Histology of Lower Digestive System 2024 PDF

12.02.2024 The Histology of The Lower Digestive System Yasemin Ersoy Çanıllıoğlu 1 The Lower Digestive System Stomach Small intestine (duodenum, jejunum, ileum) Large intestine (caecum, appendix, colon, rectum) 2 1 12.02.2024 Several processes of the gastrointestinal system (GIS) Digestion of food m...

12.02.2024 The Histology of The Lower Digestive System Yasemin Ersoy Çanıllıoğlu 1 The Lower Digestive System Stomach Small intestine (duodenum, jejunum, ileum) Large intestine (caecum, appendix, colon, rectum) 2 1 12.02.2024 Several processes of the gastrointestinal system (GIS) Digestion of food material Absorption of the end products of digestion Absorption of ingested fluids and reabsorption of secreted fluid 3 The Layers of GIS Tunica mucosa: epithelium, lamina propria, muscularis mucosa Tunica submucosa Tunica muscularis: Two layers of smooth muscle (3rd. layer in stomach) Inner circular (arranged spirally), Outer longitudinal (more elongated helix) Tunica adventitia (serosa) 4 2 12.02.2024 Mucosa ▪ Epithelium ▪ The lining epithelium invaginates to form glands, extending into the lamina propria (mucosal glands) or submucosa (submucosal glands) or ducts ▪ Lamina propria ▪ mucosa-associated lymphoid tissue (MALT) system ▪ Muscularis mucosae ▪ circular layer - inner ▪ longitudinal layer - outer 5 Submucosa ▪ a dense, irregular fibroelastic connective tissue layer ▪ houses no glands except in the duodenum ▪ enteric nervous system Meissner’s submucosal plexus ▪ houses postganglionic parasympathetic nerve cell bodies, ▪ controls the motility of the mucosa ▪ controls the secretory activities of its glands. 6 3 12.02.2024 Muscularis Externa ▪ Smooth muscle ▪ an inner circular layer and ▪ an outer longitudinal layer ▪ the enteric nervous system - Auerbach’s myenteric plexus ▪ between two muscle layers ▪ regulates the activity of the muscularis externa ▪ responsible for peristaltic activity, ▪ moves the contents of the lumen along the alimentary tract Contraction of the smooth fibers of the circular layer reduces the lumen; contraction of the fibers of the longitudinal layer shortens the tube Skeletal muscle fibers are present in the upper esophagus and the anal sphincter. 7 Serosa and Adventitia ▪ a thin connective tissue layer ▪ may or may not be surrounded by the simple squamous epithelium of the visceral peritoneum. ▪ If the region of the alimentary canal is intraperitoneal, it is invested by peritoneum, and the covering is known as the serosa. ▪ If the organ is retroperitoneal, it adheres to the body wall by its dense irregular connective tissue component and is known as the adventitia. 8 4 12.02.2024 Localised thickenings of muscle in the bowel wall act as valves and are called sphincters The sphincters of the GIS Esophagogastric sphincter Pyloric sphincter Ileocecal valve Internal anal sphincter 9 STOMACH Functions ▪ The stomach is responsible for the formation and processing of the ingested food into a thick acidic fluid known as chyme. ▪ to homogenize and chemically process the swallowed semisolid food. ▪ contractions of the muscular wall of the stomach ▪ the acid and enzymes secreted by the gastric mucosa ▪ Once the food is transformed into a thick fluid, it is released gradually into the duodenum. 