Histology of the GI Tract PDF

r Four basic mucosal types: 1. Protective • Oral cavity, pharynx, esophagus and anal canal. • A stratified mucosal lining of this type is well suited to sites of potential frictional trauma. 2. Secretory. • Occurs only in the stomach. • These glands act to produce various combinations of acid a...

r Four basic mucosal types: 1. Protective • Oral cavity, pharynx, esophagus and anal canal. • A stratified mucosal lining of this type is well suited to sites of potential frictional trauma. 2. Secretory. • Occurs only in the stomach. • These glands act to produce various combinations of acid and digestive enzymes in order to facilitate digestion of food whilst also secreting mucus to protect the mucosa itself from injury. 3. Absorptive. • This mucosal form is typical of the entire small intestine. • The mucosa is arranged into finger-like projections called villi which serve to dramatically increase surface area of the mucosa. 4. Absorptive/protective • The entire large intestine. • Cells specialized for water absorption, as well as mucus-secreting goblet cells to lubricate the passage of faeces. General layers of the alimentary Canal 1. Mucosa 2. Submucosa 3. Muscularis Externa 4. Serosa/Adventitia Stevens & Lowe's Human Histology, Chapter 11 1. Mucosa • 1. Stratified squamous epithelium. • 2. The lamina propria: - Houses glands , lymph vessels, occasional lymphoid nodules members of the gut-associated lymphoid system (GALT). • 3. Muscularis mucosae , inner circular and an outer longitudinal layer of smooth muscle. Gut-associated lymphoid system (GALT) Function: Protection GALT is prominent in the lamina propria of the small intestine. Lymphatic nodules, lymphocytes, macrophages, plasma cells, and eosinophils in the lamina propria. 2. SUBMUCOSA • Irregular fibroelastic connective tissue layer. • No glands except in the esophagus and duodenum. • Most important feature: Meissner submucosal plexus. Controls the motility of the mucosa and the secretory activities of its glands. Innervation of the GI tract Enteric nervous system: from the esophagus to the anus. Function: controls secretory and motile functions of the alimentary canal. • Meissner submucosal plexus. • Auerbach myenteric plexus. 3. Muscularis Externa • Inner circular smooth muscle layer. • Outer longitudinal smooth muscle layer. • Composed of smooth muscle (except in the upper esophagus). • The interstitial cells of Cajal (ICC) (rhythmic contractions). • The Auerbach myenteric plexus (Auerbach plexus) is situated between these two muscle layers and regulates the activity of the muscularis externa. • Responsible for peristaltic activity . 4. SEROSA or ADVENTITIA • If the region of the alimentary canal is intraperitoneal, it is invested by peritoneum, and the covering is known as the serosa . • Serosa: Simple squamous epithelium (mesothelium). Cover the organs in body cavities. If the organ is retroperitoneal, it adheres to the body wall by its dense irregular connective tissue component and is known as the adventitia . Adventitia: Connective tissue, attaches the organ to the surrounding tissue. Oral Cavity • The oral cavity is lined by stratified squamous epithelium. • The underlying submucosa: Salivary glands, which can secrete both a serous and a mucous fluid, occasional sebaceous glands. • The tongue: is a highly muscular organ. -Type of epithelium: thin non-keratinizing stratified squamous epithelium(ventral) thick keratinizing stratified (dorsal). -Characteristics: taste buds, lingual papillae, serous, mucous and mixed minor salivary glands, sublingual folds. Esophagus: • Epithelium: The esophageal mucosa has nonkeratinized stratified squamous epithelium. • Lamina propria: It houses esophageal cardiac glands: They produce a mucous secretion that coats the lining of the esophagus. • Muscularis mucosae is single layer of longitudinally oriented smooth muscle fibers. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. Submucosa: esophageal glands, mucous cells, serous cells. The muscularis externa: • Composed of both skeletal and smooth muscle fibers. • Upper one-third: skeletal muscle. • Middle portion: combination. • Lower third: smooth muscle. • The distal 1-2 cm of the esophagus, in the peritoneal cavity, is covered by serosa, the rest is enclosed by the adventitia. Esophagogastric Junction Below the diaphragm, the epithelium abruptly changes to simple columnar, a mucus-secreting epithelium similar to the epithelium of the cardia region of the stomach. Di Fiore’s Atlas of histology with functional correlations. 