Digestive System II: Esophagus and Stomach PDF

17 Digestive System II: Esophagus and Stomach General structure of GIT...

17 Digestive System II: Esophagus and Stomach General structure of GIT – Cardiac region Esophagus – Fundic or body region Stomach – Cells of fundic glands – Internal structure – Pyloric region Gastrointestinal tract (GIT) is a long muco-muscular 4. Serosa is a connective tissue layer covered by peri- tube that extends from proximal part of esophagus toneum. Adventitia is only connective tissue layer. to distal part of anal canal. GIT includes esophagus, stomach, duodenum, Mucosa ileum, jejunum, colon, vermiform appendix, and anal Mucosa consists of three layers: Epithelium, lamina canal. propria, and muscularis mucosae.Viva GIT shares common structural features that are essential for digestion, absorption, and transport Epithelium of food material. Lining of gut is simple columnar epithelium except at 1. Esophagus (lined by stratified squamous GENERAL STRUCTURE OF GIT epithelium) 2. Terminal part of anal canal (lined by stratified GIT consists of four layers (from inside outward) squamous epithelium) (Fig. 17.1 and Flowchart 17.1):Viva The major functions of epithelium are protection, 1. Mucosa: It consists of lining epithelium, lamina absorption, and secretion. propria, and muscularis mucosae. Absorptive function of gut is supported by large 2. Submucosa: Consists of dense irregular connective surface area. tissue. To increase the surface area, intestine shows 3. Muscularis externa has inner circular and outer following modification (specifically in small longitudinal smooth muscle layer. intestine):Viva, MCQ Flowchart 17.1: General structure of GIT 196 17-Digestive System II.pmd 196 07/05/2019, 3:28 PM  Digestive System II: Esophagus and Stomach 197 Fig. 17.1: Layers of intestine. 1. Plicae circularis: These are circumferential It consists of blood vessels and lymphatic vessels. submucosal folds. These vessels help in transport of absorbed nutrients. 2. Villi: These are finger-like mucosal projection. Lamina propria has gut-associated lymphatic tissue 3. Microvilli: These are cell-surface projections. (GALT) in the form of diffuse lymphatic tissue and 4. Other than these three modifications, the length lymphatic nodules. GALT plays major role in of intestine and crypts (depressions of epithelium protection, phagocytosis, and antibody secretion. In in lamina propria) also increase absorptive power ileum, lymphatic nodules occupy lamina propria and of intestine. submucosa to form Peyer's patches.Neet Intestine also performs secretory function that Lamina propria also provide space for mucosal provides lubrication factors, digestive enzymes, and depressions that form mucosal glands. For examples, even antibodies. gastric glands in stomach and crypts of Lieberkühn Secretory function of intestine is supported by in intestine. 1. Unicellular secretory cells of epithelium. 2. Simple tubular glands: These arise from lining Muscularis Mucosae epithelium and extend into lamina propria. These Muscularis mucosae is the smooth muscle cell layer are also called as crypts. that separates mucosa from submucosa. 3. Submucosal glands are compound tubuloal- Muscle cells are arranged in two layers as an inner veolar mucous secreting glands that lie in circular and outer longitudinal layer. submucosa. These are present in esophagus and Contractions of muscularis mucosae can change duodenum. shape of mucosa and it facilitates absorption and 4. Extramural glands: These are supportive glands that secretion. pour their secretion into lumen of gut through ducts. Muscularis mucosae extend into mucosal folds but These include liver (bile) and pancreas (pancreatic not into villi. juice). Embryologically, these glands are derived from endoderm of gut. Submucosa 17 Submucosa is a dense irregular connective tissue Lamina Propria layer.MCQ Lamina propria is a layer of loose connective tissue It contains numerous blood vessels, lymphatics, and that supports epithelium. occasional glands (in esophagus and duodenum). 