Histology of GI Tract PDF 2024-2025

OS 206: ABDOMEN AND PELVIS HISTOLOGY OF GI TRACT UPCM 2029 | Dr. Ronnie E. Baticulon, MD, FAFN | LU3 A.Y. 2024-2025 OUTLINE B.​ MAJOR LAYERS OF THE DIGESTIVE TRACT...

OS 206: ABDOMEN AND PELVIS HISTOLOGY OF GI TRACT UPCM 2029 | Dr. Ronnie E. Baticulon, MD, FAFN | LU3 A.Y. 2024-2025 OUTLINE B.​ MAJOR LAYERS OF THE DIGESTIVE TRACT ​ 4 layers (from inside going outward) I.​ Introduction IV.​ Transition Zones ○​ Mucosa A.​ Digestive System A.​ Esophago-gastric ○​ Submucosa B.​ Major Layers of the Junction ○​ Muscularis Digestive Tract B.​ Gastro-duodenal ○​ Serosa / Adventitia II.​ Upper Digestive Tract Junction ​ Structure and composition of these layers vary from segment to A.​ Oral Cavity V.​ References segment depending on their function B.​ Esophagus VI.​ Appendix C.​ Stomach III.​ Lower Digestive Tract A.​ Small Intestine B.​ Large Intestine C.​ Appendix I.​ INTRODUCTION A.​ DIGESTIVE SYSTEM ​ One long tube where each segment has its specific function ​ The histology of each segment changes depending on the segment’s function ​ Components (proximal to distal) ○​ Oral cavity ○​ Esophagus ○​ Stomach ○​ Small intestine ○​ Large intestine ○​ Rectum ○​ Anal canal Table 1. OVERVIEW OF THE DIGESTIVE TRACT Figure 1. Layers of the digestive tract ORGAN MAJOR FUNCTIONS OTHER FUNCTIONS Mouth ​ Ingests food ​ Moistens and MUCOSA ​ Chews and mixes food dissolves food, ​ Innermost layer ​ Begins chemical allowing you to breakdown of taste it Epithelial Lining carbohydrates ​ Cleans and ​ Usually simple columnar or stratified squamous (depending on the →​Salivary amylase lubricates the function) ​ Moves food into the teeth and oral pharynx cavity Lamina Propria ​ Begins breakdown of ​ Has some ​ Contains loose connective tissue, blood vessels, and lymphatic lipids antimicrobial vessels →​Lingual lipase activity Muscularis Mucosa Pharynx ​ Propels food from the ​ Lubricates food oral cavity to the and passageways ​ Thin layer of smooth muscle esophagus ​ Permits local and independent movement of mucosa (e.g. villi Esophagus ​ Propels food to the movement, gland secretion) stomach ​ Consists of an outer longitudinal and inner circumferential layer Stomach ​ Mixes and churns food ​ Stimulates of tendinous smooth muscle (similar to the layers of the tunica with gastric juices to protein-digesting muscularis) form chyme enzyme Remember! ​ Begins chemical ​ Secretes intrinsic ​ Muscularis is different from Muscularis mucosa. breakdown of proteins factor required for →​Muscularis = major layer ​ Releases food into the vitamin B12 →​Muscularis mucosa = sublayer of the mucosa duodenum as chyme absorption in ​ Absorbs some small intestine SUBMUCOSA fat-soluble substances ​ Dense connective tissue with larger blood and lymphatic vessels, (e.g. alcohol, aspirin) fibers and nerves ​ Possesses antimicrobial ○​ May also contain glands and significant lymphoid tissue functions (primary or secondary follicles) Small ​ Mixes chyme with ​ Provides optimal ○​ Strongest and most important in repair, hence, must always be Intestine digestive juices medium for sutured ​ Propels food at a rate enzymatic activity slow enough for Meissner’s (submucosal) plexus digestion and absorption ​ Nerve plexus of autonomic nerves which controls contraction and ​ Absorbs breakdown secretions products of ​ Also responsible for the movement of the villi carbohydrates & ​ Innervates epithelial cells and smooth muscle of muscularis proteins (duodenum mucosa; has parasympathetic innervations and jejunum), lipids (ileum), and nucleic MUSCULARIS acids, along with ​ Also called Muscularis Externa or Tunica Muscularis [2025 Trans] vitamins, minerals, and ​ Composed of