The GI tract is essentially a long tube from the mouth to the anus, with input along the way via ducts from other organs such as biliary tract and pancreas. Water is absorbed and food is processed inside the tube, with nutrients absorbed and wastes expelled.

Oral Cavity

The oral cavity is lined by a stratified squamous mucosa. Major and minor salivary glands composed of tubulo-alveolar glands produce serous and/or mucous secretions that aid in chewing and swallowing. Ducts from the major salivary glands drain to the oral cavity. The major salivary glands include the submandibular gland and the parotid gland. Salivary gland amylase provides some initial digestion of carbohydrates.

The tongue in the oral cavity is covered with stratified squamous mucosa. In the submucosa are minor salivary glands producing serous fluid. The bulk of the tongue consists of the genioglossus muscle with the muscle bundles arranged in three planes to provide movement in any direction. The squamous mucosa extends to become the gingiva at the base of the teeth. The surface of the tongue forms papillae. The filiform papillae impart a velvety texture to the dorsal (upper) surface of the tongue and allow for a scraping function. Your kitty cat has more prominent filiform papillae that impart an even rougher surface. The circumvallate papillae are arranged in a "V" pattern toward the back of the tongue and have associated taste buds. Foliate papillae are at the posterolateral aspect of the tongue and have associated taste buds. Fungiform papillae have a rounded surface and are nonkeratinized to give the appearance of a red dot pattern on the dorsum of the tongue and have associated taste buds.

Teeth have roots that anchor in the mandible and maxilla (16 upper and 16 lower for a total of 32). Each tooth has an anatomical crown covered by enamel and a root embedded in bone. Teeth can have one root or multiple roots. The overlying enamel protects the tooth and is about 95% hydroxyapatite crystal. The dentin beneath the enamel is not quite as hard--about 65% hydroxyapatite crystal. The cementum around the tooth root is about 50% hydroxyapatite crystal. The periodontal ligament is composed of collagen that interfaces the cementum to the bone of the jaw. The soft pulp is inside the tooth and is composed of loose connective tissue, vessels, nerves, and abundant intercellular ground substance. The gingiva (gum) surrounds the base of the tooth and attaches firmly at the lower enamel surface to seal off the parts of the tooth not covered by protective enamel (the clinical crown is seen above the gingival margin).

Gingival recession, seen with aging and with periondontitis and gingivitis, exposes the dentin below the enamel. The cementum and the periodontal ligament are located much lower down in the tooth socket in the bone of the jaw. Decay that is initiated by gum recession can eventually reach the pulp. When decay from any tooth surface is very deep (or traumatic injury fractures the tooth) so that bacteria have access to the pulp, or the remaining tooth material cannot hold a filling after removal of the decayed material, then a "root canal" procedure can be done. In this procecedure, the crown is removed, the root canal drilled out and replaced with inert material to seal it, and an artificial crown placed. If such severe decay is not treated, then the inflamed pulp swells and causes pain that interferes with chewing. Untreated, an abscess can occur. Through most of human history, sepsis from dental problems was a common cause of death. The degree of development of a society can, in large part, be measured by the level of dental care available to its citizens.

GI Tract Architecture

The GI tract is arranged throughout with a mucosal lining, a lamina propria with connective tissue and lymphoid tissue, a thin muscularis mucosae layer of smooth muscle, a broad submucosa containing connective tissue and lymphoid tissue, and muscularis with inner circular and outer longitudinal layers. Intraperitoneal portions of the GI tract have an outer serosa. There are autonomic nerve plexuses in small and large bowel to coordinate peristaltic movements. These include the submucosal (Meissner's) plexus and between circular and longitudinal muscle bundles is the myenteric (Auerbach's) plexus. Included in these plexuses are parasympathetic ganglion cells as well as the interstitial cells of Cajal, which are the "pacemaker" cells for peristalsis. Indistinct, solitary neuroendocrine cells are scattered throughout the mucosa.

Esophagus

The esophagus is a muscular tube lined by a stratified squamous mucosa. Predominantly voluntary striated muscle to initiate swallowing in the upper esophagus merges and changes to involuntary smooth muscle distally in lower esophagus that provides propulsive peristalsis of food and liquid boluses into the stomach. There is a physiologic lower esophageal sphincter of smooth muscle with muscle tone providing an effective barrier to regurgitation. There are minor mucous glands in the submucosa of the esophagus for lubrication. At the gastroesophageal junction, the squamous epithelium interdigitates with the glandular epithelium of the stomach.