17-Digestive System II.pmd 197 07/05/2019, 3:28 PM  198 Textbook of Human Histology Submucosa also shows few unmyelinated nerve Lumen of esophagus remains normally collapsed fibers and ganglionic cells that form submucosal and opens only during passage of a food sub- Meissner's plexus.Neet, Viva stance. Submucosa allows movement of mucosa over Lumen of esophagus has longitudinal folds. muscularis externa. Esophagus has constrictions at the following sites:Viva, MCQ Mucularis Externa 1. At the beginning of esophagus due to crico- Muscularis externa consists of smooth muscles pharyngeus muscle (pharyngoesophageal arranged in two layers: inner circular and outer sphincter) longitudinal (ICOL). 2. At crossing of arch of aorta Exceptions for this arrangement are 3. At crossing of left main bronchus – Upper third of esophagus has striated muscles, 4. At the level of entry through diaphragm. middle third of esophagus has both striated and smooth muscles. Histology of Esophagus – Stomach has inner oblique, middle circular, and Histologically, esophagus has four layers: Mucosa, outer most longitudinal muscle layer. submucosa, muscularis externa, and adventitia (Figs – In large intestine/colon, longitudinal fibers form 17.2 to 17.4 and Flowchart 17.2). prominent bundles called taenia coli. In some part, circular muscle layer forms a thick band Mucosa that works as sphincter or valves. For example, pharyngoesophageal sphincter (part of cricopharyn- Mucosa consists of the following layers: geus), lower esophageal sphincter, pyloric sphincter, 1. Epithelium: Esophagus is lined by nonkeratinized ileocecal valve, and internal anal sphincter. stratified squamous epithelium.Identification feature Between inner circular and outer longitudinal layers, 2. Lamina propria: It consist of loose areolar tissue and Auerbach's plexus (myenteric plexus) is present.Neet, few lymphatic nodules. Viva 3. Muscularis mucosae: It consist of only longitudinal Function: Contractions of muscularis externa propel smooth muscle fibers. It is thicker as compared to content of intestinal lumen. These movements are other parts of gut.Identification feature, Neet, Viva called peristalsis. Autonomic (enteric) nervous system controls movements of the muscularis externa. Serosa/Adventitia It is an outer most connective tissue covering of intestine. If intestine is covered by a layer of peritoneum (mesothelium – simple squamous epithelium), the covering layer is called serosa. If the intestine is covered only by connective tissue, the covering layer is adventitia. Large blood vessels, lymphatics, and nerves traverse through serosa. ESOPHAGUS Esophagus is a mucomuscular tube that conveys food and liquid from pharynx to stomach. Esophagus lies in the neck and thoracic region. It is fixed and separated from adjacent structures by 17 connective tissue. Fig. 17.2: Transverse section of esophagus. It consists of four Length: 25 cm. layers and has star-shaped lumen. 17-Digestive System II.pmd 198 07/05/2019, 3:28 PM  Digestive System II: Esophagus and Stomach 199 Fig. 17.3: Histology of lower part of esophagus (practice figure). Note: In upper one-third, wall of esophagus shows skeletal muscles in muscularis externa; in middle third, both skeletal and smooth muscles; whereas in lower-third only smooth muscles are present. Flowchart 17.2: Histology of esophagus Mucosa of esophagus shows longitudinal folds that Because of presence of elastic fibers, submucosa is disappear on distention of esophagus. folded, and lumen of esophagus is irregular (star- Lamina propria gives finger-like projections into shaped).Viva the epithelium. These projections prevent separation Branched tubuloalveolar mucous glands are present of epithelium from underlying connective tissue. in submucosa. Ducts of these glands pass Submucosa through lamina propria and open into lumen of 17 esophagus.Viva It has dense irregular connective tissue that consists of Glands are predominantly seen in lower part of bundles of collagen fibers, elastic fibers, large blood vessels, and nerves. esophagus. 17-Digestive System II.pmd 199 07/05/2019, 3:28 PM  200 Textbook of Human Histology Fig. 17.4: Photomicrograph. Histology of lower part of esophagus. Muscularis Externa Muscularis externa of esophagus shows the Some Interesting Facts following composition:MCQ Cardiac glands: These are mucous glands in the lamina 1. Upper third of esophagus: Skeletal muscle propria of the lower end of esophagus. It protects 2. Middle third of esophagus: Both smooth and esophagus from exposure to acidic contents of the skeletal muscles stomach. 