CHAPTER 15
Digestive Tract
293
Figure 15-11A.
Plicae circulares, villi, and microvilli.
H&E,
3
124; inset (
left
)
3
15, (
right
)
3
882
The small intestine is a long tube with three levels of folds that increase the surface area for absorption. (1)
Plicae circulares (valves
of Kerckring)
are gross folds involving the mucosa and submucosa that project into the lumen (
left inset
). (2)
Villi
are smaller folds
than the plicae circulares and involve only mucosa. The central core of each villus is formed by the lamina propria; the nutrients
absorbed from the lumen by absorptive cells are transported into the lamina propria. The lamina propria contains a
central lacteal
(a
blind-ended lymphatic vessel) and many capillaries involved in the transport of absorbed nutrients. (3)
Microvilli
(see Fig. 15-12B)
are at the apical surfaces of columnar absorptive cells, increasing the surface area at the cellular level. They appear as a pink border
in light microscopy and form a
brush border
. The drawing on the
right
shows various types of cells arranged in the epithelium of
the mucosa. These cells include
column-shaped absorptive cells
(
enterocytes
),
goblet cells
,
Paneth cells
, and
enteroendocrine cells
.
The
central lacteals
are also illustrated.
T. Yang
Lacteal
Microvilli
(brush border)
Stem
cell
Goblet
cell
Capillary
Paneth
cells
Columnar
absorptive
cell
Entero-
endocrine
cell
Goblet
Goblet
cell
cell
Goblet
cell
Columnar
Columnar
absorptive cells
absorptive cells
Columnar
absorptive cells
Lamina propria
Lamina propria
Lamina propria
Microvilli
Microvilli
Microvilli
Villus
Plicae circulare
Plicae circulare
Plicae circulare
Villus
Submucosa
Submucosa
Submucosa
Gland (crypt)
Gland (crypt)
of Lieberkühn
of Lieberkühn
Gland (crypt)
of Lieberkühn
Lumen of the
gland (crypt)
of Lieberkühn
Lamina
Lamina
propria
propria
Lamina
propria
Villi
Villi
Villi
A
Villi
Lamina propria
Submucosa
Epithelium
Serosa
Muscularis
externa
B
Figure 15-11B.
Villi of the
small intestine.
SEM,
3
170
An example of a scanning
electron
microscopy image showing ± n-
gerlike
villi
extending from the
intestinal wall and projecting into
the lumen is shown. Each villus is
composed of mucosa (
epithelium
and
lamina propria
). The lamina
propria (connective tissue) forms
the central core of the villus. Epi-
thelium with microvilli forms the
outer layer of the villus. The villi
are unique structures in the small
intestine; they are found neither
in the stomach nor in the large
intestine. The villi are tallest in
the duodenum and shortest in the
ileum. They gradually reduce in
height and size from the proximal
regions to the distal regions of the
small intestine. The
submucosa
and
muscularis externa
are also
shown here. The outermost layer
of the intestinal wall is
serosa
.
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