Chapter 23
The Digestive System
877
23
Te mucosa contains both individual follicles and
aggregated
lymphoid nodules
, the latter called
Peyer’s patches
(pi
9
erz). Peyer’s
patches are primarily located in the lamina propria but occasion-
ally protrude into the submucosa below. Teir increasing abun-
dance toward the distal end of the small intestine reflects the fact
that this region of the small intestine contains huge numbers of
bacteria that must be prevented from entering the bloodstream.
Te lymphoid tissue of the mucosa also contains proliferat-
ing B lymphocytes that leave the intestine, enter the blood, and
then home in on the intestinal lamina propria. Tere, in their
new home, they release immunoglobin A (IgA), which helps
protect against intestinal pathogens (see p. 783).
Te submucosa is typical areolar connective tissue. Elabo-
rate mucus-secreting
duodenal glands
in the submucosa of
the duodenum produce an alkaline (bicarbonate-rich) mucus
that helps neutralize acidic chyme moving in from the stomach.
When this protective mucus barrier is inadequate, the intestinal
wall erodes and
duodenal ulcers
result.
Te muscularis is typical and bilayered. Except for the bulk
of the duodenum, which is retroperitoneal and has an adventi-
tia, visceral peritoneum (serosa) covers the external intestinal
surface.
Intestinal Juice: Composition and Control
Te intestinal glands normally secrete 1 to 2 L of intestinal juice
daily. Te major stimulus for its production comes from hyper-
tonic or acidic chyme, which cause distension or irritation of the
intestinal mucosa.
Normally, intestinal juice is slightly alkaline (7.4–7.8), and
isotonic with blood plasma. Intestinal juice is largely water but
also contains some mucus, which is secreted both by the duo-
denal glands and by goblet cells of the mucosa. Intestinal juice
is enzyme-poor because intestinal enzymes are limited to the
bound enzymes of the brush border.
soF nap of a towel (±igure 23.22). Te villi are large and leaflike
in the duodenum (the intestinal site of most active absorption)
and gradually narrow and shorten along the length of the small
intestine. Te epithelial cells of the villi (called
enterocytes
) are
chiefly absorptive columnar cells. In the core of each villus is
a dense capillary bed and a wide lymphatic capillary called a
lacteal
(lak
9
te-al). Digested foodstuffs are absorbed through the
epithelial cells into both the capillary blood and the lacteal.
Te exceptionally long, densely packed
microvilli
of the ab-
sorptive cells of the mucosa give the mucosal surface a fuzzy ap-
pearance called the
brush border
(±igure 23.22b enlargement and
Figure 23.23
). Te plasma membranes of the microvilli bear en-
zymes referred to as
brush border enzymes
, which complete the
digestion of carbohydrates and proteins in the small intestine.
Histology of the Small Intestine Wall
Externally the subdivi-
sions of the small intestine appear to be nearly identical, but
their internal and microscopic anatomies reveal some impor-
tant differences. Te four tunics typical of the GI tract are also
seen here, but the mucosa and submucosa are modified to re-
flect the intestine’s functions in the digestive pathway.
Te epithelium of the villus mucosa is largely simple colum-
nar
absorptive cells
bound by tight junctions and richly endowed
with microvilli. Tese cells bear the primary responsibility for
absorbing nutrients and electrolytes. Te epithelium also has
many mucus-secreting
goblet cells.
Between the villi, the mucosa
is studded with
pits
that lead into tubular glands called
intesti-
nal crypts
(see ±igure 23.22b, c).
Crypt epithelial cells are primarily secretory cells that secrete
intestinal juice
, a watery mixture that contains mucus and serves
as a carrier fluid for absorbing nutrients from chyme. Scattered
through the crypt epithelium are
enteroendocrine cells
, the source
of the enterogastrones—secretin and cholecystokinin to name
two—and ² cells called
intraepithelial lymphocytes
(
IELs
), which
represent an important immunological defensive component.
Deep in the crypts are specialized secretory cells called
Paneth cells
, which fortify the small intestine’s defenses by re-
leasing antimicrobial agents such as defensins and
lysozyme
, an
antibacterial enzyme. Tese secretions destroy certain bacteria
and help to determine which bacteria colonize the intestinal
lumen. Te crypts decrease in number along the length of the
small intestine, but the goblet cells become more abundant.
Te various epithelial cells arise from continuously dividing
stem cells at the base of the crypts. As the daughter cells gradu-
ally migrate up the villi, they differentiate, becoming specialized
cell types—absorptive cells, goblet cells, and enteroendocrine
cells. Te fourth differentiated cell type is the Paneth cells,
which remain at the base of the crypts. Te other three types
undergo apoptosis and are shed from the villus tips, renewing
the villus epithelium every two to four days.
Homeostatic Imbalance
23.10
²reatments for cancer, such as radiation therapy and chemo-
therapy, preferentially target rapidly dividing cells. Tey kill
cancer cells, but also nearly obliterate the rapidly dividing GI
tract epithelium. Many patients suffer nausea, vomiting, and
diarrhea aFer each treatment.
Microvilli
forming the
brush border
Absorptive cell
Mucus
granules
Figure 23.23
Microvilli of the small intestine.
False-color
electron micrograph. Microvilli (28,000
3
) of absorptive cells appear
as red projections from the surface of the absorptive cell. Yellow
granules are mucus granules.
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