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them, and alert the immune system.
Starting with the small intestine, I want
to just show you this slide and make sure
that you fully understand the wall of
the gut. The gut wall was explained in another
lecture on the esophagus and stomach, but
I just want to review it here. Basically, the
wall of the gut has got the internal component
called the mucosa that lies right against
the lumen. Here, on this slide, I show you
three sections. One is the diagram of the
small intestine. The centre one is taken at
low magnifications of the small intestine,
mucosa. And on the right-hand side, you
see a higher magnification. And these really
illustrate the very specialized mucosa, the
mucosa of the gut consists of the epithelium,
the underlying lamina propria, and also the
muscularis mucosa. You can see the muscularis
mucosa as a thin reddish stained line of smooth
muscle at the base of that section in
the middle of this slide.
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Focus your attention now on the left-hand
diagram. On that diagram, you can see folds.
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They're labelled plicae circulares. These
are permanent folds in the small intestine.
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They begin about six centimeters into the
duodenum, and they are very prominent throughout
the duodenum, and also the jejunum, and then
they start to get lesser and lesser obvious
at the lower end of the ileum. Have a look
at the diagram and notice that they actually
have a central core of connective tissue called
the submucosa. The submucosa is a connective
tissue layer underneath the mucosa that gives
the gut mucosa mobility and flexibility.
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The mucosa you see is moving all the time.
As food passes along and as food is broken
down in that lumen, the mucosa is continually
moving about by contraction of muscularis
mucosa. So that submucosa underneath the mucosa
needs to be able to take in that movement.
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It needs to be flexible to accommodate the
moving mucosa adjacent to the lumen. In here,
though, it's a permanent component of the
small intestine wall. And then underneath
the submucosa, you have the muscularis externa
shown here on the left-hand diagram by those
colored lines. They represent layers of the
muscularis externa I'll describe later on
through this lecture. Now look at
the central slide. Make sure you
can identify the submucosa in this section.
Again, these are permanent folds. And on the
surface of each of these submucosal projections,
you can see the mucosa. And even at the low
magnification you see in the centre image, you can
see little tiny fine fingerlike projections
coming off the core of the submucosa. They're
called villi. And if you move across now and
focus on the right-hand section, these villi
are fingerlike projections. They're lined
by epithelial cells. They're lined mainly
by epithelial cells that will absorb all the
nutrients and other components that are broken
down in the lumen of the gut immediately adjacent
to this epithelium. Small intestine
doesn't have secretory
cells that secrete digestive enzymes, or at
least many. All the digestive enzymes come
from the pancreas. Those villi have a core
of lamina propria, the supporting connective
tissue of the epithelium. And then underneath
those little villi, there will be the muscularis
mucosa that can contract and move that mucosa
around. And as you can recall, underneath
that then is the submucosa. We're going
to look at the details of these villi in a
moment. But these fingerlike projections are
designed to increase the surface area of the
small intestine, to increase the surface area
for very effective absorption of products
from the lumen across the epithelial surface
into the lamina propria where there are many
many blood capillaries, and also
lymphatic vessels.
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The epithelial cells lining these villi also
invaginate deep into the lamina propria.
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We call those crypts. They're essentially glands.
But as we'll see, their main products are
stem cells replacing the epithelial cells
that are lost into the lumen, there's continual
renew of epithelium here. This is now higher
magnification series of three histological