smooth muscle cells organized as usually two or more water layers ​ Performs physical ○​ Inner sublayer: circular digestion via ○​ Outer sublayer: longitudinal segmentation ​ The connective tissue between muscle sublayers contains Large ​ Further breaks down ​ Food residue is ○​ Auerbach’s (myenteric) nerve plexus Intestine food residues concentrated and ○​ Blood and lymphatic vessels ​ Absorbs most residual temporarily stored Auerbach’s (myenteric) nerve plexus water, electrolytes, prior to defecation and vitamins produced ​ Mucus eases ​ Nerve plexus between muscle sublayers by enteric bacteria passage of feces ​ Has sympathetic and parasympathetic innervations ​ Propels feces toward through colon ○​ Motor to the tunica muscularis rectum ​ Responsible for peristalsis ​ Eliminates feces ○​ Contractions mechanically mix, churn, and propel food ○​ Main muscular layer responsible for movement ​ Composition and thickness would depend on segment studied Trans 3 TG18: Mariñas, Marquez, C., Marquez, M., Martires, Masa, Mejares, Mendoza TH: Punongbayan 1 of 14 ​ Variations at specific regions ○​ Muscularis is composed of skeletal muscle instead of smooth muscle in the upper portions of the GI tract (esophagus) ○​ 3 layers of muscle instead of 2 ​ As seen in the stomach where there is an additional inner oblique layer ​ Absence may lead to Hirschsprung’s Disease ○​ congenital disorder characterized by the absence of ganglion cells (GC) at the Meissner's plexus (submucosa) and Auerbach's plexus (muscularis) of the terminal rectum ADVENTITIA/SEROSA ​ Outermost layer ​ Thin layer of loose connective tissue ​ Rich in blood vessels, lymphatics, and adipose tissue Figure 4. Filiform papillae[Mescher, 2024] ○​ Adipose tissue is especially abundant in distal segments of GI tract ​ Lined by simple squamous epithelium / mesothelium Serosa ​ Covers the intraperitoneal segments of the GI tract ​ Freely-floating in peritoneal cavity ​ Secretes fluid to reduce friction ​ Covered in the outer surface with mesothelium (simple squamous epithelium) ​ Structures covered are more “free”/mobile than those covered by adventitia; they are not completely opposed to other structures Adventitia ​ Covers the retroperitoneal segments of the GI tract ​ Without mesothelium and is attached to the abdominal wall II.​ UPPER DIGESTIVE TRACT A.​ ORAL CAVITY Figure 5. Filiform papillae[Histology Guide, 2025] ​ Food is ingested, masticated, & lubricated by saliva for swallowing Fungiform Papillae TONGUE ​ 2nd most numerous ​ Muscular organ located in the oral cavity ​ “Mushroom-shaped” ​ Manipulates ingested material during mastication and swallowing ​ Larger, broader, and taller than the filiform papillae ​ Consists of a core of connective tissue with interlacing bundles of ​ Most prevalent in the anterior region of the tongue skeletal muscle fibers ○​ Interspersed among the filiform papillae ○​ Non-keratinizing stratified squamous epithelium ​ Narrow at the base with a flattened surface Papillae ​ Epithelial covering is thinner than filiform papillae due to the abundance of blood vessels and connective tissue underneath the ​ Projections on tongue's dorsal epithelium epithelium ​ Taste buds are often located on the dorsal surface Figure 6. Fungiform papillae[Ross, 2015] Figure 2. Types of papillae Filiform Papillae ​ Smallest and most numerous ​ Narrow and conical ​ Found on the entire dorsal surface of the tongue ​ Provide friction to help move food during chewing[Mescher, 2024] ​ Does NOT contain taste buds[UPCM 2028 Trans] Figure 7. Fungiform papillae[Fath El-Bab, 2006] Figure 3. Filiform papillae[Ross, 2015] OS 206 Histology of GI Tract 2 of 14 Circumvallate Papillae ​ Around 8-12 