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The gastrointestinal (GI) tract is a continuous tube beginning
at the mouth and ending at the anus (Fig. 381-1).
Its main function is to digest food and absorb nutrients and fluid.
It is subdivided into 4 regions: (1) esophagus, (2) stomach, (3)
small intestine, and (4) large intestine. The liver and pancreas
directly communicate with the GI tract via ducts that join with
the duodenum, the most anterior segment of small intestine. At the
cellular level, the tissue architecture of the gut tube is similar
throughout, consisting of 4 concentric layers.1 From
inner to outer they are (1) mucosa, (2) submucosa, (3) muscularis
propria (externa), and (4) adventitia or serosa. The mucosa is composed
of epithelium, lamina propria, and muscularis mucosae (Fig.
381-2).
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The epithelium throughout the GI tract is a highly proliferative
tissue because it continuously undergoes renewal. The source of
new epithelial cells is a stem cell compartment located in the most
basal zone.2 The stem cells give rise to rapidly
proliferating (transit amplifying) cells that migrate toward the
lumen and differentiate into the epithelial subtypes that subserve
tissue function. Conditions that result in epithelial damage often
result in regenerative changes, reflecting increased cell turnover
due to accelerated loss of epithelial cells. For example, esophagitis
is often accompanied by basal zone hyperplasia, celiac disease by
crypt hyperplasia, and colitis by branched and elongated crypts.2
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The boundary between the epithelium and the lamina propria is
formed by the basement membrane. It is made of extracellular matrix
proteins such as collagen, laminin, and fibronectin elaborated by both
the epithelium and the lamina propria cells. It serves an important
function in the maintenance of a differentiated, functional epithelium.
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The lamina propria contains fibroblasts and myofibroblasts that
regulate epithelial proliferation and differentiation. Also in the
lamina propria are immune cells, nerves, lymphatics, and blood vessels,
all of which support the epithelium and guard against invasion by
pathogens. Embedded within the lamina propria are epithelial glands
that are contiguous with the surface epithelium and that open into
the lumen. The muscularis mucosae is a thin band
of smooth muscle that separates the mucosa layer from the underlying
submucosa and provides additional structure and motility to the
mucosa.
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The submucosa consists of supporting collagenous fibers, larger
blood vessels, lymphatics, nerve fibers and ganglia, and occasional
lymphoid follicles. In the duodenum, the submucosa also contains Brunner
glands that secrete bicarbonate to neutralize gastric acidity.
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The muscularis propria consists of 2 bands of
smooth muscle with an intervening layer of nerves and ganglia, the
myenteric plexus. ...