The structure of the esophagus and gastrointestinal tract varies considerably from region to region; most of the variation occurs within the mucosa. The epithelium differs throughout the alimentary canal and is adapted to the specific function of each part of the tube. The histologic characteristics of these layers are described below in relation to specific regions of the digestive tube. The mucosa has three principal functions: protection, absorption, and secretion.

The epithelium of the mucosa serves as a barrier that separates the lumen of the alimentary canal from the rest of the organism

The epithelial barrier separates the external luminal environment of the tube from the tissues and organs of the body. The barrier aids in protection of the individual from the entry of antigens, pathogens, and other noxious sub stances. In the esophagus, a stratified squamous epithelium provides protection from physical abrasion by ingested food. In the gastrointestinal portion of the alimentary tract, tight junctions between the simple columnar epithelial cells of the mucosa serve as a selectively permeable barrier. Most epithelial cells transport products of digestion and other essential substances such as water through the cell and into the extracellular space beneath the tight junctions.

The absorptive function of the mucosa allows the movement of digested nutrients, water, and electrolytes into the blood and lymph vessels

The absorption of digested nutrients, water, and electrolytes is possible because of projections of the mucosa and submucosa into the lumen of the digestive tract. These surface projections greatly increase the surface area available for absorption and vary in size and orientation. They consist of the following structural specializations (see Fig. 16.1)

• Plicae circulares are circumferentially oriented submucosal folds present along most of the length of the small intestine.

• Villi are mucosal projections that cover the entire surface of the small intestine, the principal site of absorption of the products of digestion.

• Microvilli are tightly packed, microscopic projections of the apical surface of intestinal absorptive cells. They further increase the surface available for absorption.

In addition, the glycocalyx consists of glycoproteins that project from the apical plasma membrane of epithelial absorptive cells. It provides additional surface for adsorption and includes enzymes secreted by the absorptive cells that are essential for the final steps of digestion of proteins and sugars. The epithelium selectively absorbs the products of digestion both for its own cells and for transport into the vascular system for distribution to other tissues.

The secretory function of the mucosa provides lubrication and delivers digestive enzymes, hormones, and antibodies into the lumen of the alimentary tube.

Secretion is carried out largely by glands distributed throughout the length of the digestive tube. The various se-

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