10 5 12.02.2024 Stomach Regions (Anatomically) ▪ Cardiac region ▪ Fundus (dome shaped) ▪ Body / Corpus ▪ Greater curvature ▪ Lesser curvature ▪ Pyloric region ▪ Antrum ▪ Canal ▪ Sphincter 11 Stomach Regions (Histologically) According to the types of glands in the mucosa 1. Cardiac Region: Part near the esophageal orifice, cardiac glands 2. Fundic Region: Largest part, fundic/gastric glands 3. Pyloric Region: Proximal part of the pyloric sphincter, pyloric glands 12 6 12.02.2024 Gastric Wall ▪ Mucosa ▪ Simple columnar epithelium: ▪ Lamina propria: Fılled with tubular glands ▪ Muscularis mucosae:inner circular, outer longitudinal ▪ Submucosa ▪ Dense irregular CT ▪ Meissner’s submucosal plexus ▪ Muscularis Externa ▪ Inner oblique ▪ Middle circular ▪ Outer longitudinal ▪ Myenteric Auerbach’s plexus ▪ Tunica serosa ▪ Peritoneum 13 Mucosa ▪ Inner surface of the empty stomach shows longitudinal folds called Rugae : ▪ Are prominent in the narrow parts, poorly developed in the upper portion ▪ Are longitudinal folds of mucosa + submucosa ▪ When the stomach is full, they disappear ▪ Numerous openings in the mucosa are Gastric Pits/foveolae which are invaginations of epithelial lining into lamina propria ▪ Gastric glands open directly to the pits 14 7 12.02.2024 Rugae 15 16 8 12.02.2024 Gastric Pits and Gastric Glands SEM 17 Mucosa- Surface Epithelium ▪ Epithelium that lines the surface and gastric pits is Simple Columnar ▪ Simple columnar surface mucous cells are filled with mucinogen droplets (mucous cup) ▪ Neutral and insoluble mucus forms a thick viscous protective coat rich in bicarbonate (rough contents and acidic environment) ▪ No absorbtion in epithelium except water, salts. ▪ Alcohol, aspirin damage the epithelium and enter lamina propria 18 9 12.02.2024 Gastro-Esophageal Junction 19 20 10 12.02.2024 Mucosa - Lamina Propria ▪ simple, branched tubular glands ▪ glands are deep tubular invaginations of the surface epithelium that open into gastric pit ▪ Gland Structure: ▪ Short isthmus (cell replication) ▪ Neck ▪ Base/Fundus 21 22 11 12.02.2024 Gastric Glands-Fundic Glands Glands consists of: ▪ Mucous neck cells ▪ Parietal (oxyntic) cells ▪ Chief (zymogenic, peptic) cells ▪ Enteroendocrine/ DNES cells ▪ Stem cells 23 Gastric Glands-Fundic Glands ▪ Produce gastric juice: ▪ HCL: ▪ Produced by Parietal cells ▪ Acidic juice ▪ Converts pepsinogen to pepsin ▪ Bacteriostatic ▪ Pepsin: ▪ Proteolytic enzyme ▪ Produced by Chief cells ▪ Mucus: ▪ Acidic-protective coating ▪ Intrinsic factor: ▪ Produced by Parietal cells ▪ Glycoprotein that binds to vitamin B12 ▪ Essential for B12 absorption in the distal ileum ▪ Gastric glands also produce Paracrine and endocrine regulatory substances ▪ Gastrin and other hormone like secretions ▪ Produced by DNES cells 24 12 12.02.2024 Stomach 25 26 13 12.02.2024 27 Mucous Neck Cell ▪ Located in the neck region between parietal cells ▪ Shorter than the surface mucous cell ▪ No Mucous cap ▪ Produce soluble mucus 28 14 12.02.2024 Mucous cells 29 Parietal (Oxyntic) Cells ▪ Located among mucous neck cells in the neck region and also upper and middle portions of the neck ▪ Large, sometimes binucleated ▪ Eosinophilic cytoplasm ▪ Intracellular canalicular system ▪ H+,K+-dependent ATPase -rich tubulovesicular system ▪ Mitochondria, provided ATP requeried to pump H ions into lumen of the intracellular canaliculus ▪ Secretes HCl and Intrinsic Factor ▪ Contains Gastrin, Histamine H2, Acetylcholine M3 receptors that induce HCl secretion 30 15 12.02.2024 Parietal Oxyntic cells 31 Chief Cells (Zymogen/Peptic cells) ▪ Typical protein-secreting cell morphology ▪ Located in the deeper part of the fundic glands ▪ Apical eosinophilia (zymogen granules) ▪ Basal basophilia (rER) ▪ Secrete Pepsinogen and a weak lipase ▪ Acidic gastric juice activates pepsinogen-pepsin 32 16 12.02.2024 Chief Zymogenic cells 33 Chief and parietal cells 34 17 12.02.2024 Parietal cells Chief cells 35 Enteroendocrine Cells (DNES) ▪ Found at every level of the gland but more prevalent in the base ▪ Small cells that may not reach the surface ▪ Contain small secretory granules ▪ Stains with silver salts and Chromium (named: enterochromaffin/argentaffin/ar gyrophil cells) ▪ Secretes hormones like Gastrin, VIP (Vasoactive Intestinal Peptide), Somatostatin 36 