Let me now describe the histology of the large
intestine. It's a fairly simple organ to
describe histologically because its functions
are merely to store the unwanted and undigested
products as feces and also to absorb water.
On the left hand section, you can see a low
magnification section through the colon or
large intestine. Identify the lumen and the
folded mucosa. See if you can make out the
submucosa, and then finally, the muscularis
externa. The muscularis externa has a specialized
longitudinal band called the teniae coli.
They're three bands running along the length
of the colon. And when the colon is empty
or almost empty, components can fold up, form
circulations because of the contraction of
these teniae coli. There are no
villi in the large intestine,
in the colon. The folds you see are submucosal folds,
and the mucosa only consists of invaginations
of the epithelial surface into glands. In
the middle section, you can see an example
of this. They're straight tubular glands.
They're quite long. They embed deep into
the lamina propria so that the cells there that
are absorbing water, which is their function,
are away from the hostile environment in the
lumen. These glands are often referred to
as the Crypts of Lieberkuhn. Besides having
cells there, enterocytes there that absorb
the water back into the body, there's a
lot more goblet cells appearing now.
You see those nicely stained in that special
section on the right hand side. Notice how
many goblet cells there are. The epithelium
is dominated in parts by these goblet cells.
They are going to lubricate the feces as they
pass out of the body or help lubricate
the feces. So really, the large intestine is a
simple organ, stores unwanted products, undigested
products, and absorbs water back into the
system, and the mucus of the goblet cells
helps to lubricate the feces.
Finally, let's have a look at the rectum