Hey y'all. Dr. Brown back again with another thrilling video, this time about
the histology of the stomach. So just in case you haven't had Anatomy, or you
haven't had Anatomy in a hot minute, just a reminder of the stomach is this pouch
like extension of the alimentary canal. and it's located kind of to...mostly to the
left of the midline and the superior portion of the abdominal cavity, and the
stomach can be divided into four primary distinct regions. So we have the cardia
or cardiac region; that is the region where the esophagus meets the stomach. We
have the fundus, which is the superior kind of distinction of the stomach, and
you'll find that there are other organs that have kind of a superior hump on
them called a fundus. And then the main part of the body...sorry...the main part of
the stomach is called the body. So we see that right here, and then this is divided
into the two parts of the pylorus. So the pyloric Antrim and pyloric canal are
both parts of the pylorus or pyloric region of the stomach, and that's simply
where it narrows down as it meets the proximal most portion of the small
intestine. Histologically, the stomach is very different from any of the other
primary digestive organs. So what we're gonna do is look at a cartoon of stomach
histology first, and kind of go over the the general features, and then we'll
actually look at real micrographs and see how those four layers and it's
actually not all four, but two of them in particular are modified to enable the
stomach to do its job of chemical and mechanical digestion of swallowed food.
So the first thing you'll notice about the mucosa of the stomach is that the
epithelium and the lamina propria are not in a nice kind of wavy single layer
like we saw in the esophagus. Instead the epithelium and lamina propria of the
mucosa are invaginated or pulled downward into kind of a pocket, so that
you can see up here at the surface we have these kind of holes, and then in
cross-section we can see these holes lead down into this invagination of the
epithelium. So most of epithelial surface area is not exposed
to the lumen of the stomach, which is full of gastric juice, and gastric juice
is this combination of primarily hydrochloric acid, very powerful
digestive enzymes, and mucus. I like to call it murder juice; it's got a pH of
anywhere from 1 to 3 and it contains very powerful protein...proteases, and I
don't know if you guys have checked but you're kind of made out of proteins. So a
lot of the modifications we're gonna see in the mucosa are a trade-off between
needing to make these substances that can digest meat, and dealing with the
fact that we're made out of meat, and therefore could be digested with these
things. And so the first kind of adaptation to that is the folding inward
of the gastric epithelium into these gastric pits up here, and in the gastric
glands which are what line these invaginations of the epithelium. Up at
the top of the gastric pit you'll see it's largely covered with surface
epithelial cells, and the surface epithelial cells are primarily mucous
secreting cells. So there's always gonna be kind a, a thin layer of protective
mucus covering the parts of the stomach that are actually coming into contact
with the gastric juice. As we move down into the gastric glands, we see some of
the cells that actually produce the gastric huge...gastric juice. I can't
say words. So the first of these is the parietal cell seen here. These cells are
very large they tend to stain red in standard histological preparations,
and these are the cells that are producing both the hydrochloric acid and
a substance called intrinsic factor, which is necessary for the absorption of
vitamin b12. Then on down we have our chief cells, and these contain little
granules called zymogen granules, and the zymogen granules are going to become
active protease enzymes once they reach the acidic environment of the lumen.