18 12.02.2024 DNES, Parietal and Chief (peptic) cell 37 Pylor- Gastrin cells Immunohistochemistry 38 19 12.02.2024 Stem Cells ▪ Columnar undifferentiated proliferating cell in isthmus and neck ▪ Lifespan of surface mucous cells: 3-5 days, parietal cells: 150-200 days; chief and DNES cells:60-90 days, mucous neck cell: 6 days ▪ Continuous cell renewal in stomach and intestines ▪ They migrate upwards and downwards to take place of shed cells 39 The cell types of the gastric epithelium Mucous cells Surface mucous cells Neck mucous cells Acid producing cells (oxyntic or parietal cells) Enzyme producing cells (chief or peptic cells) Stem cells Enteroendocrine cells 40 20 12.02.2024 Cardiac Glands ▪ Shallow pits ▪ Base of glands highly coiled ▪ Mostly surface lining mucous cells ▪ NO CHIEF CELLS ▪ Occasional DNES cells 41 Pyloric Glands ▪ Deep pits ▪ Branched, coiled tubular glands ▪ Mostly mucous secreting cells ▪ DNES cells ▪ Occasional Parietal cells ▪ Secrete lysozyme (antibacterial) ▪ NO CHIEF CELLS 42 21 12.02.2024 43 Muscularis Externa and Serosa ▪ Inner Oblique ▪ Middle Circular ▪ Outer Longitudinal ▪ Randomly oriented than forming layers ▪ Myenteric (Auerbach’s) plexus ▪ Serosa : Mesothelium and underlying CT 44 22 12.02.2024 45 Small Intestine Three regions: ▪ Duodenum ▪ Jejunum ▪ Ileum 46 23 12.02.2024 Small Intestine ▪ digests food material ▪ absorbs end products of the digestive process. ▪ the duodenum, receives enzymes and an alkaline buffer from the pancreas and bile from the liver. ▪ epithelial cells and glands of the mucosa contribute buffers and enzymes to facilitate digestion. 47 Intestinal Wall Histological Layers: 1. Mucosa ▪ Epithelium ▪ Lamina Propria ▪ Muscularis Mucosae (inner circular, outer longitudinal) 2. Submucosa 3. Muscularis externa 4. Adventitia or Serosa 48 24 12.02.2024 Modifications of the Luminal Surface of SI plicae circulares, villi, microvilli, and crypts of Lieberkühn 49 Modifications of the Luminal Surface of SI Foldings increase the absorptive surface: 1. Plicae circulares (circular folds, valves of Kerkring)x3 2. Intestinal Villi- x10 3. Microvilli on the surface of enterocytes- x30 4. Intestinal Glands (crypts of Lieberkühn) 50 25 12.02.2024 51 The Intestinal Villi ▪ Projection of the mucosa ▪ A core of loose CT covered by the epithelium: 1. Epithelium: simple columnar epithelium 2. Lamina Propria of the intestinal villi: ▪ Fibroblasts ▪ Smooth muscle cells (derived from the muscularis mucosae) ▪ Lymphocytes, plasma cells, eosinophils, macrophages ▪ A network of capillaries ▪ A central, blind ending lymphatic capillary; the lacteal 52 26 12.02.2024 53 54 27 12.02.2024 Intestinal Epithelium ▪ Simple columnar epithelium with microvilli that also lines the intestinal glands ▪ Contains 6 Major Cell Types: 1. Enterocytes (Absorptive Cells) 2. Goblet Cells (Unicellular mucin-secreting gland) 3. Enteroendocrine Cells (produces paracrine and endocrine hormones) 4. Regenerative Cells (at the intestinal glands) 5. Paneth Cells (Responsible for mucosal innate immunity) 6. M Cells (Microfold cells- modified enterocytes that cover enlarged lymphatic nodules) 55 56 28 12.02.2024 Enterocytes / Absorptive Cells ▪ Functions: terminal digestion and absorption of water and nutrients ▪ Apical surface: brush border ▪ Microvilli ▪ An actin filament core ▪ 0.5-1μm length ▪ Attached to Terminal web ▪ Contain intramembranous enzymes: ▪ Lactase (Lactose intolerance) ▪ Maltase ▪ Sucrase ▪ Enterokinase (activates trypsinogen) ▪ Aminopeptidase 57 Goblet cells/Caliciform cells: ▪ Endoepithelial, unicellular gland (mucus production) ▪ Form a protective layer lining the lumen ▪ Has two domains: ▪ Apical domain ▪ Basal domain ▪ Their numbers increase towards the large intestine 58 29 12.02.2024 59 Enteroendocrine / DNES cells ▪ Similar to the ones in the stomach ▪ Secrete peptide hormones controlling several functions of the GI tract ▪ Although they are concentrated in the lower part of glands, they can be found at all levels of each villus 60 30 12.02.2024 61 Regenerative / Stem Cells ▪ Are found in the lower half of intestinal glands ▪ Bottom third of the crypts in large intestine ▪ Cells divide, migrate upward and downward, and differentiate into lost or exfoliated cells ▪ The turnover rate: Enterocytes : 6 days Enteroendocrine and Goblet cells: 4 weeks 62 31 12.02.2024 Paneth Cells ▪ Pyramidal cells found in the bases of intestinal glands ▪ Play a role in regulating normal bacterial flora ▪ Provide host defense against microbes in the small intestine. ▪ They are functionally similar to neutrophils. ▪ to maintenance of the gastrointestinal barrier ▪ Produce lysozyme, defensins, TNF-α 63 64 32 12.02.2024 Ileum- Paneth cell 65 M / Microfold Cells ▪ Overlies Peyer’s patches and other large lymphatic nodules ▪ phagocytose and transport antigens from the lumen to the lamina propria ▪ Have microfolds on their apical surface that capture antigens and transport them to the vicinity of T Cells which in turn, stimulates a response in the GALT 66 33 12.02.2024 Protection of the Small Intestine Several Defensive Mechanisms 1. Peyer’s patches and M cells perform the cellular surveillance of antigens 2. IgA- Plasma cells: neutralizes antigens 3. Paneth cells: control of the microbial flora 4. Acidity of gastric juice: inactivates ingested organisms 5. Peristalsis: prevents bacterial colonization 67 Muscularis Mucosae ▪ Consists of an inner circular and outer longitudinal smooth muscle layer ▪ Smooth muscle cells extend from the muscularis mucosae into the lamina propria and convoy the lacteal 68 34 12.02.2024 Submucosa ▪ Dense irregular connective tissue ▪ Meissner’s plexus: Submucosal plexus ▪ Duodenal submucosal glands: Brunner’s glands ▪ Branched tubuloalveolar mucous glands ▪ Secretion of these glands has a pH: 8.8-9.3 ▪ This highly alkaline secretion neutralizes the acidic chyme ▪ Peyer’s patches: specialized lymphoid follicles found in the mucosa and part of the submucosa of the ileum 69 Muscularis Externa 1. Inner circular smooth muscle layer: squeezes and in some places forms sphincters 2. Outer longitudinal smooth muscle layer: shortens gut The Myenteric Plexus (Auerbach’s plexus) is located in between these two muscle layers 70 35 12.02.2024 Muscularis Externa Duodenum Jejunum 71 Serosa ▪Simple squamous epithelium (mesothelium) ▪ Thin layer of areolar connective tissue underneath ▪Exceptions: ▪ Parts not in peritoneal cavity have adventitia and lack serosa 72 36 12.02.2024 Some organs are “retroperitoneal” ▪ Are “behind the peritoneum” ▪ Fused to posterior (dorsal) abdominal wall ▪ Lack a mesentery ▪ Include: ▪ ▪ ▪ ▪ ▪ Most of duodenum (1st part of small intestine) Ascending colon Descending colon Rectum Pancreas ▪ Have an adventitia layer 73 Serosa Small intestine 74 37 12.02.2024 The Histological Layers of the Small Intestine - Duodenum 75 Duodenum: has Brunner’s glands in the submucosa and the villi are broad and short (leaf-like), incomplete serosa and extensive adventitia The sphincter of Oddi is present at the terminal ampullary portion (Vaterin)of two converging ducts (bile duct and pancreatic duct) Jejunum: Long finger-like villi, well developed lacteal, No Brunner’s glands and No Peyer’s patches Ileum: shorter finger-like villi, Peyer’s patches in the mucosa and a part of the submucosa 76 38 12.02.2024 http://www.lab.anhb.uwa.edu.au/mb140/CorePages/GIT/git.htm#topicsmallintestine 77 http://www.lab.anhb.uwa.edu.au/mb140/CorePages/GIT/git.htm#topicsmallintestine 78 39 12.02.2024 Ileum- Peyer’s patches http://www.pathologyoutlines.com/topic/smallbowelsuperpagenontumor.html 