when I spoke about the
three characteristics of epithelia that they
have an apex, that they have a lateral border and
they sit on a basement membrane. So these
cells have polarity. Recall from an earlier
part of this lecture, I explained that epithelial
cells exhibit polarity and that means they
have apical surfaces or apical domains. They
have lateral domains and basal domains. Now
if you look at the diagram shown here and
you look very very carefully at the surface
of the epithelium in the section imaged here,
again you can see that on the surface of the
cells, there is a specialization called microvilli,
shown in the diagram. Microvilli are very
fine structures that are one example of the
surface epithelial specializations. So I am
going to now look at the apical borders of
some cells and describe some of these surface
specializations. They are unique to epithelia.
They are different and they have very important
functions. They include microvilli that we
saw previously on this columnar epithelium.
01:33
These microvilli are often called the brush
border, particularly when we talk about tubes
in the kidney or striated border sometimes
when we talk about epithelial cells such as
we see here lining the intestinal tract or
parts of the intestinal tract. Another surface
specialization, which is really like microvilli,
but they are much longer are stereocilia, and
we find those in parts of the male reproductive
system, particularly the ductus epididymidis.But
also you see these very special stereocilia
in the hair cells in the ear, and we will talk
about that more when we look at the ear at
a later lecture. And then, on the surface of the
cells on the right hand side, I showed these
previously in pseudostratified epithelium.
02:31
There are cilia, long hair like structures projecting
from the cell that help to move things along
the cell surface, and sometimes we call this
a pseudostratified ciliated epithelium because
of the presence of the cilia in certain locations
of the body, particularly the respiratory tract.
02:53
Well, let's look at the surface specializations
or modifications in a bit more detail, starting
with microvilli or the microvillus. If you
look at the histological section shown here,
find the elongated nuclei of the epithelial
cells. And then towards the surface, you can
see some pale pink cytoplasm. But right against
the clear lumen shown here, you can see just
a fine darker pink line, that represents the
microvillous border, the brush border or the
striated border. On the right hand side is
a diagram illustrating the structure of these
microvilli or individual microvillus. It has
a central core of actin filaments and those
actin filaments project down into the apex
of the cell and are connected to a parellel
network of actin filaments called the terminal
web. The actin filaments are project up,
the microvillus are kept and linked to the tip of
the microvillous or the tip of the cell membrane
by the protein villin. And myosin also joins
the actin filaments to the cell membrane called
the plasmalemma. And along the terminal web
not shown here, is lots of myosin tube fibres
and they are contractile. And what they do
is that they can contract and while contracting,
they can fan out these microvilli like fingers
fanning out in my hand. And that increases
the distance between microvilli, and therefore
exposes greater surface for absorption because
these structures are specialized for very
efficient absorption of water. They increase
the surface area for that transport. Stereocilia
are very similar. They are longer because
the actin filaments are longer. They branch
as well. And also they use different anchoring
proteins to hold the actin filaments together
into the terminal web and also to the cell
membrane shown here. But essentially they
are similar structures specialized for absorption
of fluid. Let's have a look at cilia. They
are totally different structure altogether.
05:33
Again look at the epithelial surface, you
can see elongated nuclei of the epithelial
cells, some clear staining cells. Well, right on
the surface you can see some little hair like
structures and a little dark line. Those hair
like structures are cilia and the dark lines
you see are the basal bodies of all these
cilia. The cilia are composed of what we call
the axoneme. It's a special arrangement of
nine plus two pairs of microtubules and those
microtubules enable the cilia to be motile.
And the microtubules are inserted into the
basal bodies at the apex of the cell and all
those basal bodies are actually connected
in many ways to each other. And that allows
the cilia to be in unison together in one
direction or the other, and therefore move
along the surface of the epithelium, foreign
bodies, secretory products or even cells.
Let's now look and see how epithelial cells