present on the tongue ​ Found on the posterior region ​ Largest and surrounded by furrows ○​ Presence of furrows differentiates circumvallate papillae from fungiform papillae (have no furrows) ○​ Allow food to come into contact with the taste buds on its lateral surface ​ Lined up in a V-shape parallel to the sulcus terminalis ​ Von Ebner Glands ○​ Found at the base of the furrows ○​ Serous, tubular-acinar glands ​ Serous glands appear more eosinophilic than mucus glands ○​ Secretions dissolve and wash away food particles so tongue can Figure 12. Foliate papillae[Histology Guide, 2025] process new taste stimuli Taste Buds ○​ Secretes epidermal growth factor (EGF) for regeneration of epithelium ​ Allows for taste (salty, sour, sweet, bitter, umami) ​ Oval-shaped, elongated cells perpendicular to the epithelium ○​ Has microvilli pointing toward the surface to form the taste pore ​ Different locations depending on the papillae ○​ Fungiform: at the surface ○​ Circumvallate: along the sides facing the furrows Figure 8. Circumvallate papillae[Ross, 2015] Figure 13. Taste buds[Eroschenko, 1993] Figure 9. Circumvallate papillae[Mescher, 2024] Figure 14. Taste buds from foliate (L) and circumvallate (R) papillae [Histology Guide, 2025] Types of Taste Bud Cells ​ Supporting/sustentacular cells ○​ Elongated, have a darker cytoplasm and a slender, dark nucleus ​ Taste/gustatory cells ○​ Light cytoplasm, more oval, lighter nucleus ​ Basal cells ○​ At the periphery of the taste bud near the basement membrane ○​ Able to regenerate ○​ Give rise to both the sustentacular and gustatory cells Figure 10. Circumvallate papillae[Histology Guide, 2025] Foliate Papillae ​ Rudimentary in humans ​ Consists of several parallel ridges on each side of the tongue anterior to the sulcus terminalis[Mescher, 2024] ​ Best developed in young children[Mescher, 2024] Figure 15. Cells of taste buds[Mescher, 2024] B.​ ESOPHAGUS ​ The adult esophagus typically measures 23-37 cm and about 1-2 cm in diameter ○​ 10 inches long[Trans 2028] ​ Transfers swallowed food from pharynx to stomach ​ Proximal third is voluntary because swallowing is a voluntary process ○​ Upper third is entirely skeletal (2-4 cm) made of adventitial lining ​ Distal third is involuntary made entirely of smooth muscle ○​ From proximal to distal, smooth muscles become predominant ​ Has four layers ○​ Mucosa ○​ Submucosa Figure 11. Foliate papillae[Ross, 2015] ○​ Muscularis ○​ Adventitia OS 206 Histology of GI Tract 3 of 14  C.​ STOMACH ​ Expandable fibromuscular sac that stores and mixes food ​ Derived from foregut ​ Fundus ○​ Area in the stomach above where the esophagus connects to the stomach ○​ If you draw a horizontal line from the end of the esophageal sphincter, everything that is above is the fundus[Trans 2028] Figure 18. Anatomy and parts of the stomach GROSS ANATOMY ​ Cardiac region at the concave lesser curvature Figure 16. Layers of esophagus. ​ Pyloric region at the greater curvature ○​ Contains the pyloric antrum and pyloric sphincter MUCOSA ○​ Most distal: pylorus with a boundary of the pyloric sphincter ​ Facing the lumen ​ Fundus and the body are two additional anatomic regions ○​ Composed of non-keratinized stratified squamous epithelium ​ Rugae ​ Lamina propria ○​ Seen when empty ○​ Between epithelium and the muscularis ○​ Made by folds of the mucosa and submucosa ○​ Esophageal cardiac glands which secrete mucus for lubrication ○​ Allows for distention of up to 3-4 liters when full of epithelium ○​ Contains glands, connective tissues, blood vessels FUNCTIONS ​ Muscularis mucosa ​ Secretes additional enzymes and gastric juices ○​ Boundary ○​ Liquifies the bolus into chyme ○​ Highly acidic ​Stomach lining