How we’re going to get
molecules both across a cell,
across a vessel, and
across a tissue?
This is going to be very helpful
for reabsorption, absorption, and
then also delivery of substances
from the blood to various tissues.
The surface of epithelial
tissue is very important.
It has a number of modifications
that have been made
to it to increase its
likelihood to do a good job.
These apical membranes, and the apical
membrane is the membrane closest to the tube.
This oftentimes has been modified
to include more surface area
and how it is included more
surface area is by projections,
things like villi
even having more invaginations
associated with the membrane
allows for a greater surface
area within a given volume.
The other way that there can be
apical modifications is via cilia.
These sometimes will be motive in nature,
meaning that they will want to help
something move along like the mucociliary
elevator in the respiratory tract,
or sometimes it is sensory in nature
like what is in the renal epithelial
tissue in the distal convoluted
tubule next to the macula densa.
The lateral aspects of epithelial tissue are
as equally or maybe even more important.
It’s hard to think about the junctions
between cells as being important.
They just seem like they are there
to hold themselves together.
But this can be an important process to
move solutes through various tissues.
They form something
called tight junctions.
Tight junctions can either be tight or
tighter and they have high electrical
resistances and help you establish
gradients across the epithelial tissue.
There are also leakier
and I know the wording is a
little bit confusing here.
They are called tight junctions,
but some tight junctions
are not as tight as others,
so we call those leakier.
The ones tight junctions that
are tighter we call tighter.
So leakier tight junctions have a
lower electrical resistance and
they oftentimes are there to help
isosmotic movement of fluids.
The lateral aspects also have
some anchoring junctions.
Now, these are the ones
to help hold it together.
They have cell-to-cell
which involves these
adhering junction and
desmosomes, as well as
having gap junctions.
Gap junctions help two cells
communicate between each other
across this distance of
having both plasma membranes.
The basolateral membrane of epithelial
surfaces also can have surface
area enhancing structures such
as invaginations or infoldings.
But they are very important to be
able to hold tight to something else,
usually, from the epithelial
surface to another cell surface.
They use hemidesmosomes for
this and focal adhesions.
Epithelial transport allows
for multiple ways in which
you’re going to get across
that membrane surface.
You can either move
between a cell
by those lateral mechanisms and
that’s known as paracellular flow,
or you can move from the apical
membrane to the basolateral
membrane and that is
called transcellular flow.
It is good to look at an example
of these two types of flow.
Let’s look at the flow
of water or fluid flow.
You could travel through an
aquaporin in the apical membrane
then travel through a second aquaporin
in the basolateral membrane.
This would be a good example of a
transcellular flow through aquaporins.
Another way to get water
through from the apical
membrane to the basolateral
is between a cell,
and this paracellular flow can
happen in leakier tight junctions.
Epithelial cells also need to be thought
of as having two different polar ends.
How we will discuss this is
first talk about a symmetrical
cell that has transporters all
the way through that cell.
And in this case you might have
things like potassium channels,
sodium channels, and a
They are located throughout
the cell or around that cell.
In contrast, epithelial cells have
their transporters polarized.
So, you will have the sodium-potassium
ATPases on the basolateral membrane.
You might have some channels like potassium
channels also on the basolateral membrane.
But then you’ll have
maybe at the top, like
these sodium channels.
So you concentrate your channels on
different sides of the membrane.
This allows for the transport to
occur from the apical membrane
into the cell and then out
on the basolateral membrane.
So this is a good example to think
about where the transporters are.
If you have transporters that
are like epithelial on one side
versus the other, it creates
something called vector transport
and this is moving something
across a cell layer.
So where would you want to move
something across a cell layer?
Well, here, it might be to transport,
let’s say, food stuff from the gut
or from the GI’s tract
into the interstitial fluid
to be picked up by the blood.
You also sometimes generate voltage
differences across the epithelial cell
because some of the
transporters are located on
one side and a different
set is on the other.
Sometimes they won’t transport things at
the same rate or maybe certain ions cross
that are either
positive or negative,
and that sets up a gradient of
positive and negative charges.
Remember, voltage differences
are very important and
they are set up by these ion