11th ediition.. STOMACH All the gastric regions display rugae, longitudinal folds of the mucosa and submucosa which disappear in the distended stomach. Rugae permit expansion of the stomach as it fills with food and gastric juices. Stevens & Lowe's Human Histology, Chapter 11 Stomach (histologically) • The cardia is a small area of mucussecreting glands surrounding the entrance of the esophagus. • The mucosa of the fundus and body consists of glands that secrete acidpepsin gastric juices as well as some protective mucus. • The glands of the pylorus secrete mucus of two different types and there are associated endocrine cells which secrete the hormone gastrin. . Fundic Mucosa • 1. Epithelium lining the lumen. • 2. Underlying connective tissue (The lamina propria). • 4. The smooth muscle layers forming the muscularis mucosae. Surface Mucous cells: Function: production of a gel-like substance that adheres to the lining of the stomach and protects it from autodigestion. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. • The epithelial lining of the stomach invaginates into the mucosa, forming gastric pits (shallowest - cardiac region and deepest - pyloric region). • Increase the surface area of the gastric lining. • Five to seven gastric glands of the lamina propria empty into the bottom of each gastric pit. Di Fiore’s Atlas of histology with functional correlations. 11th ediition.. Lamina Propria: • Fundic Glands: • Extends from the muscularis mucosae to the base of the gastric pit and is subdivided into three regions: 1. Isthmus. 2. Neck. 3. Base. • Composed of six cell types. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. • Regenerative cells proliferate to replace all of the specialized cells lining the fundic glands, gastric pits, and luminal surface (every 5 to 7 days). • Mucous neck cells produce a soluble mucus that is mixed with and lubricates the chyme, reducing friction as it moves along the digestive tract. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. Parietal (Oxyntic) Cells • These cells produce HCl and gastric intrinsic factor (necessary for vitamin B 12 absorption from the ileum). ClINICAL APPLICATION For various reasons, including autoimmunity, parietal cells may be damaged to the extent that insufficient quantities of intrinsic factor are secreted and vitamin B 12 is not absorbed adequately. This vitamin is a cofactor required for DNA synthesis; low levels of vitamin B12 can reduce proliferation of erythroblasts, producing pernicious anemia. Anthony L. Mescher. “Junqueira’s basic Histology Text and Atlas”. CHAPTER 15 Digestive tract 14th edition. Lange Mc Graw Hill. Chief (Zymogenic) Cells Contain secretory granules that house the proenzyme pepsinogen precursor of pepsin. Gastric lipase: digestion of many lipids. They also manufacture leptin, that acts on the arcuate nucleus of the hypothalamus to inhibit the sensation of hunger. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. Enteroendocrine cells (DNES/APUD) • There are at least 13 different DNES cell types, only some of which are located in the mucosa of the stomach. • Cells of the DNES have been localized also in the respiratory system and in the endocrine pancreas. • Additionally, some of the secretory products synthesized and released by these DNES cells are identical with neurosecretions localized in the CNS. Characteristics of the stomach • There are three layers of muscle in the stomach wall: innermost oblique layer, middle circular layer, and outer longitudinal layer. • The oblique layer (internal) -> assist in the churning action to mix food with the secretions of the gastric mucosa. Differences in the Mucosa • Cardiac region: no chief cells. • Pyloric Region: predominant cell type in the pylorus is the mucous neck cell they produce lysozyme , a bactericidal enzyme. • The gastric pits are deeper. • The entire stomach is invested by a serosa. • This external covering provides an almost friction-free environment during the churning movements of the stomach. Pyloric-Duodenal Junction The pylorus of the stomach is separated from the duodenum of the small intestine by a thick smooth muscle layer called the pyloric sphincter that is formed by the thickened circular layer of the muscularis