Let's now look and see how epithelial cells
are joined together. Well there are three different
types of junctional complexes and they have
three different sorts of functions. The first
juntional complex is at the very apex of the
cells, and they are called occluding or tight
junctions. They are barrier. They prevent
material passing between the cells. It's
important that the body can control what passes
across or what it is absorbed across the epithelial
surface. And so these occluding or tight junction
prevent material passing through between cells.
As sometimes these occluding junctions can
become a little bit permeable and do allow
things to pass through, but that's controlled
by the epithelial cells in certain locations.
But essentially they present themselves as
a barrier. They are also a barrier to the
migration of components to the cell membrane.
They seperate the apical border of the cell
from the lateral border of the cell. And that's
very important because these molecules in
the apex of the cell and on the lateral surface
of the cell that have very important functions.
So it's important to seperate these two domains.
And one function of the tight junction or
the occluding junctions is to act as maintain
the integrity of these two domains. Lower
down in the diagram, you can see anchoring
junctions and there are two types of anchoring
junctions. Some are called the zonula adherens,
meaning they form a belt like structure around
the cell, the occluding junction do the
same. These zonula adherens are actually attached
to the actin filaments of the cytoskeleton,
and they are linked to the cytoskeleton from
one cell to its neigbouring cell. The other
top of anchoring junction is the desmosome
or macula adherens. These are little spot
junctional complexes, spot weld, if you like
and they are attached to the intermediate
filaments of the cytoskeleton, of one cell
and also to another cell. So these anchoring
junctions link the cells together so that
the epithelium acts as a cohesive unit. And
the remaining type of junction is the communicating
junction or gap junction. These consists of
proteins, connexons which enable things to
pass between the cells, things like anions,
nutrients and various other chemical signals.
So cells can communicate with each other via
these gap junctions. And that's very important
for smooth muscle cells to be able to contract
in a sequential manner, and also cardiac muscle
to contract in a similar sequential manner.