Esophagus,+Stomach,+and+Intestines

=**Objectives**=

1) esophagus- transport 2) stomach- mechanical disruption, absorption of water and alcohol 3) small intestine – chemical and mechanical digestion / absorption. 4) large intestine – absorb electrolytes (salts and water) and vitamins 5) rectum and anus – defecation
 * 1. Describe the overall plan of the alimentary canal.**

1) Mucosa: 3 layers are always present in GI tube with varying thickness 2) Submucosa: contains vessels, some nerves. Contains glands in 2 regions of GI (esophagus and duodenum). Also contains neuronal plexus = submucosal plexus(Meissner's) in most portions of GI tract. 3) Muscularis: has 2 layers of smooth muscle, an inner circular and an outer longitudinal with the myenteric plexus(Auerbach's) of nerves in between. 4) Serosa: is two layers, connective tissue internally, and has epithelium covering it, usually squamous epithelium, that corresponds to the visceral peritoneum.
 * 2. Describe the features of each layer of the alimentary canal.**
 * epithelium
 * lamina propria- CT
 * muscularis mucosae – thin layer of muscle

All enteric neurons in the plexus and support cells are derived from the neural crest. Thus they are intrinsic neurons(inside GI tract). The GI does still receive signals from extrinsic sources but only modify the activity. Parasympathetic inputs from the Vagus nerves release acetylcholine, which increases motility in the GI tract. Sympathetic inputs from the splanchnic nerves release norepinephrine, which relaxes the gut.
 * 3. Describe the innervation of the alimentary canal.**

Scattered in epithelium are enteroendocrine cells (EC). They secrete mainly serotonin. The stretch (presence of food) on these EC cells cause the release of serotonin to stimulate peristalsis from the plexuses, and the release of gastrin and histamine to parietal cells to stimulate HCl production.

Myenteric plexus(Auerbach's Plexus): mostly motor, regulates peristalsis. Located between muscle layers in muscularis externa.

Submucosal plexus(Meissner’s plexus): Is secretory-motor. Release hormones and peptides. Innervate the mucosa and are located in the submucosa. Also communicate with the Myenteric plexus via hormone release to initiate peristalsis.


 * 4. Describe the specializations of each region of the alimentary canal.**

__Esophagus__ Has esophageal sero-mucus glands in the sub-mucosa( secrete mucous, pepsinogen, and lysozyme) that are responsive to HCl. Also esophageal-cardiac mucous glands in the mucosa. //Memory hint:// //**S**////ero-////**M**////ucous(name),// //**S**////ub-////**M**////ucosa(location),// //**S**////erous-////**M**////ucous(secretion). Cardiac// //**M**////ucosa secretes only// //**M**////ucous.//

Upper region has skeletal muscle that phases out into smooth muscle on the way down, allowing voluntary control over swallowing. Inner lining is Stratified Squamous Epithelium. Contains BOTH neural plexuses.

__Cardiac Stomach__ Contains no chief cells, but many surface lining cells. Has highly coiled glands with shallow gastric pits. Regenerative cells are found on the junction of the pit and gland, near the lumen. Glands are contained in the Lamina Propria of the mucosa. S//tomach (all parts) DO NOT have the Submucosal Plexus, only the Myenteric Plexus.//

__Fundic/Body Stomach__ Contains long, straight glands with short gastric pits. Chief cells in body release pepsinogen, renin, and gastric lipase. Many regenerative cells are found on the junction of the pit and gland, near the lumen.

__Pyloric Stomach__ Mainly mucous neck cells with very few chief cells. Contains branched, deep glands with deep gastric pits. Regenerative cells are found on the junction of the pit and gland.

__Duodenum__ Villi are broad, numerous, and tall. Few goblet cells. Brunner’s glands in the submucosa secrete alkaline mucous fluid to neutralize the acidic contents coming out of the stomach. Contains plicae circularis, villi, and microvilli to increase surface area. Has Crypts of Leieberkuhn in between villi with Paneth cells and regenerative cells at the bottom

__Jejunum__ Villi are narrower, shorter, and less dense than in the duodenum. Many goblet cells. Contains plicae circularis, villi, microvilli and crypts of Leieberkuhn inbetween villi.

__Ileum__ Villi are the shortest, narrowest and the fewest in number. Many goblet cells. Contains Peyer’s patches that are located opposite to where the mesentery attaches. Microfold cells are present to pickup and transport antigens to Peyer’s patches. Contains plicae circularis, villi, microvilli and crypts of Leieberkuhn inbetween villi.

__Colon and Cecum__ No villi or Paneth cells. Comprised of crypts of Leieberkuhn. Crypts contain goblet cells to protect large intestinal lumen. Muscularis externa is Teniae coli, which are constantly contracted to produce haustra and plicae semilunaris that form structural compartments.

__Rectum__ Crypts of Leieberkuhn are fewer but deeper. Increased number of goblet cells compared to the colon. At the rectal-anal junction, epithelium become stratified squamous non-keritinized and contains branched circumanal glands.

__Anal Canal__ Longtitudinal muscles in the rectum disappear to an elastich sheath. Contains the internal and external anal sphincter. Anal column forms perpendicular folds that increase flexibility. Pectinate line is where the rectal-anal junction changes epithelium from simple columnar to stratified squamous non-keratinized. Epithelium at the anal skin is stratified squamous keratinized.

