Large intestine
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General features
Large intestine is last organ of
digestive tract proper
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divided into 3 or 4 regions
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cecum
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appendix in humans
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colon
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rectum
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h. Large intestine
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No villi
lumenal epithelium has
microvilli
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Mucosal folds present
brush border
sometimes incomplete
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difficult to detect.
some nutrient
absorption
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lower than small
absorbs most of the
remaining H2O
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Large intestine
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Large intestine
Abundant intestinal glands
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simple tubular glands with glandular epithelium
mucus-producing cells
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like goblet cells; wider at base
glands densely-packed
lamina propria; composed of unusually cellular, dense C.T.
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Large intestine
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Muscularis mucosae; thicker than in small intestine.
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Lymphatic nodules; common
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form first in (the lamina propria) the mucosa
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spread into the submucosa.
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Large intestine
Submucosa; not significantly different from that in small intestine.
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Muscularis externa ;
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outer, longitudinal; incomplete
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inner, circular; thick , subdivided into masses.
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Large intestine
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Special features
Cecum: first region of large
intestine
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is a side-pouch at the beginning
harbors commensal
bacteria
help digestion of fibrous
plant foods
producing cellulase
almost no animals themselves
can produce
Cellulose digestion in
humans is insignificant
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Large intestine
Plant materials pass through the small
intestine relatively undigested
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lodge in the cecum
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cellulose; broken down,
exposing cells to the other digestive
enzymes
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some of the digestion products
absorbed by microorganisms; some
into the wall of the cecum
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Vita K; glucose
animals with a high proportion of plant
materials in diet have a larger cecum.
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h. Large intestine
In humans part of
cecum is vestigial
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the appendix.
Most of the cecum
not significantly
different from colon.
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h. Large intestine
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Appendix
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Not found in most mammals
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found in humans
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Vestigial structure?
recolonization of intestinal
flora
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lost much of the typical
organization of the large intestine
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primary function; immune defense
evolutionarily become a part of the
immune-system.
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h. Large intestine
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Mucosa of appendix
Lumenal epithelium: simple
columnar epithelium + scattered
goblet cells; brush border absent.
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Intestinal glands: relatively sparse;
cells with mucus vacuoles.
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Lamina propria: much more cellular
than large or small intestine
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greater abundance of immunedefense cells
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Muscularis mucosae: thin;
sometimes discontinuous
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often not distinct except on high
power.
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Large intestine
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Submucosa of appendix
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Mostly lymphoid tissue
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Dense lymphoid tissue.
Nodules are continuous.
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Typical submucosal
components limited
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Interface between
submucosa and muscularis
externa not distinct.
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SMFs of the circular layer
are intermingled with
dense CT of the
submucosa.
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Large intestine
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Muscularis externa
Some loss of layering
pattern.
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SMT less compact.
Outer muscularis externa;
intermingling of SMT with
dense CT of the TA
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Tunica adventitia (TA)
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No special features.
Loss of typical features due to
immune-defense function.
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Large intestine
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Colon
Longest region of the
large intestine; shorter
than the small intestine
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ascending,
transverse, and
descending
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Distal descending
portion joins the
rectum, last region
of the large intestine.
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Large intestine
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Main functions:
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Reabsorption of water
Water reabsorption determines
the consistency of the feces
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< normal → diarrhea
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> normal → constipation
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Large intestine
Factors determine water reabsorption
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lumen contents speed
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osmotic pressure of the lumen solution
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ability of the mucosa to absorb solutes
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infection of the lumenal epithelium
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water reabsorption can be severely impaired
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severe dehydration may occur
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dehydration can be life-threatening
Putrefaction of wastes
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Commensal bacteria break down some of undigested materials.
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Compaction of undigested materials (feces)--in last portions of region.
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Large intestine
Rectum and recto-anal
junction
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Rectum is a short final region
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More H20 reabsorption;
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More compaction of feces
wall receptors when
stretched (usually due to
fecal mass)
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sends impulse signals to the
brain that result in the
sensation of needing to
defecate
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Large intestine
Histology of the rectum is similar
to colon mucosa
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submucosal folds absent.
Where the rectum transitions into
anal canal, muscularis mucosae
ends
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inner, circular layer of the
muscularis externa becomes
thickened; internal anal
sphincter.
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Nearby the outer, longitudinal layer
of the muscularis externa ends.
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Large intestine
Lumenal epithelium changes
abruptly to the stratified
squamous epithelium of the
anal canal; intestinal glands
end.
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Distant from the lumen and
closer to the end of the anal
canal
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external anal sphincter
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skeletal muscle
after infancy; under
voluntary control.
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No distinct outer surface to
tunica adventitia.
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Rectum
Large intestine
Anal canal
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