externa of the stomach . Small intestine • 4-7 meter long. • Function: The small intestine digests food material and absorbs end products of the digestive process. • Doudenum (receives enzymes and an alkaline buffer). • Jejunum. • Ileum. Gut MODIFICATIONS TO THE LUMINAL SURFACE • Plicae circulares (valves of Kerckring) are transverse folds of the submucosa and mucosa that form semicircular to helical elevations, some as large as 8 mm in height and 5 cm in length. • Unlike rugae of the stomach, these are permanent fixtures of the duodenum and jejunum and end in the proximal half of the ileum. Villi • Villi are epithelial covered, finger-like protrusions of the lamina propria. • Their numbers are greater in the duodenum than in the jejunum or the ileum, and their height decreases from 1.5 mm in the duodenum to about 0.5 mm in the ileum. • Villi increase the surface area of the small intestine by a factor of 10. • Microvilli, modifications of the apical plasmalemma of the epithelial cells covering the intestinal villi, increase the surface area of the small intestine by a factor of 20. • The three types of intestinal surface modifications increase the total surface area available for absorption of nutrients by a factor of 400 to 600. • Crypts of Lieberkühn Invaginations of the epithelium into the lamina propria between the villi form intestinal glands, which also augment the surface area of the small intestine. Modifications of the Luminal Surface Abraham L. Kierszenbaum. Histology and cell biology. 3rd edition. Elsevier. P.510. Intestinal Mucosa • Epithelium: • The simple columnar epithelium covering the villi and the surface of the intervillar spaces is composed of enterocytes, goblet cells, and DNES cells, Paneth cells, regenerative cells. • ENTEROCYTES function in terminal digestion and absorption of water and nutrients . • Additionally, these cells reesterify fatty acids into triglycerides, form chylomicrons. • Goblet Cells: • The duodenum has the smallest number of goblet cells, and their number increases toward the ileum. • These cells manufacture mucinogen, whose hydrated form is mucin , a component of mucus , a protective layer lining the lumen. • Paneth cells: These pyramid-shaped cells manufacture the antibacterial agent lysozyme , defensive proteins (defensins) , and tumor necrosis factor-α. The submucosa of the duodenum is unusual because it houses glands known as Brunner glands (duodenal glands) . Brunner glands • The ducts of these glands penetrate the muscularis mucosae and usually pierce the base of the crypts of Lieberkühn to deliver their secretory product into the lumen of the duodenum. • Brunner glands secrete a mucous, alkaline fluid in response to parasympathetic stimulation. This fluid helps neutralize the acidic chyme that enters the duodenum from the pyloric stomach. Muscularis Externa and Serosa The muscularis externa: • Auerbach myenteric plexus (responsible for the peristaltic activity of the small intestine.) Except for the second and third parts of the duodenum which have adventitia , the entire small intestine is invested by a serosa . Differences between the duodenum, jejunum, and ileum • Presence of Brunner glands in the submucosa of duodenum. • The villi of the jejunum are narrower, shorter, and sparser than those of the duodenum. The number of goblet cells per unit area is greater. • Ileum: Greatest development of GALT: Peyer's patches, lymphoid follicles (also called nodules). Histology and cell biology Abraham L. Kierszenbaum. 3rd edition. P.510. Large Intestine • It absorbs most of the water and ions from the chyme it receives from the small intestine as well as much of the gases present in its luminal content and compacts the chyme into feces for elimination. The cecum and the colon are indistinguishable histologically. Large Intestine • The colon has no villi but is richly endowed with crypts of Lieberkühn but there are no Paneth cells. • The number of goblet cells of its simple columnar epithelium increases from the cecum to the sigmoid colon. • DNES cells, including L cells that secrete the appetite reducing hormone peptide YY (PYY) . • Rapid mitotic activity of the regenerative cells replaces the epithelial lining of the crypts and of the mucosal surface every 6 to 7 days. Large intestine The muscularis externa: The muscularis externa is unusual in that the outer longitudinal layer is not of continuous