3. Lower third of esophagus: Smooth muscles Lower esophageal sphincter (LES): It is present at the only junction of esophagus with stomach. LES is only a Muscles are arranged as inner circular and outer physiological sphincter as it does not exist longitudinal layer. anatomically. Between two layers, myenteric plexus (Auerbach’s plexus) is present. Muscularis externa is toughest layer of Summary (Examination Guide) esophagus.Neet Esophagus has the following layers: Mucosae: It consists of stratified squamous epithelium Adventitia/Serosa (nonkeratinized), lamina propria, and muscularis mucosa Esophagus is surrounded by dense fibrous tissue (thick, longitudinal smooth muscles). (adventitia). Submucosa: It consists of dense irregular connective Serosa: Lower 1 inch of esophagus is covered by tissue with elastic fibers and compound tubuloalveolar peritoneum (serosa). glands. Muscularis externa: In upper third of esophagus, it has Functions of Esophagus skeletal muscle fibers; in middle third, both skeletal and smooth muscle fibers; whereas in lower third, only 1. Transport of food and liquid from pharynx to smooth muscle fibers. stomach. 17 Adventitia: Esophagus is surrounded by connective 2. Esophageal glands secrete mucus that protects tissue. Only lowest 1 inch, it is surrounded by serosa esophagus from acidic contents of stomach. (peritoneum). 3. No absorption, no digestion. 17-Digestive System II.pmd 200 07/05/2019, 3:28 PM  Digestive System II: Esophagus and Stomach 201 Clinical Correlation 3. Pyloric region: It connects stomach with duo- Achalasia cardia: It is also called cardiospasm. In acha- denum. lasia cardia, because of neuromuscular dysfunctions, Histologically, stomach shows usual four layers of lower esophageal sphincter (cardiac sphincter) fails gut, that is, mucosa, submucosa, muscularis externa, to relax. It results in megaesophagus (dilatation of and serosa. Gastric mucosa shows presence of gastric esophagus) and patient complaints of dysphagia glands (Fig. 17.6). (difficulty in swallowing) and in severe cases there may be a regurgitation of undigested food in Internal Structure of Stomach respiratory tract. Rugae: Inner surface of stomach shows longitudinal Gastroesophageal reflux disease (GERD): In GERD, mucosal folds/ridges called rugae. These rugae allow because of failure of lower esophageal sphincter stomach to distend when filled and rugae disappear mechanism, the acidic contents of stomach enter in on stomach distention.Viva the lower part of esophagus. It causes heart burn (chest Gastric pits: Mucosal surface of stomach shows pain) and inflammation of esophagus. numerous depressions called gastric pits. These are Barrett's esophagus: In Barrett's esophagus, lining openings of simple tubular gastric glands.Viva epithelium of esophagus changes from stratified Gastric canals: Mucosal folds lie longitudinally along squamous to simple columnar epithelium with goblet lesser curvature of stomach. These are called gastric cells. The main cause of Barrett's esophagus is GERD. canals. This is a precancerous condition. Mucus coating/visible mucus: Surface epithelial cells of stomach secrete thick, gel-like mucus that covers STOMACH the internal surface of stomach, and protects stomach from acidic contents. Stomach is a hollow muscular organ that lies between esophagus and duodenum. Anatomically, stomach has fundus, body, and Some Interesting Facts pylorus parts, and cardiac and pyloric ends. Prostaglandins (PGE2) enhance mucus and bicarbo- Histologically, stomach shows three regions: cardiac, nate secretion, and thus protect gastric mucosa from fundic, and pyloric regions (Fig. 17.5). acidic damage. 1. Cardiac region: It is a small part of stomach that Nonsteroidal antiinflammatory drugs (NSAIDS), such lies at cardiac end (at gastroesophageal junction). as, aspirin suppress production of prostaglandins 2. Fundic (body) region: It includes fundus and body and make gastric mucosa prone to damage gastric of stomach. Histological features of fundus and ulcers. body parts are similar. Fig. 17.5: Regions of stomach for histological description. Fig. 17.6: Location of gastric glands in lamina propria. 