Right? We keep them inactive until we get out of here, and up
into here where we have some protection from that gastric juice. Then the acid in
the gastric juice can...can turn...can turn the zymogen into enzymes such as pepsin,
and then finally down here in the bottom part of these gastric glands, we see some
enteroendocrine cells. Now these are also often called G-cells and they're
called that because enteroendocrine means they're creating digestive
hormones, and the hormone that's produced most often of the most well...well-known
initially was gastrin. So these are gastrin secreting cells so they're also
called G-cells. We now know in fact that they're also secreting the hormone
somatostatin. So enteroendocrine cell is a more appropriate name for them. All
right, we've seen it in a cartoon, now let's actually see these things in real
life. Let's look at some actual histological preparations. So here we can
see a very low mag image of the stomach, but we can already make out some of
those distinctive features that I mentioned in the last picture. So here we
can see the gastric pits, right? And you notice these surface epithelial cells
have a very distinct appearance they are essentially goblet cells. These are
columnar mucous secreting cells, and they're going to be making the mucus
layer that's going to protect everything up here. As we move downward into the
gastric glands, we see some cells known as mucous neck cells. So these little
patches of darker cells down here, and here; they're kind of small and very dark
staining. They're very basophilic, and these little guys are going to be
producing more mucus, and they're going to be protecting the more delicate
tissues further down in the gastric glands. So here you can see a gastric
gland. Coming down. So just outline this little gastric gland as we come down. Now
keep in mind these are three-dimensional structures, so you see it appears to end
here and that's probably because we're looking at a two-dimensional section, and
part of it either went away from us or toward us. Now you can also see as you
move downward. So from the surface of the mucosa deeper down toward the lamina
propria, that the epithelial cells appear to
change color, and that's because the parietal cells, this is mostly in here,
parietal cell, these tend to be more eosinophilic, so there are reder color and
then the chief cells lower down are going to be more purplely or blue. So we
can kind of see this trend even at this low mag image, but let's go up in
magnification and look even closer at these gastric glands. So in this higher
magnification image we can see the distinctive surface epithelial cells up
at the top, and I'm probably going to need a different color pen than red for
this. So let's go ahead and switch to...we'll go back to black. Poop. And then as
we move downward we can see some of the mucous neck cells down in here, and then
we very distinctly see the parietal cells so you know these guys are
parietal cells. They're large, they're red they tend to be pretty round. You can see
they're they're fairly circular in shape, and further down still we see the blue
chief cells. So if you get a really good section at a good magnification, it's not
really hard to distinguish between the parietal cells higher up in the gastric
glands and the chief cells lower down in the gastric glands. What is hard to see
in a typical stomach is the enteroendocrine cells, and that's because
they're fairly rare. So if we look closer, we can see a very distinct parietal cell
here, with its characteristic central nucleus and red or pink staining, and
then we can also see a couple of chief cells right here. And you notice that
they tend to be more columnar cells, their nuclei tend to be more basal down
toward the bottom of the cell, and of course they stain more purple or blue.
You still can't see any enteroendocrine cells in here; like I said they're kind
of hard to spot. So what we're gonna do is cheat a little bit this is a section
through a stomach with somebody who has I think it's called G-cell sarcoma,
but it's cancer where you produce too many enteroendocrine cells. So since
this person has essentially G-cell cancer. We can see a G-cell here. I'm
going to switch back to red so it's easier to see. So these large kind of
distended cells with the centrally located nuclei,
but they're not parietal cells they don't...they're...they're not red in color.
These are the enteroendocrine cells. So like I said these are harder to find
unless you have a patient with G-cell cancer. Next we're gonna look at the
other layer that's heavily modified. So we've seen so far how the mucosa is
modified with it's enfolded epithelium and lamina propria with the
stratification of the cells, so that we have the surface epithelial cells and
the mucous neck cells closer to the caustic environment of the gastric juice.
Then the parietal cells making the hydrochloric acid, and then the chief
cells even further down making the zymogen granules, which don't become
active until they hit the hydrochloric acid, which is going to be higher up. So
this is going to protect our underlying tissues. The lamina propria is folded. The
muscularis mucosa is about the same as in any other organ. The submucosa is
about the same, but muscularis externa is also modified rather heavily in the
stomach. So if we look at the muscularis externa you can see in the cartoon on
the left we have a longitudinal layer. Right. That's the outermost layer. Got it.
Check. We got a circular layer we're also used to seeing that, but what we're not
used to seeing is this innermost oblique layer. So there's a third layer of smooth
muscle in the muscularis externa of the stomach, and while it's not easy to see, I
think once I point it out you'll be able to see the distinction. In the micrograph
on the right, you can see right about here, is the borderline between the
oblique layer above and the circular layer below. So this is circular and then
finally we can see one more transition and then the longitudinal layer is down
here. So the the modifications to the mucosa are largely to facilitate
chemical digestion of the ingested food. The acid will denature proteins opening
up their set...their tertiary and secondary structures, so that the
protease enzymes can get in there and break the proteins up into smaller
pieces. The modifications to musclaris are going to facilitate
mechanical digestion, having that third layer of smooth muscle running at a 45
degree angle to the other two is going to allow the stomach to exhibit a
characteristic motion called churning, which is going to allow it to really
work those digestive enzymes into the chewed and swallowed food creating an
acidic paste called chyme, which will then exit the stomach via the pyloric
canal, and be further chemically digested by enzymes in the proximal portion of
the small intestine. So that's about it for now we've seen how we modify those
layers in the stomach to allow the stomach to be a stomach, and not an
esophagus, and not a small intestine. That's it for now.
Study hard. We'll see you later.