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The gastrointestinal (GI) tract is a continuous tube beginning at the mouth and ending at the anus (Fig. 376-1). 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. The main function of the GI tract is to digest food and absorb nutrients and fluid. At the cellular level, the tissue architecture of the gut tube is similar throughout, consisting of 4 concentric layers; from the inner layer to outer layer, 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. 376-2).

Figure 376-1

The gastrointestinal tract.

Figure 376-2

Tissue architecture of the gastrointestinal tract. (Reproduced with permission from Gartner L, Hiatt J: Color Textbook of Histology. Philadelphia: Saunders; 2007.)

The epithelium throughout the GI tract is a highly proliferative tissue that continuously undergoes renewal. The source of new epithelial cells is a stem cell compartment located in the most basal zone. 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.

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.

The lamina propria contains fibroblasts and myofibroblasts that regulate epithelial proliferation and differentiation. Also found 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 mucosal layer from the underlying submucosa and provides additional structure and motility to the mucosa.

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.

The muscularis propria consists of 2 bands of smooth ...

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