17 17-Digestive System II.pmd 201 07/05/2019, 3:28 PM  202 Textbook of Human Histology CARDIAC REGION OF STOMACH appearance on H&E staining). Ducts of cardiac glands open as the gastric pits. Q. Write a short note on histology of cardiac region Muscularis mucosae consists of smooth muscle cells of stomach. that are arranged as inner circular and outer longi- A section of stomach in cardiac region shows four tudinal layers. layers: Mucosa, submucosa, muscularis externa, and serosa (Figs 17.7, 17.8 and Flowchart 17.3). Submucosa Mucosa Submucosa consists of dense irregular connective tissue. It has blood vessels, nerve fibers, and Mucosa of cardiac region shows three layers: Meissner’s (submucosal) plexus. Epithelium, lamina propria, and muscularis mucosae. Flowchart 17.3: Histology of cardiac part of stomach Epithelium: Simple columnar epithelium with basal oval nuclei. Epithelial cells secrete mucus and hence also called mucous surface cells. These cells show vacuolated cytoplasm as mucus is washed during the slide preparation.Viva Note: There are no goblet cells and microvilli in stomach (microvilli and goblet cells are present in small intestine).Viva Lamina propria: Lamina propria contains mucous secreting, simple tubular cardiac glands. These glands are lined by simple columnar epithelium with flattened basal nuclei. Apical cytoplasm of these cells is filled with mucin granules (may show vacuolated 17 Fig. 17.7: Histology of gastroesophageal junction (longitudinal section, practice figure). Fig. 17.8: Photomicrograph. Histology of cardiac part of stomach (low magnification). 17-Digestive System II.pmd 202 07/05/2019, 3:28 PM  Digestive System II: Esophagus and Stomach 203 Muscularis Externa Previous concept: Smooth muscles of stomach are arranged as an outer longitudinal, middle circular, and inner oblique layer. New concept: Histologically distinct layers of muscularis externa cannot be differentiated. Smooth muscles of muscularis externa are oriented randomly.Viva Myenteric or Auerbach’s plexus is present between bundles of smooth muscle fibers. Serosa Stomach is covered by a connective tissue layer with mesothelial lining of visceral peritoneum. Along lesser and greater curvatures, peritoneum continues as lesser and greater omentum, gastro- splenic and gastrophrenic ligaments. FUNDIC OR BODY REGION OF STOMACH Q. Write a short note on the histology of fundic or body region of the stomach. A section of stomach in fundic or body region shows four layers: Mucosa, submucosa, muscularis externa, and serosa (Figs 17.9, 17.10 and Flowchart 17.4). Mucosa Mucosa of fundic (body) region shows three layers: Epithelium, lamina propria, and muscularis Fig. 17.9: Histology of body/fundus of stomach at low mucosae. magnification (practice figure). Fig. 17.10: Photomicrograph. Fundus/body region of stomach. (entire stomach wall on left, X40 and low magnification of gastric mucosa on right, X100; H&E stain). 17 17-Digestive System II.pmd 203 07/05/2019, 3:28 PM  204 Textbook of Human Histology Flowchart 17.4: Histology of fundic (body) region of stomach CELLS OF FUNDIC GLANDS OF STOMACH Q. List the cells of fundic glands of stomach. Fundus or body of stomach has branched tubular glands that have mucous neck cells, chief cells, parietal (oxyntic) cells, enteroendocrine cells, and some undifferentiated stem cells (Figs 17.11 to 17.13 and Flowchart 17.5).Viva Mucous Neck Cells These are located in the neck region of fundic glands, just beneath gastric pits.Neet These are columnar cell with basal oval nucleus. Cytoplasm shows apical mucinogen granules (foamy appearance on H&E staining).Neet These cells produce soluble mucus (Note: Surface mucous cells produce insoluble/cloudy mucus).Neet Epithelium: Stomach is lined by simple columnar surface mucous cells. These cells show presence of mucinogen granules in apical part and basal oval nucleus. H&E staining: Surface mucous cells show vacuolated apical cytoplasm as mucus is removed during slide preparation. Periodic acid–Schiff (PAS) staining: Mucus takes deep magenta stain. Toluidine blue staining: Blue-colored glycoproteins of mucus is seen. Lamina propria shows presence of simple branched tubular glands called gastric or fundic glands. These glands extend from bottom of gastric pit (surface epithelium) to muscularis mucosae. Each gastric gland is connected with gastric pit by a small narrow segment called isthmus. Isthmus of gastric gland has stem cells that divide and migrate toward gastric pit and toward gastric glands. Each gastric gland has – Neck segment – It lies adjacent to isthmus. It is a narrow and long segment. – Fundic segment/base – It is a short and wide segment. Fundic segment usually divides into two or three branches. Gastric glands are lined by mucous neck cells, chief cells, parietal/oxyntic cells, enteroendocrine cells, and some undifferentiated cells. Muscularis Mucosae, Submucosa, Muscle Layer, Serosa Structures of muscularis mucosae, submucosa, 17 muscle layer, and serosa in fundic or body region of stomach are same as that of cardiac region of Fig. 17.11: Gastric mucosa at high magnification showing stomach. gastric glands (practice figure). 