682
UNIT FIVE
The epithelial cells that form the lining of the small intestine are con-
tinually replaced. New cells form in the intestinal glands by mitosis
and migrate outward onto the villus surface. When the migrating
cells reach the tip of the villus, they are shed. This
cellular turnover
renews the small intestine’s epithelial lining every three to six days.
As a result, nearly one-quarter of the bulk of feces consists of dead
epithelial cells from the small intestine.
Secretions of the Small Intestine
In addition to the mucous-secreting goblet cells, abundant
throughout the mucosa of the small intestine, many specialized
mucous-secreting glands
(Brunner’s glands) are in the submu-
cosa in the proximal portion of the duodenum. These glands
secrete a thick, alkaline mucus in response to certain stimuli.
The intestinal glands at the bases of the villi secrete large
volumes of a watery fl uid (see F g. 17.35). The villi rapidly
reabsorb this fl uid, which carries digestive products into the
villi. The fl uid the intestinal glands secrete has a nearly neu-
tral pH (6.5–7.5), and it lacks digestive enzymes. However,
the epithelial cells of the intestinal mucosa have digestive
enzymes embedded in the membranes of the microvilli on
their luminal surfaces. These enzymes break down food mole-
cules just before absorption takes place. The enzymes include
peptidases,
which split peptides into their constituent amino
acids;
sucrase, maltase,
and
lactase,
which split the disaccha-
membrane. The deeper layers of the small intestinal wall are
much like those of other parts of the alimentary canal in that
they include a submucosa, a muscular layer, and a serosa.
The lining of the small intestine has many circular folds
of mucosa, called
plicae circulares,
that are especially well
developed in the lower duodenum and upper jejunum
(f g.
17.38)
. With the villi and microvilli, these folds help increase
the surface area of the intestinal lining.
Lumen
Villus
Intestinal
gland
FIGURE 17.36
Light micrograph of intestinal villi from the wall of
the duodenum (50×).
Microvilli
Cell membrane
Mitochondrion
Golgi apparatus
Nucleolus
Nucleus
Rough
endoplasmic
reticulum
(a)
Lumen
Microvilli
Cytoplasm of
epithelial cell
(b)
FIGURE 17.37
Intestinal epithelium. (
a
) Microvilli increase the surface area of intestinal epithelial cells. (
b
) Transmission electron micrograph of
microvilli (16,000×).
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