is protected by mucus, made up of mucin which is a glycoprotein ​ Secretes intrinsic factor ○​ Necessary for the absorption of vitamin B12 ○​ Megaloblastic anemia ​ Vitamin B12 deficiency ​ Continues the digestion of carbohydrates by salivary amylase ​ Promotes protein digestion through pepsin ​ Begins digestion of triglycerides by lipase Figure 17. Mucosal layer of esophagus. MUCOSA SUBMUCOSA ​ Surface epithelium ​ Contains cardiac glands and larger blood vessels ○​ Simple columnar epithelium that extends into and lines gastric ​ Esophageal glands proper pits ○​ Contains pepsinogen for protein digestion and lysozyme for ​ Lamina propria protection ○​ Loose connective tissue that fills the spaces between gastric ​ Ducts with cuboidal or stratified cuboidal epithelium glands ○​ Blood vessels, lymphatic tissues MUSCULARIS EXTERNA ○​ Everything within the gastric gland which extends to muscularis mucosa[Trans 2028] Layers ​ Muscularis mucosae ​ Inner = circular; Outermost = longitudinal ○​ Thin smooth muscle that forms the outer boundary of the ○​ Mnemonic: LOCI mucosa ​ Auerbach (myenteric plexus) ○​ Consists of an inner circular and outer longitudinal layer ○​ Between inner circular and outer longitudinal muscles ​ In the cardia and pylorus regions ○​ Mucosa also contains tubular glands, with long pits, branching Parts into coiled secretory portions, called cardiac glands and pyloric ​ Upper third glands ○​ Made of striated skeletal muscle (voluntary) ○​ These glands lack both parietal and chief cells, primarily ​ Middle third secreting abundant mucus ○​ Transition from skeletal to smooth muscle ​ Lower third ○​ Made of smooth muscle (involuntary) Determining Location in the Esophagus ​ Look at the muscles ○​ Skeletal muscle ​ Multinucleated and striated ​ Cross-sectional cut ​Nucleus in periphery ○​ Smooth muscle ​ Uninucleate without striation ​ Cross-sectional cut ​Nucleus at center ADVENTITIA/SEROSA ​ Adventitia​ ○​ Upper esophagus or in thoracic cavity (connective tissue) ​ Serosa ○​ Lower esophagus in the abdominal cavity (simple squamous mesothelium) Figure 19. Layers of stomach (Mucosa, Submucosa, Muscularis Externa, Serosa).[diFiore] OS 206 Histology of GI Tract 4 of 14  Neuroendocrine Cells CELLS OF THE GASTRIC GLANDS ​ Part of the diffuse neuroendocrine system ​ Found in the base of the glands ○​ But not readily identifiable in H&E stained slides ​ Produce locally-acting hormones ○​ Usually secrete peptides with specific functions ​ Examples ○​ In the fundus ​ Small enteroendocrine cells secrete serotonin (5-hydroxytryptamine) ​ Found at the basal lamina of the gastric glands ○​ In the pylorus ​ Enteroendocrine cells are located in contact with the glandular lumens, including G cells ​ Produces the peptide gastrin ​ There are many other enteroendocrine cells in the rest of the GI tract Stem Cells ​ Found mainly in the neck of the gastric glands ​ Undifferentiated cells that divide continuously to replace all other cell types in the glands (e.g., regenerate mucosa) ​ Cells in the stomach are replaced every 5-7 days ○​ Can be a good and a bad thing Figure 20. Cells of the Gastric Glands ○​ Collateral damage in chemotherapy, because they are also rapidly dividing Surface Mucous Cells ​ Not easily identified in sections of normal gastric mucosa ​ Appear clear or ”empty-looking” on H&E ○​ But become very prominent with plentiful mitotic figures after ○​ Mucous material is washed off during staining damage to the mucosa has occurred ​ Cover the luminal surface of the stomach & partly line the gastric SUBMUCOSA[Trans 2028] pits ​ Have short surface microvilli ​ Contains dense connective tissue and more collagen than lamina ​ Produce mucus