17-Digestive System II.pmd 204 09/05/2019, 12:53 PM  Digestive System II: Esophagus and Stomach 205 Fig. 17.12: Cells in glands of stomach. Flowchart 17.5: Cells of gastric glands Some mucous neck cells give rise to stem cells in the Apical portion of the cell has secretory zymogen neck region of gland that later replace surface granules that impart slight apical eosinophilia. mucous cells. Function: Secretion of pepsinogen – a precursor of Mucous cells are replaced every 5–7 days. proteolytic enzyme pepsin.Neet Electron microscopy: Cell shows abundant rER in basal Chief or Zymogenic Cells part and secretory granules (Golgi apparatus) in These are blue in color on H&E staining and also apical part. Luminal surface of cells shows numerous called peptic cells. small microvilli. These are located in deeper/basal part (lower one- third) of the gastric glands in fundic and body part Parietal (Oxyntic) Cells of stomach.Neet, Viva These are mostly found in upper part and neck These cells synthesize proteins (pepsinogen); hence region of gastric glands in fundus and body part of their cytoplasm contains a large quantity of rough the stomach.Neet, Viva 17 endoplasmic reticulum (rER). This rER takes baso- These are large triangular cells with a spherical philic (blue /hematoxylin) stain in basal part of cell nucleus. Apex of cell is directed toward lumen of (basal basophilia).Identification feature gland and base toward basal lamina. 17-Digestive System II.pmd 205 09/05/2019, 12:54 PM  206 Textbook of Human Histology Lifespan of parietal cell is 150–200 days.Neet Parietal cells have gastrin receptors, histamine (H2) receptors, and acetylcholine receptors. Enteroendocrine Cells These cells are mostly present in lower portion of gastric glands. Types of enteroendocrine cells 1. Closed type: These cells rest on basal lamina and do not reach lumen of gland. 2. Open type: These cells have microvilli that reach lumen of glands. H&E staining: These cells remain unstained on H&E staining; hence, difficult to identify. Silver staining: These cells are argentaffin or argyro- philic cells (stained by silver salts). Functions: Secretion of gastrin, serotonin, and histamine directly into the blood vessels. Stem Cells Some undifferentiated stem cells are present in neck region of gastric glands. These cells divide and move upward and downward to replace other cells. PYLORIC REGION OF STOMACH Q. Write a short note on histology of pyloric region of stomach. A section of stomach in pyloric region shows four layers: Mucosa, submucosa, muscularis externa, and Fig. 17.13: Photomicrograph. Gastric mucosa at high magni- serosa (Figs 17.14, 17.15 and Flowchart 17.6). fication showing gastric glands (at high magnification only a portion of gland can be seen, hence in this photomicrograph, Mucosa two separate views of gastric gland are shown, upper image Mucosa of pyloric region shows three layers: shows superficial part and lower image shows deeper part of Epithelium, lamina propria, and muscularis the gland). mucosae. H&E staining: These cells take dark acidic (pink) Surface epithelium is lined by tall columnar surface stain. As these cells stain strongly with eosin, they mucous cells. are called oxyntic cells. Lamina propria shows presence of pyloric glands. Electron microscopy: Parietal cells have an intracellular canalicular system and tubulovesicular membrane Flowchart 17.6: Histology of pyloric part of stomach system. Apical cell membrane shows several invagi- nations that form intracellular canaliculi. Wall of canaliculi shows a number of small microvilli. Cytoplasm contains numerous mitochondria. Immunohistochemistry: Wall of canaliculi contains a number of proton (H+) pumps and Cl– pumps. These pumps help in secretion of HCl. Function: Parietal cells secrete HCl and intrinsic 17 factor.Neet Note: Intrinsic factor binds with vitamin B 12 and helps in its absorption. Deficiency of intrinsic factor leads to vitamin B12 deficiency and pernicious anemia. 