Stomach

The stomach has a columnar mucous epithelium lining broader surface pits and longer, narrower glands. The pits are shorter in the fundus and longer in the antrum. The glands in the gastric fundal region contain parietal cells secreting hydrochloric acid and intrinsic factor. Fundal glands also have chief cells secreting pepsinogen, a proteolytic enzyme. Cuboidal mucous neck cells in the glands protect against the acid and enzyme secretions. In the antrum and pyloric region distally in the stomach, there are columnar mucous cells in pits and glands.

A thick band of smooth muscle forms the pylorus, which regulates passage of food into the small intestine.

Small Intestine

The small intestinal mucosa has surface villi lined by columnar cells. The villi terminate in the lamina propria as glandular lumina known as crypts of Lieberkuhn. The villi greatly increase the surface absorbtive area. In the duodenum, the lamina propria and the submucosa have proportionately more lymphoid tissue than the rest of the GI tract. The duodenum has prominent rounded pale Brunner's glands with mucous-secreting cells in the submucosa. The jejunum has more prominent folds (plicae) of the mucosa to increase absorbtive area. Each intestinal villus contains a blind-ended lymphatic channel known as a lacteal. Lipids are taken up to form chylomicrons that preferentially congregate in the lacteals after a fatty meal and are grossly visible as small 1 to 3 mm pale tan to yellow areas on the mucosal surface.

The ileum has more prominent submucsal lymphoid tissue, which appears as small nodules or as elongated ovoid structure known as Peyer's patches. If all the lymphoid tissue in the GI tract were coalesced, it would constitute the largest lymphoid organ in the body. The major immunoglobulin secreted by plasma cells of the gastrointestinal tract (and respiratory tract) is IgA, so-called "secretory" IgA. This is bound to protein on the glycocalyx overlying the microvilli of the brush border to neutralize harmful agents such as infectious organisms. Persons who are deficient in IgA are more likely to have gastrointestinal and respiratory tract infections.

Proceeding distally in the small intestine, there are more Paneth cells at the base of the glands. Paneth cells contain prominent red zymogen granules.

There are several lesser known cells. The neuroendocrine (enteroendocrine) cells scattered in the glands become more numerous distally in small intestine. There are a variety of enteroendocrine cells in the mucosa of the gut. When food passes into the small intestine, some of them release cholecystokinin, which promotes digestion with release of cholecystokinin which causes the gallbladder to contract and release bile that aids in digestion of fats. Cholecystokinin also causes release of a variety of enzymes from pancreatic acinar cells.

Interstitial cells of Cajal (ICC) are the pacemaker cells in gastrointestinal (GI) muscles. They also mediate or transduce inputs from enteric motor nerves to the smooth muscle syncytium. What is known about functional roles of ICC comes from developmental studies based on the discovery that ICC express c-kit, the proto-oncogene that encodes the receptor tyrosine kinase Kit, and proper development of ICC depends upon signalling via the Kit receptor pathway. ICC can only be identified by immunohistochemical staining. Melanocytes, hematopoietic cells, and germ cells may also depend upon c-kit for development. Loss of ICC disrupts normal GI tract motility. Auerbach's myenteric plexus has an intrinsic function to control peristalsis, though it also receives parasympathetic innervation through the vagus nerve. Sympathetic impulses inhibit peristalsis. If parasympathetic or sympathetic innervation to the gut is lost, peristalsis continues because of the ICC.

Submucosal (Meisner's) plexuses along with the myenteric (Auerbach's) plexuses contain ganglia that function to maintain peristalsis through internal and external stimuli.

The microfold cells, or M cells (membranous cells), have phagocytic properties and help to process antigen and present it to lymphoid cells. The M cells are found over aggregates of lymphoid tissue in the intestine.

Appendix

The appendix has a narrow lumen. The mucosa is similar to that of the colon. The submucosa has prominent lymphoid tissue.

Colon

The colon has a mucosal architecture of long tubular glands (crypts of Lieberkuhn) lined by columnar mucous cells. Goblet cells are numerous and provide lubrication for the transit of stool. Lymphoid nodules are present in the lamina propria and submucosa. The outer longitudinal layer of muscle is coalesced into long bands known as the taenia coli.

At the anorectal junction, there is a mucosal transition from columnar cells lining crypts to stratified squamous mucosa. Both above and below the junction are prominent submucosal veins (internal and external hemorroidal veins) which when dilated form hemorrhoids with itching and bleeding. A layer of skeletal muscle is present at the anus to provide a sphincter under voluntary control.

The function of the colon is primarily to absorb water and electrolytes to concentrate the volume of stool. There is about 7 to 10 liters of gas traversing the colon each day, though only about half a liter is passed as flatus. The gaseous components include swallowed air (nitrogen and oxygen) along with methane and hydrogen from digestive processes and bacterial growth. Complex undigestible sugars such as raffinose and stachyose are food for bacteria, which turn these sugars into gas. Hence, this is one of the side effects of eating certain foods, such as beans.