thickness along the surface, but most of it is gathered into three narrow ribbons of muscle fascicles, known as taeniae coli . The constant tonus maintained by the taeniae coli puckers the large intestine into sacculations, called haustra coli . The serosa displays numerous fat-filled pouches, called appendices epiploicae. Rectum and Anal Canal • Histologically, the rectum resembles the colon, but the crypts of Lieberkühn are deeper and number fewer per unit area. Anal Mucosa • The epithelium is simple cuboidal from the rectum to the pectinate line (at the level of the anal valves). • Stratified squamous nonkeratinized from the pectinate line to the external anal orifice. • Stratified squamous keratinized (epidermis) at the anus. • The lamina propria, houses anal glands at the rectoanal junction and circumanal glands at the distal end of the anal canal. • Additionally, hair follicles and sebaceous glands are present at the anus. The vermiform appendix • Is a 5- to 6-cm-long diverticulum. • The mucosa of the appendix is composed of a simple columnar epithelium, consisting of surface absorptive cells, goblet cells, and M cells where lymphoid nodules adjoin the epithelium. • Crypts of Lieberkühn. DNES cells, and infrequent Paneth cells. • Serosa. HISTOLOGY OF THE GI TRACT (ACCESSORY GLANDS) Dra. Diana Vera Estrada MAJOR SALIVARY GLANDS • Parotid. • Sublingual. • submandibular. Branched (compound) tubuloacinar glands. Salivary Gland Schematic diagram of salivary gland acini, ducts, and cell types. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. • Serous cells: seromucous cells, they secrete both proteins and a considerable amount of polysaccharides. • -Secretory granules rich in salivary amylase. • Mucous cells: Secretory granules housing mucinogen. • Myoepithelial cells (basket cells): are rich in actin and myosin. As the processes of myoepithelial cells contract, they press on the acinus, facilitating release of the secretory product into the duct of the gland. FUNCTION The major salivary glands produce approximately 700 to 1100 mL of saliva per day. Minor salivary glands are located in the mucosa and submucosa of the oral cavity, but they contribute only 3 to 5 mL to the total daily salivary output. LUBRICATIVE: mucosa, dissolve food, moisten food for easily swallowing. PROTECTIVE: antibacterial function of lysozyme, lactoferrin, IgA. DIGESTIVE: Amylase ---- digestion carbohydrates Lingual Lipase ---hydrolisis of lipids. Parotid Gland • The parotids are composed entirely of serous cells rich in zymogen granules. • The two parotid glands produce only about 30% of the total salivary output. • The saliva produced by the parotid gland has high levels of the enzyme salivary amylase and secretory IgA. The sublingual glands • The sublingual gland produces a mixed, but mostly mucous, saliva. • The gland is composed of mucous tubular secretory units, many of which are capped by a small cluster of serous cells, known as serous demilunes. • Intercalated and striated ducts are poorly developed. Photomicrograph of the monkey sublingual gland. Submandibular Gland • Produces approximately 60% of the total salivary output. • The number of serous demilunes is limited. • Mucus cells secrete highly glycosylated mucins rich in sialic acid and sulfate that lubricate tissue surfaces forming a thin protective film called pellicle that act as a barrier against acids and regulate adherence of bacteria and yeast. Minor Salivary Glands • Small, nonencapsulated salivary glands are distributed throughout the oral mucosa and submucosa with short ducts to the oral cavity. • Predominantly composed of mucous cells, but some serous cells may be present. Pancreas Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. Secretory and duct portions Acinar cells have many secretory granules, known as zymogen granules. The basal cell membranes of acinar cells have receptors for the hormone CCK. Photomicrograph of the monkey exocrine pancreas. CC, Centroacinar cell; Se, septum; SA, serous acinus (×540). • Centroacinar cells and intercalated ducts both have receptors on their basal plasmalemma for the hormone secretin , released by DNES cells of the small intestine, and acetylcholine. Enzymes produced by the acinar cells • Pancreatic amylase breaks down starches, carbohydrates, and glycogen into disaccharides. • Pancreatic lipase breaks down fats into fatty acids an monoglycerides. • Pancreatic cholesterol esterase breaks down cholesterol esters into cholesterol and fatty acids. • DNase and RNase break down DNA and RNA, respectively. • Elastase breaks down the major component of elastic fibers, namely elastin. Lighter centroacinar cells project into the lumen of each acinus. 