to protect the mucosal layer of the stomach propria ​ Secrete protective bicarbonate ions directly into the deeper layers ​ Contains lymph vessels, capillaries, large arterioles, venules of the surface mucous coat ○​ Important for absorbing nutrients from the stomach ○​ Prevents the mucosa from being digested by the gastric juices of ​ Most blood vessels are here the stomach Submucosal (Meissner’s) nerve plexus Neck Mucous Cells ​ Isolated clusters of parasympathetic ganglia seen deeper in the ​ Squeezed between the parietal cells in the neck and base of the submucosa gastric glands ​ Have larger secretory granules and more polyribosomes MUSCULARIS EXTERNA ​ Manufacture soluble mucus that becomes part of chyme and ​ Three layers of smooth muscle: lubricates the alimentary canal ○​ Inner Oblique, Middle Circular, Outer Longitudinal Parietal/Oxyntic Cells Inner Oblique ​ Distributed along the length of the glands but most numerous in ​ Note present in all slides the isthmus ​ Large rounded cells which have an extensive eosinophilic Middle Circular cytoplasm and a centrally located nucleus ​ Thickened at the pylorus to form the pyloric sphincter ​ Look like “fried eggs” ○​ Prevents food from directly going to the duodenum ​ Secretions ​ Pylorus opens once digested particles need to go to the small ○​ Gastric acid (hydrochloric acid) intestine ​ Maintains the acidity of the stomach ​ Important for food digestion Outer Longitudinal ​ Parasympathetic stimulation by vagus nerve produces HCl ​ Isolated clusters of parasympathetic ganglia seen deeper in the ​ Histamine causes HCl production submucosa ○​ Intrinsic factor ​ Binds to Vitamin B12 (Cobalamin) before absorption in terminal Myenetic (Auerbach’s) Nerve Plexus ileum ​ Found between middle circular and outer longitudinal layers ​Pathology lacking intrinsic factor: Vitamin B12 deficiency or ​ Consists of parasympathetic ganglia and nerve fibers megaloblastic anemia ○​ Important for propelling food ○​ When stomach is removed, the patient is prone to Vitamin B12 deficiency thus needing replacement Chief/Peptic/Zymogenic Cells ​ Located towards the bases of the glands ​ Condensed, basally-located (not centered) nuclei and strongly basophilic granular cytoplasm, reflecting large content of ribosomes ​ Secretes pepsinogen which is converted to pepsin ○​ Important in protein digestion ○​ Otherwise inactive to prevent auto-digestion of stomach Figure 22. Histological Diagram of Myenteric Nerve Plexus SEROSA ​ Thin outer layer of connective tissue that overlies the muscularis externa ​ Covered by a simple squamous mesothelium of visceral peritoneum ​ Can contain adipose cells Figure 21. Chief Cells OS 206 Histology of GI Tract 5 of 14  III.​ LOWER DIGESTIVE TRACT Microvilli ​ Surface feature of enterocytes/epithelium A.​ SMALL INTESTINES ​ Cytoplasmic extensions that cover the apices of the intestinal ​ Site where digestive processes are completed absorptive cells ○​ Nutrients are absorbed by cells of epithelial lining ​ Forms the “brush border” or fuzzy border under the microscope ​ Three regions ​ Coated by a glycoprotein called glycocalyx ○​ Duodenum (shortest) ○​ Contains different enzymes ○​ Jejunum (first ⅖) ​ Lactase (digests lactose) ○​ Ileum (last ⅗) ​ Peptidases (digests dipeptides) ​ Absorption ​ Sucrase (digests sucrose) ○​ Duodenum and jejunum ​ Lipase (digest triglycerides) ​ Mainly carbohydrates and proteins ○​ No enzymes = no nutrient absorption = malnutrition ○​ Ileum (terminal ileum) ​ Dietary lipids Figure 25 Microvilli of Small Intestine(UOHSC) Figure 23. Histological Diagram of Small Intestine (Mucosa to Serosa)(UPCM Trans 2028) MUCOSA Figure 26 Comparison of Villi and Microvilli Figure 