Hey y'all. Browns back and we're gonna be learning some more histology, this time
about the elementary canal wall. So the elementary canal is that tube that
sometimes called the GI tract, or the digestive tract, and it is the canal, or
tract, or tube that can that goes continually from mouth to anus. So it's a
continuous tube traveling through your body, in which nutrients are digested and
absorbed and feces...from which feces is defecated. And all of those digestive
processes that you've learned already, ingestion, mechanical digestion, chemical
digestion, absorption, propulsion, and defecation, all of those are carried out
inside the alimentary canal by the primary digestive organs, that is the
organs that make up the alimentary canal including the esophagus, the stomach, the
small intestine and the large intestine, and the functions of those different
organs come from the histology of the wall of this tube, the wall of this canal.
So if you understand how the canal is modified from one organ to another, then
you can understand how histology the histological structure imparts function
to those different organs. Now you've been listening to me talk for a minute
and a half, and you may be wondering why are there tacos on the screen, and that's
because I like the analogy of the alimentary canal wall is similar to the
menu at kind of a derivative generic American tex-mex restaurant. That is they
take the same four basic ingredients, so you got a meat, you've got some cheese,
you got some veggies, and you've got a tortilla, but you change those up to go
from one menu item to the next. So to go from a soft taco to a hard taco you fry
the tortilla shell, or you use a flour tortilla versus a fried corn tortilla.
Maybe you have cotija cheese in one taco, and you have American cheese in another.
Or maybe you're making sub-basin instead of the tortilla being thin and folded
it's a thick little patty, but by changing details about those four
ingredients you get pretty much the entire menu at one of these restaurants.
So what we're gonna do is look at quite closely the four layers that make up the
alimentary canal wall, and what those layers tend to contain throughout the
alimentary canal, and then in future modules...or sorry...future sections of
this module you'll be able to see how those layers are modified in order to
impart various functions to the different organs of the alimentary canal.
So let's look at these four layers. So you are looking at a cross-section
through the esophagus. This is the organ we're going to be using. The picture on
the left is slightly lower magnification then the picture on the right, but in
both we can see all the layers of the alimentary canal. So the layer facing the
lumen, which is essentially a surface, remember the contents of the alimentary
canal are considered to be external. That...that is part of the external environment
it is not subject to immune attack it is surrounded by the elementary canal, but
it's external. So like we have in most most things facing an external
environment we're gonna have some sort of mucous membrane. So the topmost layer
here is called the mucosa, and switch pen colors. We're gonna go to a black pen. So
the top layer here is called the mucosa.
Over here and the image on the right the mucosa extends about yay far, and I
promise I'm going to show you how to tell these things apart. I'm not just
gonna label them and walk off. Underneath the mucosa here and here in this image,
we have a sub mucosa. That's hard isn't it. Sometimes this isn't all rocket
surgery. Underneath the submucosa we have two layers of smooth muscle here and in
this one, well that's a bad bracket, you get the idea here this is called the
muscularis externa, because it's moving away from this mucosa layer. And then
finally underneath the bottom most layer, so kind of this stuff here in the
picture on the right, and this bottom most layer here is an Adventitial.
In most of the esophagus but over the rest of your GI organs this is called
the serosa. Okay. So let's dive in a little bit, and look at how we tell these...
..the components of these three individual layers, and we're gonna start with the
mucosa. So the mucosa is always going to have at its very top facing the lumen
an epithelium. So in the case of the esophagus this is a stratified squamous
epithelium. So that's gonna extend from the surface all the way down to these
basal cells here. Okay. So there's the epithelium. Underneath the epithelium
you're gonna have a thin layer of connective tissue. So it's usually loose
or areolar connective tissue, and this is the lamina propria. And then underneath
that we have some thin little spotty bits of smooth muscle. So you can see
this is kind of in staining a rather intense pink color. You can see the the
infrequent nuclei in here. So this layer of smooth muscle that separates the
mucosa from the submucosa underneath is called muscularis mucosae or muscularis
mucosa depending on the textbook. So all of these things so from the top of the
epithelium to the bottom of muscularis mucosa, that is the mucosa layer.
Everything between muscularis mucosa and the beginnings of muscularis externa is
submucosa. So all of the stuff in here is submucosa. So the mucosa is always going to
have an epithelium, but what that epithelium is is going to vary depending
on which organ you are in. So in this case it's stratified squamous. In the
stomach it is a simple cuboidal. In the intestines it's a pseudo stratified
columnar or a simple columnar depending on where you are in the intestines.
You can also find additional cells in here besides just your regular
epithelial cells. In the intestines, particularly your
see lots of goblet cells, which are going to secrete a protective mucus. You're
often gonna see endocrine cells as part of the mucosa, and that's because certain
regions of the elementary canal actually secrete their own hormones.
