CHAPTER 15
Digestive Tract
297
Goblet
Goblet
cells
cells
Goblet
cells
Germinal center
Germinal center
of lymphatic nodules
of lymphatic nodules
Germinal center
of lymphatic nodules
C
Glands of
Glands of
Lieberkühn
Lieberkühn
Glands of
Lieberkühn
Submucosa
Muscularis externa
Villi
Villi
Villi
Mucosa
Mucosa
Mucosa
Inner circular muscle
Outer longitudinal muscle
Muscularis
mucosae
Glands of
Glands of
Lieberkühn
Lieberkühn
Glands of
Lieberkühn
Villus
Villus
Villus
A
Figure 15-15A.
Jejunum, small intestine.
H&E,
3
34; inset
3
103
The
jejunum
is a segment of small intestine between the duode-
num and the ileum. It is similar in general structure and layers to
the other regions of the small intestine. It contains
mucosa
,
sub-
mucosa
,
muscularis externa
, and
serosa
. The jejunum has neither
Brunner glands
nor
Peyer patches
. The cells of the epithelium of
the mucosa are similar to those of the epithelium of other regions
of the small intestine (see Figs. 15-12A to 15-13B). Goblet cells
steadily increase in number along the entire length of the small
intestine from the duodenum to the ileum. Paneth cells are often
found at the base of the
glands of Lieberkühn
(
lower inset
). The
glands of Lieberkühn are intestinal glands (simple tubular glands),
which extend from the spaces between the bases of the villi deep
into the lamina propria.
Figure 15-15C.
Mucosa of the ileum, small intestine.
H&E,
3
68
Here is an example of the
mucosa
of the
ileum
showing
numerous
goblet cells
in the surface epithelium. In this section, two lym-
phatic nodules are located in the lamina propria. These lymphatic
nodules have
germinal centers
and are, therefore, secondary
lymphatic nodules. These nodules may extend into the submu-
cosa (Fig. 15-15B). The lymphatic nodules and Peyer patches
are locations where lymphocytes can interact with antigens and,
therefore, play important roles in immunological function. Naive
B cells (B lymphocytes) within these lymphoid patches are primed
and awaiting exposure to unique epitopes. When stimulated by a
speci± c antigen from the intestinal mucosa, they differentiate into
plasma cells and memory B cells. In response, the plasma cells pro-
duce large quantities of immunoglobulins ([Ig] antibodies), espe-
cially IgA to combat mucosal infection. The memory B cells live
on in the
Peyer patches
to retain immunity to a speci± c antigen.
Peyer
Peyer
patch
patch
Peyer
patches
Mucosa
Mucosa
Mucosa
Submucosa
Submucosa
Submucosa
Muscularis enterna
Muscularis enterna
Muscularis externa
Serosa
Serosa
Serosa
B
Figure 15-15B.
Ileum with Peyer patches, small intestine.
H&E,
3
13
The
ileum
is the longest segment of the small intestine making up
three ± fths of the 6 to 7 m length of the small intestine. One of the
unique features of the ileum is the presence of clusters of lymphatic
nodules called
Peyer patches
. These are most numerous in the dis-
tal portion of the ileum. Some isolated lymphatic nodules may be
found in other parts of the digestive tract but not in aggregations
of clusters of nodules like Peyer patches. The
villi
in the ileum are
shorter and smaller than in other parts of the small intestine. The
numbers of
goblet cells
are greatly increased in the ileum. The
inset
shows an endoscopic image of the ileum with its relatively smooth
surface.
Vitamin K is absorbed in both the jejunum and the ileum, but
vitamin B12 is only absorbed in the ileum (especially the ter-
minal ileum). The absorption of vitamin B12 requires coupling
with gastric intrinsic factor, which is produced by parietal cells in
the stomach; these two substances become intimately associated
with the wall of the ileum. If a large portion of the stomach or
ileum is surgically removed,
vitamin B12 de±
ciency
may result,
leading to
megaloblastic anemia
and
neurologic symptoms
.
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