24. Plicae circulares, vili and layers From UPCM 2028 Trans: of small Intestine(UPCM Trans 2028) ​ Dairy and Acute Gastroenteritis (Diarrhea) ○​ Relates to importance of microvilli and glycocalyx Plicae Circulares ○​ Enzymes are washed-off → absence of lactase → promote ​ AKA Valves of Kerckring / Valvular conniventes / Horizontal more diarrhea mucosal folds / Circular folds ○​ Lactose NOT absorbed → ATTRACT water → goes to large ​ Permanent spiral folds or elevations of the mucosa with a intestine → interact with gut bacteria → gas submucosal core that extends into the intestinal lumen ○​ End Result: Diarrhea and flatulence ​ Folds increase surface area and enhance nutrient absorption ○​ Rapid turnover of epithelial cells allows for drinking milk again right after diarrhea Lamina Propria: Core Mucosa ​ Virus attacking microvilli ​ Serves as core of villi ○​ Causes severe malabsorption ​ Contents ○​ Blood vessels Crypts of Lieberkuhn (Intestinal Crypts) ○​ Lymphatic capillaries ​ AKA Intestinal crypts ○​ Nerves ​ Indentations in mucosa ○​ Smooth muscle ○​ Pits within circular folds ○​ Loose irregular tissue ​ Act as openings of the short tubular glands ​ Transference of nutrients (flow) ○​ Enterocytes → Capillaries → Veins → Reabsorb in venous circulation → Portal Vein → Liver ​ Lacteals ○​ Lymphatic vessels ○​ Function: absorb lipids ○​ Destination: Cisterna Chyli (Thoracic Ducts) ○​ Histological appearance: clear-staining ​ Arterioles, venules, and capillaries ○​ Help in absorption of lipids in lamina propria ​ Lymphoid follicles ○​ Contains Lymphocytes, plasma cells, tissue eosinophils, macrophages, and mast cells ○​ Part of mucosa-associated lymphoid tissue (MALT) Villi ​ Permanent finger-like mucosal projections of lamina propria that extend to intestinal lumen Figure 27 Intestinal Crypts (UPCM Trans 2028) ​ Increases the surface area of small intestine to increase absorption Muscularis Mucosae ○​ Similar to stomach having rugae ​ Smooth muscle fibers extending into core of individual villi and are ​ Lined by simple columnar epithelium responsible for their movement ​ Components of connective tissue core: ​ Increases the contacts of the villi with digested food products in ○​ Lacteals the intestine ○​ Blood capillaries ○​ Done to promote interaction between epithelial cells ○​ Individual strands of smooth muscles ​ Enterocytes ○​ Absorptive cells in epithelium ○​ Contains microvilli ​ Submucosal plexus ○​ Core smooth muscle that helps villi move OS 206 Histology of GI Tract 6 of 14  Figure 29. Small Intestine: (A-B) Longitudinal section of villi showing the muscularis mucosa (Mu), simple columnar epithelium, capillaries (C), and goblet cells, (C) Cross section of villi showing the lacteal (L) and T lymphocytes (Ly) Figure 28. Drawn Histological Slide of Small Intestine Cells in Mucosa Figure 30. Sections of Crypt of Lieberkuhn showing Paneth Cells (P). Other parts ​ Found in Crypts of Lieberkühn shown are Endocrine cells (E), and Goblet cells (G) ​ Enterocytes ○​ Most numerous cell type, main absorptive cells ○​ Contains microvilli ○​ Coated with glycocalyx ​ Goblet cells ○​ Scattered among enterocytes ○​ Produces mucin for lubrication of intestinal contents and protection of epithelium ○​ Empty looking because contents are washed away during slide Figure 31. Diagram showing Mucosal Cells processing SUBMUCOSA ​ Paneth cells ​ Moderately dense loose connective tissue ○​ Found at the base of the crypts and are distinguished by their ​ May contain large group of lymphoid nodules / Peyer’s Patches prominent eosinophilic apical granules (reddish-orange)​ (mostly in ileum), which are important in inflammation ○​ Presence of granules indicate actively