The digestive tract (alimentary canal) is an approximately 9-m-long, hollow, tubular structure that extends from the oral cavity to the anus whose

wall is modified along its length to perform the various facets of digestion.

•The oral cavity receives food and, via mastication and bolus formation, delivers it into the oral pharynx, from where it enters the esophagus and eventually the stomach.

•The gastric contents are reduced to an acidic chyme, which is transferred in small aliquots into the small intestine, where most digestion and absorption occur.

•The liquefied food residue passes into the large intestine, where the digestion is completed and water is resorbed.

•The solidified feces are then passed to the rectum for elimination through the anus.

A common architectural plan is evident for the digestive tract from the esophagus to the anus, in that four distinct concentric layers may be recognized to constitute the wall of this long, tubular structure.

LAYERS OF THE WALL OF

THE DIGESTIVE TRACT

The layers of the digestive tract are the mucosa, submucosa, muscularis externa, and the serosa/adventitia. These layers are described from the lumen outward, and they form the general plan of the digestive tract. The cellular composition and the general plan are modified along the digestive tract as one proceeds from the esophagus to the anus (see Table 14-1, which depicts these alterations).

•The innermost layer directly surrounding the lumen is known as the mucosa, which is composed of three concentric layers:

a wet epithelial lining with secretory and absorptive functions;

a connective tissue lamina propria containing glands and components of the circulatory system;

and a muscularis mucosae, usually consisting of two thin, smooth muscle layers, responsible for the mobility of the mucosa.

•The submucosa is a coarser connective tissue component that physically supports the mucosa and provides neural, vascular, and lymphatic supply to the mucosa. Moreover, in some regions of the digestive tract, the submucosa houses glands.

•The muscularis externa usually consists of an inner circular and an outer longitudinal smooth muscle layer, which is modified in certain regions of the digestive tract.

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Although these layers are described as circularly or longitudinally arranged, they are actually wrapped around the digestive tract in tight and loose helices, respectively.

Vascular and neural plexuses (Auerbach’s and Meissner’s) reside between the muscle layers.

The muscularis externa functions in churning and propelling the luminal contents along the digestive tract via peristaltic action.

Thus, as the circular muscles reduce the diameter of the lumen, preventing the movement of the luminal contents in a proximal direction (toward the mouth), the longitudinal muscles contract in such a fashion as to push the luminal contents in a distal direction (toward the anus).

•The outermost layer of the digestive tract is either a serosa or an adventitia.

The intraperitoneal regions of the digestive tract, that is, those that are suspended by peritoneum, possess a serosa. This structure consists of connective tissue covered by a mesothelium (simple squamous epithelium), which reduces frictional forces during digestive movements.

Other regions of the digestive tract are firmly attached to surrounding structures by connective tissue fibers. These regions possess an adventitia.

REGIONS OF THE DIGESTIVE TRACT

Esophagus

The esophagus is a short, muscular tube whose lumen is usually collapsed unless a bolus of food is traversing its length for delivery from the pharynx into the stomach.

•The mucosa of the esophagus is composed of a stratified squamous nonkeratinized epithelium; the lamina propria, a loose type of connective tissue, housing mucus-producing esophageal cardiac glands; and a muscularis mucosae composed only of longitudinally oriented smooth muscle fibers.

•The submucosa of this organ is composed of dense, irregular collagenous connective tissue interspersed with elastic fibers. This is one of the two regions of the digestive tract (the other is the duodenum) that houses glands in its submucosa. These glands are the mucus-producing esophageal glands proper.

•The muscularis externa of the esophagus is composed of inner circular and outer longitudinal layers. Those in the proximal (upper) one-third are skeletal; those in the middle one-third are skeletal and smooth, whereas those in the distal (lower) one-third are smooth muscle.

TABLE 14-1 • Selected Histological Features of the Alimentary Canal

*The muscularis mucosae is composed entirely of smooth muscle throughout the alimentary canal.

†The muscularis externa is composed entirely of smooth muscle in all regions except the esophagus. The upper third of the esophageal muscularis externa is all skeletal muscle; the middle third is a mixture of skeletal and smooth muscle; and the lower third is all smooth muscle.

CT, connective tissue.

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Stomach

The stomach functions in acidifying and converting the semisolid bolus into the viscous fluid, chyme, which undergoes initial digestion and is delivered into the duodenum in small quantities.

The gastric mucosa is lined by a simple columnar epithelium whose surface lining cells (not goblet cells) produce a mucous substance that coats and protects the stomach lining from the low pH environment and from autodigestion.