__Appendix__ Contains Peyer’s patches on all sides, not just opposite to where the mesentery attaches.


 * 5. Describe/list the functions of the alimentary canal by region and cell type.**


 * Region || Cell Type || Function ||
 * Esophagus || Stratified squamous non-keratinized, sero-mucous and mucous glands, skeletal and smooth muscle || Transportation of bolus ||
 * Cardiac Stomach || Simple columnar, surface lining cells || Mechanical and chemical digestion ||
 * Fundic/Body Stomach || Simple columnar with Rugae, surface lining cells, chief cells, parietal cells, EC cells || Mechanical and chemical digestion ||
 * Pyloric Stomach || Simple columnar with Rugae, mainly Neck cells || Mechanical and chemical digestion ||
 * Duodenum || Simple columnar with goblet cells, Villi/Microvilli on Pliae Circ, Paneth cells, EC cells || Enzymatic digestion and absorption ||
 * Jejunum || Simple columnar with goblet cells, Villi/mV/PC, Paneth cells, EC cells || Absorption ||
 * Ileum || Simple columnar with goblet cells, Villi/mV/PC, Paneth cells, EC cells || Absorption ||
 * Colon and Cecum || Simple columnar with goblet cells, Absorptive, Goblet, Regenerative, EC cells || Water reabsorption ||
 * Rectum || Simple columnar with goblet cells || Water reabsorption and preparation for defecation ||
 * Anal Canal || Simple cuboidal; stratified squamous non-keratinized; stratified squamous keratinized || Preparation for defecation ||
 * Appendix || Simple columnar with goblet cells || Absorption ||


 * 6. Describe the function of the specialized cells of the gut, e.g. digestive glands, gastric pits, enteroendocrine cells, paneth cells, mucus neck cells etc.**

__Myenteric/Auerbach’s Plexus__ Derived from neural crest cells. Mostly motor nerves regulating peristalsis. Located between muscle layers in muscularis externa. Present in all parts of tract.

__Submucosal (Meissner’s) Plexus__ Derived from neural crest cells. Mainly secretory-motor, release hormones and peptides. Innervates the mucosa. Located in the submucosa. Absent in stomach.

__Esophageal-cardiac Glands__ Mucous glands in the esophagus at the pharynx and gastric junction.

__Esophageal Sero-Mucous Glands__ Serous-mucous glands in the submucosa of the esophagus. Secretes pepsinogen and lysozyme.

__Gastric Pits__ Invaginations of the stomach epithelium that lead to gastric glands at the bottom. Contain surface lining cells and regenerative cells.

__Rugae__ Folds in the mucosa and submucosa of the stomach.

__Gastric Glands__ Found in the lamina propria of the stomach and open into the gastric pits. Contains regenerative cells, mucous neck cells, parietal cells, chief cells, and enteroendocrine cells.

__Parietal Cells__ Stomach gastric gland cells that secrete hydrochloric acid and gastric intrinsic factor (for vitamin B12 metabolism).

__Chief Cells__ Stomach gastric gland cells that secrete pepsinogen, gastric lipase, and rennin.

__Enteroendocrine Cells__ Triangular-shaped cells with large nucleus. May contain granules. Found in glands and crypts. Secretes serotonin, gastrin, histamine and other hormones. Also secretes glucagon and gastric inhibitory peptide in gastric glands to control parietal cells' HCl production.

__Plicae Circularis__ Large folds in the small intestine, including the mucosa and submucosa. Lined with Villi, which are lined with microvilli.

__Villi__ Projection of epithelium on small intestine. Contains individual smooth muscle strands(originating in musc. mucosa), blood vessels, and lymph vessels.

__Paneth Cells__ Found in the small intestine at the base of crypts. Secretes lysozyme, which breaks down bacterial walls. Exact function unknown, but diagnostic of small intestine.

__Peyer’s Patches__ Lymph organs found in the ileum and appendix.

__Crypts of Lieberkuhn__ Invaginations between villi in the small intestine and in the epithelium of the large intestine, containing regenerative cells and paneth cells(small intestine ONLY) at the bottom.

__Brunner’s Glands__ Found in the submucosa of the duodenum. Branched tubulo-alveolar glands that secrete alkaline mucous fluid to neutralize acidic chyme from the stomach.

__Teniae coli__ Discontinuous muscularis externa on the large intestine that are continually contracted, forming haustra and plicae semilunaris.


 * 7. Describe the structural and functional regions of the esophagus.**

Esophageal mucosa is stratified squamous nonkeratinized epithelium. The muscularis mucosae is discontinuous single layer of muscle. The lamina propria is dense irregular CT, has large amount of diffuse lymphoid tissue, arterioles and venuoles. The esophagus contains esophageal-cardiac glands located at junction of esophagus and stomach and at junction of esophagus and pharynx and sero-mucous glands along entire length of esophagus. The submucosa consists of dense fibroelastic CT, has esophageal glands and serous glands secreting pepsinogen and lysozyme. The muscularis externa of the upper region is all skeletal, the middle region transitions from skeletal to smooth and the lower region is all smooth muscle. The esophagus has adventitia connecting it to the diaphragm superiorly.