00:00
So it's important that you understand all
these concepts. First of all, I wish to
describe the histology of the esophagus.
Here is a rather image filled piece of
information, there's two pictures here. One
on the left is a diagram that starts to
explain all the major components that make
up the wall of the gut. And I'm
going to go through this very carefully because,
as I mentioned earlier, it's an
essential feature of understanding the
histology all the way along the
elementary tract. On the right-hand side,
there is a section taken through the
esophagus and then viewed using a scanning
electron microscope. And I want
to take you through this very slowly,
because as I stressed a couple of times
now, it's very important to identify all
the different components of the wall of the
gut and then understand their function.
When looking at sections through a
tube, such as the gut tube here, the tube
through the esophagus, try and locate the
lumen first. So, locate the lumen on
the right-hand side scanning electron
micrograph image of the esophagus. It's
a star-shaped or rather folded shaped
lumen here because the esophagus
is collapsed.
01:34
It's only going to open out when a bolus
of food is passing down through it, as part
of the swallowing process. The surface of
the lumen is where you find the mucosa.
01:49
The mucosa is that component of the gut
wall that is adjacent to the lumen, the
cells open on to the lumen. So the mucosa
consists of the epithelium of the gut
wall, the gut tube, and also the supporting
connective tissue, the lamina propria.
02:16
And often we include the very small thin
muscle layer around the mucosa called the
muscularis mucosa. So just going through
that once again, the lumen is in the
centre, and then you have the mucosa which
consists of the epithelium that lines
that lumen, supported by a lamina propria.
And then we have the muscularis mucosa.
02:44
Sometimes, I will refer to both those
components, the mucosa and muscularis
mucosa as just the mucosa. Then there is
a space of loose connective tissue we
call the submucosa. That's a very
important layer because it contains
nerves and blood vessels that are going
to branch and provide the mucosa, the very busy
epithelial surface, with all the blood
nutrients that need to carry out the
functions all the way along the gut.
03:26
Then you have on the outside, the muscularis
externa. And all the way along
the gut tube, that muscularis externa
consists of at least two layers, mostly
it's an inner circular layer and an outer
longitudinal layer. So let's work back from that
muscularis externa and just describe some
of the main roles of each of these
layers, generally, and then we'll see how
they become specialized when we look here at
the esophagus, and then again, at the stomach.
And in later lectures, I will describe the roles
they serve when looking at other parts
of the elementary tract. Well, the
muscularis externa, consisting of those
two layers, is responsible for
contracting and moving food along the gut
tube. And by using both contraction of
circular muscle and then longitudinal
muscle, the gut wall can have a
peristaltic wave of contraction that forces
the food along. That's the major role of
the muscularis externa. On the outside of
that muscularis externa is going to be
connective tissue called adventitia, which
blends or holds that part of the
gut wall against surrounding tissues. Or
sometimes, as indicated here, it's called
the serosa, because the surface is lined
or free and lined by part of the peritoneum.
05:07
Now, that muscularis externa is working
independently of what's going on in the
lumen. The epithelial cells, particularly
in the stomach and further down the gut, are
busy carrying out functions, secreting,
absorbing all the necessary jobs they
have to do, supported by the
lamina propria.
05:32
The muscularis mucosa around that mucosa
helps local mixing. It contracts and
relaxes, and therefore, changes the
dimensions of the environment
immediately at the luminal surface. And
allows this local mixing, and therefore,
the mixing of secretory products to aid
digestion and also the mixing of
products to enhance or increase the
surface area for absorption.
06:04
That muscularis mucosa works independently
of the muscularis externa, and they're
controlled by different components of the
enteric nervous system. So think of
one tube inside another tube, and the inner
tube is working independently from
the outer tube. But overall, the outer tube
is responsible for moving the food along
as it's broken down and as the nutrients
are absorbed. So, the submucosa
acts as a sort of a cushion. It allows a
lot of movement within the lumen, of course
by the muscularis mucosa. It allows the
lumen to expand, as you see here, if a bolus
of food passes down the lumen, and therefore,
that submucosa is going to
allow that expansion. And it's also going
to give the freedom of movement from the
inner tube and then the outer muscularis
tube. So again, let me emphasize those
important components of the wall of the
gut. You should know them because they're
going to change, particularly the mucosa,
as we move down along the gut tube.