The lamina propria of the stomach houses gastric glands; depending on the region of the stomach, these are cardiac, fundic, or pyloric (see Graphic 14-1).

•The mucosa of the empty stomach is thrown into longitudinal folds, known as rugae. The luminal surface, lined by a simple columnar epithelium (surface lining cells), displays foveolae (gastric pits), whose base is perforated by several gastric glands of the lamina propria. All gastric glands are composed of parietal cells (oxyntic cells), mucous neck cells, surface lining cells, diffuse neuroendocrine system (DNES, formerly known as APUD) cells, and regenerative cells. Fundic glands, in addition, also house chief (zymogenic) cells.

Parietal cells live for approximately 200 days before being replaced by stem cells.

They secrete hydrochloric acid (HCl) into their intracellular canaliculi. These cells alter their morphology during HCl secretion, in that they increase their number of microvilli that project into the intracellular canaliculi. It is believed that these microvilli are stored as the tubulovesicular system, flanking the intracellular canaliculi when the cell is not secreting HCl. The production of

HCl is dependent on gastrin, histamine H2, and acetylcholine M3 binding to their respective receptors on the parietal cell basal membrane.

Parietal cells also secrete gastric intrinsic factor, a glycoprotein that binds to and forms a complex

with vitamin B12 in the gastric lumen. When this complex reaches the ileum, it binds to specific receptors on the surface absorptive cells, and the vitamin becomes absorbed (see Table 14-2).

Mucous neck cells live for approximately 6 days. They are located in the neck of gastric glands, and they manufacture soluble mucus that becomes part of and lubricates chyme.

Surface lining cells live for about 3 to 5 days and manufacture visible mucus that adheres to the lining of the stomach, protecting it from autodigestion.

The various types of DNES cells live for about 60 to 90 days. They produce hormones such as gastrin, somatostatin, secretin, and cholecystokinin.

Table 14-3 presents other hormones produced by the entire digestive tract.

Regenerative cells, located mainly in the neck and isthmus, replace the epithelial lining of the stomach and the cells of the glands.

Chief cells, located in the base of the fundic glands, live for about 60 to 90 days. They produce precursors of enzymes (pepsin, rennin, and lipase).

Small Intestine

The small intestine is composed of the duodenum, jejunum, and ileum.The mucosa of all three regions displays villi (singular: villus), extensions of the lamina propria, covered by a simple columnar type of epithelium. The epithelium is composed of goblet, surface absorptive, and DNES cells.

•Goblet cells produce mucinogen that becomes hydrated to form mucin, which, when mixed with the luminal contents of the stomach, becomes known as mucus.

•DNES cells release various hormones (e.g., secretin, motilin, neurotensin, cholecystokinin, gastric inhibitory peptide, and gastrin) (see Table 14-3 for hormones produced by the digestive tract).

•The tall, columnar surface absorptive cells possess dense accumulations of microvilli, forming the striated border. Their tips have a thick coat of glycocalyx,rich in disaccharidases and dipeptidases. These cells function in absorption of sugars, amino acids, fatty acids, monoglycerides, electrolytes, water, and many other substances. These epithelial cells also participate in the immune defense of the body by manufacturing secretory protein, which binds to the J protein component of the antibody and protects immunoglobulin A (IgA) as it traverses the epithelial cell and enters the intestinal lumen. Long-chained lipids, in the form of chylomicrons, are delivered to the lacteals, blindly ending lymphatic channels of the villus.

Simple tubular glands of the mucosa, the crypts of Lieberkühn, open into the intervillar spaces. These crypts are composed of simple columnar cells (similar to surface absorptive cells), goblet (and oligomucous) cells, DNES, and regenerative cells, as well as Paneth’s cells. The last are located in the base of the crypts and house large secretory granules believed to contain lysozyme, defensin and TNF-a.

•The lamina propria of the ileum houses large accumulations of lymphatic nodules, Peyer’s patches. The surface epithelium interposed between Peyer’s patches and the lumen of the ileum instead of being composed of simple columnar cells is formed by microfold cells (M cells) (see below).

The submucosa of the duodenum contains numerous glands, duodenal (Brunner’s) glands, that produce an alkaline, mucin-containing fluid that protects the intestinal lining.They also manufacture human epidermal growth factor (also known as urogastrone), a polypeptide that inhibits HCl production and enhances epithelial cell division.

Large Intestine

The large intestine is subdivided into the cecum, the colon (ascending, transverse, descending, and sigmoid), the rectum, the anal canal, and the appendix (see Graphic 14-2). The large intestine possesses no villi but does house crypts of Lieberkühn in its lamina propria.