700×. Toluidine blue, plastic section LM showing pancreatic acini at high magnification.. 825×. H&E. Netter Essential Histology 2nd Edition. LIVER • Endocrine and exocrine function. • Functional unit: The hepatocyte. The liver is completely enveloped by peritoneum, which forms a simple squamous epithelium covering over the dense, irregular collagenous connective tissue -> Glisson capsule. - Classical Lobules • Hepatocytes are arranged in hexagon-shaped lobules (classical lobules) about 2 mm in length and 700 µm in diameter. Photomicrograph of a dog liver displaying the central vein (CV), liver plates (LP), and sinusoids (Si). This animal was injected with India ink that was phagocytosed by Kupffer cells (KC), which, consequently, appear as black spots (×270). The blood flowing in these wide vessels is prevented from coming in contact with the hepatocytes by the presence of an endothelial lining composed of sinusoidal lining cells. Resident macrophages, known as Kupffer cells, are intermingled with the sinusoidal lining cells of the sinusoids. Perisinusoidal Space of Disse • Occupied by microvilli of the hepatocytes. • It facilitates exchange of materials between the plasma escaping from the bloodstream and the hepatocytes. Perisinusoidal Space of Disse • Contains type III collagen fibers (reticular fibers) that support the sinusoids (limited amount of type I and type IV collagen fibers), a basal lamina is absent. • Perisinusoidal stellate cells (also known as Ito cells and fat storing cells.) • Store vitamin A. • Manufacture and release type III collagen into the space of Disse, secrete growth factors required by the liver for generating new hepatocytes, and by differentiating into myofibroblasts, form fibrous connective tissue to replace toxically damaged hepatocytes. Hepatocyte • Life span - 150 days. • The plasma membranes have two domains -> lateral and sinusoidal. • The lateral domain of the hepatocyte cell membrane contact the adjacent hepatocyte and form labyrinthine intercellular spaces (1 to 2 µm in diameter) known as bile canaliculi. • These cells manufacture primary bile, which is modified by the epithelial cells lining the bile ducts and gallbladder and become known as bile. Hepatocyte • Actively synthesize proteins for their own use as well as for export (abundance of free ribosomes, RER, and Golgi apparatus). • Hepatocytes contain varying amounts of inclusions in the form of lipid droplets and glycogen. • The enzyme glucuronyltransferase, located in the SER of the hepatocyte, catalyzes the conjugation of bilirubin with glucuronide to form the water-soluble. Anthony L. Mescher. “Junqueira’s basic Histology Text and Atlas”, 14th edition. Lange Mc Graw Hill. Normal liver zones, microscopic: Edward C. Klatt. MD. Robbins and Cotran Atlas of Pathology. The liver chapter 8. 3rd edition. Elsevier. Gallbladder • The gallbladder wall has a mucosa which comprises an absorptive epithelium resting on a highly vascular lamina propria. • It is composed of 4 layers: 1. 2. 3. 4. Epithelium Lamina propria Smooth muscle Serosa/adventitia Simple columnar epithelium , composed of two types of cells: the more common clear cells and the infrequent brush cells. Textbook of histolog y. Gartner, Leslie P. PhD. 2017 Elsevier. Structure of the Gallbladder • Lamina propria: • It is composed of a vascularized loose connective tissue. • It is abundant with elastic and collagen fibers. • In the neck of the gallbladder, the lamina propria is made up of simple tubuloacinar glands → these produce a small amount of mucus to lubricate the lumen of this constricted region. • There are no lymphatic vessels in this layer. The gallbladder does not have a muscularis mucosae or submucosa. Muscularis externa: Contraction of the smooth muscle reduces the volume of the bladder, forcing its contents out through the cystic duct. • The layer of tissue where the gallbladder attaches to the liver surface is referred to as the adventitia. • The unattached surface is covered by a serosa or visceral peritoneum consisting of a layer of mesothelium and a thin layer of loose connective tissue.