17-Digestive System II.pmd 206 07/05/2019, 3:28 PM  Digestive System II: Esophagus and Stomach 207 Fig. 17.15: Photomicrograph. Histology of pyloric part of stomach (low magnification on left, High magnification on right, H&E stain). Muscularis Externa It has thicker circular smooth muscle coat that forms pyloric sphincter.Identification feature Serosa Fig. 17.14: Histology of pyloric part of stomach showing only Stomach is covered by a connective tissue layer with mucosa and part of submucosa. Note: Muscularis externa and mesothelial lining of visceral peritoneum. serosa are not shown in this figure (practice figure). The differences between histology of fundic (body) Gastric pits of pyloric glands are deep and occupy about and pyloric region of stomach are listed in Table 17.1. the two-third thickness of mucosa.Identification feature Pyloric glands are shorter than fundic Clinical Correlation glands.Identification feature Glands occupy only lower Gastritis is inflammation of lining epithelium of half or third of the mucosa. stomach. The most common symptoms of gastritis are Pyloric glands are coiled branched tubular glands upper abdominal discomfort (pain), heartburn, nausea, with wider lumen than fundic glands. and vomiting. Pyloric glands are lined by only one type of cell that Gastric ulcer is a discontinuity of epithelial lining of are similar to the neck mucous cells.Identification feature stomach. These ulcers produce abdominal pain that These cells produce mucus. improves with eating. Common causes of gastric ulcers Occasional enteroendocrine cells are also present. are Helicobacter pylori bacterial infection, G cells are present in the deeper part of pyloric glands nonsteroidal antiinflammatory drugs, tobacco, and in duodenum. These cells secrete gastrin.Viva smoking, and stress. Pernicious anemia: Failure of parietal cells to produce Muscularis Mucosae, Lamina Propria, and intrinsic factor results in deficiency of vitamin B12. Submucosa 17 Intrinsic factor is essential for absorption of vitamin The structure of muscularis mucosae, lamina propria, B12 in the small intestine. Deficiency of vitamin B 12 and submucosa in the pyloric region of stomach are produces pernicious anemia. Treatment: Injection of same as that of the cardiac region of stomach. vitamin B12 (cobalamin). 17-Digestive System II.pmd 207 07/05/2019, 3:28 PM  208 Textbook of Human Histology Q. List the differences between histology of fundic (body) and pyloric part of stomach.Viva Table 17.1: Differences between histology of fundic (body) and pyloric region of stomach Feature Fundus or body of stomach Pyloric region of stomach Gastric pit Shallow, occupy upper third thickness of mucosa Deeper, occupy about two-third thickness of mucosa Glands Occupy lower 2/3rd thickness of mucosa Occupy lower 1/3rd thickness of mucosa – Coiling at base Less coiled More coiled – Lumen Narrow Wider – Cells Parietal cells, chief cells, mucous neck cells Mostly one type; Mucus-secreting cells (chief cells are absent) Summary (Examination Guide) A section of stomach shows four layers: Mucosa, submucosa, muscularis externa, and serosa. Mucosa: It is lined by surface mucous cells. These are simple columnar cells with basal oval nuclei and secrete visible mucus. Mucosa shows gastric pits (openings of glands). – Fundus/body: Short gastric pits – Pylorus: Long gastric pits Glands: Branched tubular glands. – Cardiac: Cardiac glands shows mucus-secreting cells. – Fundus/body: Long gastric glands that show pink oxyntic cells (HCl, intrinsic factor), neck mucous cells (mucus), blue colored deeper chief cells (pepsinogen), enteroendocrine cells (histamine, gastrin). – Pylorus: Pyloric glands are short and mostly lined by mucus-secreting cells. Muscularis mucosae shows inner circular and outer longitudinal smooth muscles. Submucosa: It consists of dense irregular connective tissue and has blood vessels, nerve fibers, and Meissner's (submucosal) plexus. Muscularis externa: Randomly oriented smooth muscle cells: Inner oblique, middle circular, and outer longitudinal layers. In pylorus, thick circular fibers form pyloric sphincter. Serosa: Stomach is covered by a connective tissue layer with mesothelial lining of visceral peritoneum. 17 17-Digestive System II.pmd 208 07/05/2019, 3:28 PM