On the right-hand side of this slide is an
electron micrograph taken of this brush border.
It consists of numerous microvilli. A microvillus
is labelled. Each microvillus has a central core
of actin filaments. And those actin filaments
interact with other actin filaments running
parallel to the apex of the cell.
There's also myosin filaments running along
the apex of the cell. And when this contracts,
because it's intermingled with the actin
filaments, it causes the microvilli to slightly
spread apart. And this allows a greater surface
area of the microvilli for absorption.
Their main role is absorption, and that's the
main role of these enterocytes in the small
intestine. The diagram on the left-hand side isn't
meant to be too detailed and too descriptive
for a histology course. So don't panic about
the content. The reason why I've put it
here is to just emphasize how important these
microvilli are in the absorption of proteins,
and also later on, the absorption of lipids.
If you look at the diagram, you can see it
explains the breakdown of proteins in the
lumen of the intestine by pancreatic enzymes.
And those proteins, when broken down, are
then taken into the surface of the microvilli
by attaching to a certain carrier of proteins
and other structures. And then they get internalized
into the cell, at the apex of the cell, and
they move through the cell to where they
finally dealt with inside the cell cytoplasm.
So again, let me stress not to worry about
the details of the physiology described here,
but it's just to emphasize how important
these microvilli are in the absorption of
proteins from the lumen of intestine.
On this slide, you can see an illustration
on the right-hand side of how important these
microvilli are in absorbing lipids. This description
of the process is labelled one to five of this
absorption process, and the numbers coincide
with various stages through the cell on the
right-hand side. Again, I don't want you
to get too involved with the details here.
It's physiology. But I just want to stress
how important again these microvilli are.
The lipids are broken down in the lumen of the
intestine by bile salts, and also pancreatic
lipase. They are then diffused towards the
microvilli where they're taken in through
the microvilli. You can see them as little
yellow representations in this diagram.
And they get internalized also in the apex of the cell.
And that's labelled 2 on the diagram.
They then move towards the Golgi apparatus where
they're turned in or transformed into chylomicrons.
And also in the Golgi apparatus, again labelled
3 here, they're coated with a membrane.
And then they can diffuse to the basolateral
surfaces of the enterocytes. And the membrane
that binds these chylomicrons and the content
then fuses with those lateral borders of the
cell, and so the chylomicron is released into
the intercellular space. They can't leak
back into the lumen again because of the junctional
complexes towards the apex of the cell, as
explained in a lecture on epithelia that I
gave in this histology course.
So, the only place for them to go is to go
into the basolateral component and then diffuse
into the interstitium behind the cells where
the capillaries are and where the lacteals are.
Down the bottom of the diagram, you can see
a lacteal in the villus of the duodenum.
These lacteals are blind-ended vessels that
absorb all the excessive interstitial fluid,
and they eventually form a hall network called
lymphatic vessels. And they finally drain
back into the venous system up towards the
neck and shoulder region. This is where these
chylomicrons and where lipids make their way
to be absorbed back into the body. They go
into these lymphatic vessels, these lacteals,
and find their way into the vascular system
through this way. Here again you see
diagram on the left, and
on the right, it just illustrates again these
microvilli, but also the junctional complexes.
Hard to see in the electron micrograph but
they're at the apex of the cell. So, all
the movement of these digested lipids passes
down the lateral borders as I described in
the previous slide. This slide shows again
the diagram illustrate the absorption
of lipids across the enterocyte on the left-hand
side. But it also shows in the right-hand
side a light microscope picture taken through
a villus. You can see the brush or striated
border on the enterocytes, and you can see
a lacteal shown in the middle of the villus,
in the lamina propria. These lacteals
are blind-ended tubes that
collect all the interstitial fluid or excessive
interstitial fluid and return that fluid along
lymphatic channels, lymphatic vessels to finally
deposit that content, that lymph into veins
up in the shoulder or neck region. These lymphatic
vessels, therefore, transport the lipid that's
absorbed across the microvilli into the cell
and then pass out of the cell in the basolateral
borders, and then find their way through the
interstitium into these lacteals or lymphatic
vessels. That's how lipid is absorbed and
returned back into the vascular system.