Okay? So epithelium, lamina propria, muscularis mucosae, that is the mucosa.
The submucosa, like we said is everything from muscularis mucosa. So in this
picture on the left, oh, that is an inadvertent ink mark. Where's my eraser.
There it is. There we go. So this is muscularis mucosa. This is the smooth muscle of
muscularis mucosa here. So everything from here down to here is mucosa. So the
submucosa is largely dense irregular connective tissue. So most of this is
type 1 collagen, but it's also going to contain the vasculature for the
alimentary canal wall. So you can see a small artery here and here. You can see
some veins. There's an irregularly shaped vein here and another vein here.
Neurovascular is also found here. So the myenteric plexus, the group of
nerves that supply the the wall of the alimentary canal can be found in the
submucosa, and we also find glance. So if we look at the picture on the right here
you'll see this large bicarbonate secreting gland. This is from lower down
in the esophagus, close to the stomach, and you can also see very well the
smooth muscle of muscularis mucosa. This is an artifact right in here. That's just
where the tissue tore. So submucosa, largely specialized to provide
vasculature to the canal wall, both blood as well as lymphatic vasculature as well
as neuro vasculature. Underneath we have muscularis externa, which itself is
comprised of two layers. So just to kind of orient you to where we are, this is
the submucosa here, and then everything down here is muscularis externa, and you
can probably tell by looking that there are actually two layers to muscularis
externa. This upper layer called the circular or
circular layer has muscle fibers that you can see these smooth muscle fibers
appear to be running right-to-left in this cross section, and that's because
you know this is a tube. Right? So imagine if we have a tube like so, we've cut it
in cross-section. These fibers are running in a circular pattern. They
running this way around the entire circumference of this inner muscular
layer. Okay. So that's why when we take this in cross section what we're looking
at here is comes from like this. These fibers appear to run right-to-left.
Underneath you can see the shape of the bundles of smooth muscle fibers looks
very different, and that's because this is the longitudinal layer down here. The
longitudinal layer has fibers...so if we just kind of make this more of a 3D...the
fibers of the longitudinal layer are running lengthwise, longitudinally, along
the entire elementary canal wall, and so when you take them in cross-section what
you get are just the little dots. Okay. So submucosa circular layer
longitudinal layer. Finally, I don't have a good picture of it here, but the
outermost layer is called the adventitia for most of the length of the esophagus,
and that's, because it's inside the mediastinum, that layer of connective
tissue that runs kind of down the middle of your thoracic cavity and separates
the right and left pleural cavities from one another, but once the...once the
esophagus makes its way through the esophageal hiatus and the diaphragm and
enters the abdominal pelvic cavity that loose connective tissue is covered with
a mesothelium that secretes serous fluid so we call it a serosa. Okay. So it's only
called an adventitia when it's in the thoracic cavity; as soon as it makes it
into the peritoneal or abdominal pelvic cavity we call it a serosa. So finally,
let's draw this out, let's draw a little sketch. So I expect you to be able to
draw a sketch of this when it's all done, so
I should expect me to be able to draw a sketch of this. So our first layer is
gonna be the mucosa. So we're gonna need, first of all, an epithelium. So here's our
little epithelium here, and underneath the epithelium we're gonna have some
loose connective tissue to make up the lamina propria, and then finally those
discontinuous patches smooth muscle providing tone to the overlying
epithelium and lamina propria, and that's muscularis mucosa. So we have an
epithelium here, we have a lamina propria here, and we have muscularis mucosa here.
All of these things collectively make up the mucosa or mucous membrane. Underneath
that we have some dense connective tissue. We also have a little arteriole
maybe, and put a venule in there like so. Now nerves. So this is kind of like the
trophic center of the organs, so that is the submucosa. Under the submucosa we're
gonna have two layers of smooth muscle, so we're gonna have one layer running
back and forth like so. So that's the circular or circular layer. Ok we're
gonna have another one where the muscle fibers are, you know, looking at the
screen like this they'd be kind of coming at us. Right? They're coming right
for us. So this would be another layer of muscle. This is the longitudinal layer
smooth muscle, and these two collectively make-up muscularis externa,
and then finally underneath that lies another layer of connective tissue,
and, since most of your digestive organs are not the upper two-thirds of the
esophagus, we'll cover this or if that mesothelium, and this will be the
serosa. So now that you know the four layers of the alimentary canal wall, and
some of the features of those layers, we can move in and talk about each organ
that makes up the alimentary canal. All of the primary digestive organs, and
see how these four layers are modified. We're really finally getting into the
business of seeing how histological structure imparts function. So I think
you're gonna find these last few modules pretty interesting as we tie together
all of those different organs that we learned about earlier on in the course.
Well that's it for now. Hope you had fun. Study hard. We'll see you later.