Introduction

Basic Organization of the Gastrointestinal Tract

• The mucosa surrounds the lumen of the GI tract and consists of an epithelial cell layer supported by a thin layer of connective tissue known as the lamina propria. The muscularis mucosa is a thin layer of smooth muscle that supports the mucosa and provides it with the ability to move and fold.
• The submucosa is a thick connective tissue layer that contains arteries, veins, lymphatics, and nerves.
• The muscularis externa surrounds the submucosa and is composed of two muscle layers, the inner circular layer and outer longitudinal layer. These two layers move perpendicularly to one another and form the basis of peristalsis.
• The adventitia consists of connective tissue containing blood vessels, nerves, and fat. In the portions of the tract within the peritoneal cavity, it is lined by the mesothelium. Recall from the Laboratory on Epithelia that the mesothelium is a specially named layer of simple squamous epithelial cells. In these tissues, the adventitia is referred to as serosa.

• Protective mucosa is characterized by a stratified squamous epithelium. It is found in the oral cavity, pharynx, esophagus, and anal canal.
• Secretory mucosa contains cells that are responsible for the secretion of digestive enzymes. It is found exclusively in the stomach.
• Absorptive mucosa contains two key structures, crypts and villi, and is responsible primarily for absorbing digested nutrients. It is found along the entirety of the small intestine.
• Absorptive/protective mucosa specializes in water absorption and mucous secretion. It is found in the large intestine.

Gastrointestinal Motility

• The interstitial cells of Cajal serve as the pacemaker cells of the GI tract and are modulated by the parasympathetic nervous system.
• The submucosal plexus, or Meissner's plexus, is located in the submucosal layer and controls the mucosal glands and the muscularis mucosa.
• The myenteric plexus, or Auerbach's plexus, is located between the two layers of the muscularis externa and is responsible for peristaltic movements.

Oral Cavity and Esophagus

Stomach

• Mucous-secreting cells produce mucous and bicarbonate ions, which protect the stomach epithelium from the damaging effects of stomach acid. They appear pale and contain obvious mucous droplets. These cells are located in all layers of the gastric gland, but are particularly prominent in the gastric pit and in the neck of the glands.
• Parietal cells secrete hydrochloric acid and intrinsic factor, which is important for the absorption of vitamin B12 in the terminal ileum. They have a characteristic pyramidal shape and are usually found in the isthmus of the gastric gland. Parietal cells have a characteristic tubulovesicular system that fuses with their membrane canaliculi when they become activated.
• Chief cells produce pepsinogen, which is stored in large apical secretory granules. After pepsinogen is secreted, it is converted by the acidic environment of the stomach to pepsin that is an active protease. Chief cells are found in the base of the gastric glands.
• Enterochromaffin-like (ECL) cells produce histamine, which is important in the release of stomach acid. They are typically found in the base of the gastric glands.
• G-cells secrete the peptide hormone gastrin into the blood stream.
• Stem cells divide to replace the other cells in the gastric glands. They are located primarily in the neck of the glands.

• The cardia is a circumferential ring about 3 centimeters deep distal to the gastro-esophageal junction. Its glands tend to be convoluted and are lined primarily by mucous-secreting cells.
• The body is the main part of the stomach and is bounded by the greater and lesser curvatures. Its glands are straight with limited branching and are lined by a smaller population of mucous-secreting cells than those of the cardia. It also contains chief and parietal cells.
• The pylorus is the distal third of the stomach and terminates at the beginning of the duodenum. It possesses glands with deeper pits and large amounts of coiling and branching. These glands contain many mucous-secreting cells, ECL cells, and G-cells.

Small Intestine

• At a gross level, the small intestine is a long tube into whose lumen projects the plicae circularis, circular folds of the mucosal epithelium, lamina propria, muscularis mucosa, and submucosa.
• Villi, finger-like projections involving only the epithelium and lamina propria, project into the lumen. They have a central lymphatic vessel known as a lacteal, which is crucial for the absorption of lipids from the intestine.
• Microvilli make up a brush border on the surface of the columnar cells of the mucosal epithelium.

• In the duodenum, pancreatic juice and bile are released into the lumen. Digestion is completed by enzymes in the pancreatic juice and on the surface of the epithelial cells where the products of digestion are absorbed. The glands of Brunner, extensive mucous glands that are found in the submucosa, are found exclusively in this segment. The villi of the duodenum are also exceptionally long.
• The jejunum is the site of a great deal of nutrient absorption and has the most prominent plicae circularis.
• The ileum has the shortest villi and is the site of vitamin B12 absorption. It is characterized by abundant Peyer's patches, which are clumps of diffuse lymphoid tissue, in the submucosa.

• Enterocytes are the tall columnar epithelial cells that make up most of the intestinal lining and perform most of the intestinal digestive and absorptive functions.
• Goblet cells store and secrete mucous.
• Paneth cells serve an immune function and are found at the base of the crypts.
• Enteroendocrine cells produce hormones that govern motility and secretion, just as they do in the stomach.
• Stem cells replenish the other cell types and are found at the base of the crypts.

Large Intestine