79 Peyer’s Patches ▪ Located in the ileum ▪ The main component of the GALT (GutAssociated Lymphoid Tissue) ▪ Has two components: 1. The dome: contains B cells 2. A germinal center: contains IgA (+) B cells, CD4 (+) T cells and antigen presenting cells ▪ They are lined by Follicle Associated Epithelium (FAE) consisting of M cells (antigen representing cells) and enterocytes 80 40 12.02.2024 Large Intestine ▪ Function: ▪ reabsorbtion of water electrolites, ions ▪Parts ▪ Cecum ▪ Vermiform appendix ▪ elimination of undigested residues and waste in feces ▪ lubricate and compact contents by mucus ▪ Colon ▪ Ascending colon ▪ Transverse colon ▪ Descending colon ▪ Sigmoid colon ▪ Rectum ▪ Anus 81 82 41 12.02.2024 Large Intestine ▪ No villus in the large intestine ▪ No plicae circularis ▪ More goblet cells ▪ Deeper crypts of Lieberkühn ▪ In the cecum and colon but not in the rectum, outer longitudinal muscle layer forms 3 separate longitudinal bands called Taeniae coli (TC) ▪ Contraction of TC and circular muscles draws the colon into sacculations called ▪ No Paneth cells, a few in cecum Haustra coli ▪ Few DNES cells ▪ Absorptive cells have irregular microvilli (No striated border) ▪ More lymphoid tissue ▪ Descending colon is invested by adventitia ▪ Transverse colon is invested by serosa ▪ Ascending colon is invested by adventitia 83 Mucosa 1. A surface columnar epithelium ▪ Enterocytes: transport of ions and water ▪ Goblet cells: secrete mucus to lubricate and protect the mucosa ▪ Stem cells and DNES cells 2. Lamina Propria: ▪ Numerous straight tubular glands (crypts of Lieberkühn) ▪ Lymphoid follicles (well developed GALT) ▪ Absence of lymphatic vessels in the core of lamina propria 3. Muscularis Mucosae ▪ Lymphatic vessels form a network around smooth muscles ▪Does not form villi!!! 84 42 12.02.2024 85 86 43 12.02.2024 Large intestine 87 Submucosa ▪ Dense irregular CT ▪ No Glands!!! ▪ Lymphoid follicles penetrate the submucosa ▪ Lymhatic vessels, blood vessels, nerves ▪ No Plicae circulares!!! 88 44 12.02.2024 Muscularis Externa ▪ Inner circular and outer longitudinal smooth muscle ▪ The bundles of outer longt. layer fuse and form Taeniae coli ▪ Taeniae coli: ▪ 3 longitudinally oriented bands, 1cm wide ▪ The contraction of taeniae coli draws the colon into sacculations called Haustra TC: Taeniae Coli; HC: Haustra Coli, OA: Omental Appendices (small peritoneal projections filled with fat), Arrows: Mucosal semilunar folds 89 Taeniae Coli 90 45 12.02.2024 Adventitia and Serosa ▪ Has scattered sacs of adipose tissue, the appendices epiploicae 91 Serosa 92 46 12.02.2024 Cecum and Appendix ▪ The cecum is a blind pouch distal to the iliocecal valve ▪ The appendix is a thin, finger-like extension of cecum ▪ The appendix has large numbers of lymphatic follicles that extend into the submucosa 93 Appendix 94 47 12.02.2024 Appendix 95 Rectum and Anal Canal ▪ The rectum is the dilated distal part of GI tract ▪ Consists of two parts 1. Its upper part; rectum proper. ▪ ▪ ▪ Contains transverse rectal folds, no teniae coli, fewer but deeper crypts of Lieberkühn. 2. The lower part; anal canal ▪ Has longitudinal folds called anal columns (anal sinuses in between). 96 48 12.02.2024 Anal Canal ▪ Anal glands extend into the submucosa and even muscularis externa ▪ No taeniae coli in the rectum! ▪ Large apocrine glands; the circumanal glands, are found in the skin surrounding the anal orifice. ▪ The muscularis mucosae disappears at the ATZ, where the circular muscularis externa thickens to form the internal anal sphincter ▪ The submucosa of anal columns contain superior rectal artery and the rectal venous plexus→ enlargement→ internal hemorrhoids ▪ The external anal sphincter is formed by the striated muscle of the pelvic floor 97 98 49 12.02.2024 99 50