producing proteins ○​ If it outgrows the blood supply, intestinal walls get perforated ​ Granules contain ​ May lead to death if untreated in typhoid cases ​Antimicrobial peptides (defensins) ​ Perforated ileitis ​Protective enzymes (Lysozyme, Phospholipase A) ○​ Function: defense and protection Meissner Plexus / Submucosal Plexus ​ Neuroendocrine cells ​ Autonomic ○​ Produced locally acting hormones to regulate GI motility and ​ Controls the smooth muscles of the lamina propria and muscularis secretion mucosae ○​ Examples (Gastrin, CKK) ○​ Moves the villi and help propel lymph through the lacteals ○​ Not seen with H&E stain ​ Stem cells Brunner’s Glands ○​ Found at the base of the crypt, divide continuously to replenish ​ Pale staining compound tubulo-alveolar all of the above four cell types ​ Secrete alkaline mucus and epidermal growth factor (modulates ○​ Cannot be distinguished from the others histologically parietal cell secretion and rate of proliferation) ○​ Makes the organ prone to uncontrolled cell division or cancer ​ Only found in the duodenum ​ Intraepithelial lymphocytes ○​ Mostly T cells MUSCULARIS EXTERNA ○​ Provide defense against invasive organisms ​ Has two layers: ○​ Outer longitudinal Table 2. SUMMARY OF THE CELLS OF THE SMALL INTESTINAL MUCOSA ○​ Inner Circular LOCATION IN ​ Thickened terminal ileum forming ileocecal sphincter CELL TYPE FUNCTION MUCOSA Auerbach (Myenteric) Nerve Plexus Absorptive Epithelium / Digestion and absorption of ​ Autonomic Intestinal Glands nutrients in chyme ​ Strong peristalsis and segmental contraction of intestines ​ Between the 2 layers of muscle: Goblet Epithelium / Secretion of mucus ○​ Smooth muscle in the inner circular layer Intestinal Glands ○​ Outer longitudinal layer of muscularis ​ Important for peristalsis Paneth Intestinal Glands Secretion of bacterial enzyme DIFFERENTIATING SMALL INTESTINE SEGMENTS lysozyme; phagocytosis Table 3. CHARACTERISTICS OF THE 3 SEGMENTS OF THE LARGE INTESTINE ENTEROENDOCRINE CELLS (NOT SEEN IN H&E) SEGMENT KEY FEATURE CHARACTERISTICS G cells Intestinal glands Secretion of the hormone of duodenum internal gastrin Duodenum Contain branched ​ Lined by columnar duodenal (Brunner’s) epithelium with microvilli I cells Intestinal glands Secretion of the hormone gland with ​ Villi are more prominent of duodenum cholecystokinin, which mucus-secreting broad and tall (1.5 mm) stimulates release of cells in the and numerous pancreatic juices and bile submucosa ​ Fewer goblet cells in the epithelium K cells Intestinal glands Secretion of the hormone glucose-dependent Jejenum ​ Shorter (1 mm), narrower, insulinotropic peptide, which and fewer villi than the stimulates the the release of Nothing to be noted duodenum insulin ​ More goblet cells in the epithelium M cells Intestinal glands Secretion of the hormone of duodenum and motilin, which accelerates Ileum Lymphatic nodules ​ Few villi that are narrow jejunum gastric emptying, stimulates are particularly large and short (0.5 mm) intestinal peristalsis, and and numerous; they ​ More goblet cells than in stimulates the production of aggregate in the duodenum and jejunum pepsin lamina propria and submucosa to form S cells Intestinal glands Secretion of the hormone prominent Peyer’s secretin Patches OS 206 Histology of GI Tract 7 of 14 From UPCM 2027 Trans: ​ Distinct features of the Small Intestine: ○​ Absence of chief cells and parietal cells ○​ Presence of Crypts with villi and Paneth cells ○​ From duodenum to ileum: ​ Decreasing height of villi ​ Increasing number of goblet cells ​ Villi + Submucosal Cardiac Glands → Esophagus ​ Villi - Glands → Stomach ​ Villi + Brunner’s Glands → Duodenum ​ Fewer Villi - Glands → Jejunum ​ Villi + more Goblet Cells + Peyer’s Patches → Ileum Figure 36. Section of Jejunum, compare villi to Duodenum and Ileum Figure 32. Section of Duodenum from Mucosa to Muscularis Externa Figure 37. Section of Ileum showing Mucosa to Serosa, notice Peyer patches Figure 38. Sections of Ileum showing Payer’s Patches, shorter villi, serosa. INTEGRATION: DIGESTION OF LIPIDS Figure 33. Section of Duodenum from Mucosa to Serosa Figure 34. Sections of Duodenum showing Villi and Glands. (Bottom Left) Lymphoid follicle in lamina propria (black circle), and Brunner's Glands in submucosa (green circle) [UPCM 2026 trans] Figure 39.. Absorption in an enterocyte (Summary of Lipid Digestion) Steps in Lipid Digestion 1.​ Lipids in the lumen of the small intestine are broken down by pancreatic lipase to fatty acids and monoglycerides 2.​Monoglycerides and fatty acids are emulsified by bile, forming micelles that move into the surface absorbing cells. Glycerol diffuses directly into the surface absorbing cells. 3.​Monoglycerides and fatty acids are esterified into triglycerides (TAGs) within the smooth ER 4.​TAGs are complexed with protein within the Golgi apparatus, forming chylomicrons that are released into the lacteals. Figure 35. Section of Jejunum showing Villi and Lack of Glands. SM = submucosa, P (Apo-protein of chylomicron: ApoB48) = plicae, V = villi, M = muscularis, S = serosa 5.​Glycerol and short- and medium-chain fatty acids are absorbed directly into the blood. Summary Points ​ Absorption of Lipids ○​ Most of what we eat are dietary triglycerides ​ Broken down as fatty acids and monoglycerides to be absorbed in the enterocyte ○​ Reforming in the enterocyte as triglycerides then form chylomicrons ○​ Chylomicrons go to lymphatic capillaries / lacteals to be absorbed OS 206 Histology of GI Tract 8 of 14  B.​ LARGE INTESTINE ​ Recovers water and salt from feces ​ Propels increasingly solid feces to the rectum prior to defecation ​ Dysfunction leads to diarrhea or loss of water and electrolytes[Trans 2028] Figure 44. Submucosa and submucosal/Meissner plexus[2028 Trans] MUSCULARIS EXTERNA ​ Inner circular layer ○​ Continuous in the colon wall ​ Outer longitudinal layer Figure 40. Gross anatomy of the large intestine showing the cecum, colon, ○​ Condensed into three broad longitudinal bands called taeniae and rectum coli ​ Causes formation of shallow sacculations (Haustra) ​ Auerbach/Myenteric Nerve Plexus ○​ Between internal circular and external longitudinal layer ○​ Innervates muscularis externa Figure 41. Cell population of the large intestine Figure 45. Muscularis externa[2028 Trans] Figure 42. Layers of the Large Intestine [2026 Trans] Figure 46. Taenia coli and haustra of colon[Alexander, 2003] MUCOSA ADVENTITIA/SEROSA ​ Epithelium lining: simple columnar epithelium ​ Adventitia ○​ Consists of absorptive cells with microvilli ○​ Covers the ascending and descending colon ​ Absorbs mostly water (as opposed to cells founds in small ​ Retroperitoneal intestine) ​ Serosa ○​ Contains a lot more goblet cells instead of enterocytes ○​ Covers the transverse and sigmoid colon ​ Produce mucus that lubricates the lumen of the large intestine ​ Intraperitoneal to facilitate the passage of feces C.​ APPENDIX ​ As water is removed from the feces, it would be more solid and difficult to propel ​ Slender worm-shaped tube that projects from the blind end of the ​ Contents: cecum ○​ Intestinal glands ​ NOT a vestigial organ; has an immunologic function ○​ Lamina propria ○​ Part of mucosa-associated lymphoid tissue (MALT) or ​ ALWAYS contain lymphoid follicles gut-associated lymphoid tissue (GALT) ○​ Muscularis mucosae ​ Appendicular wall ​ Thin external layer ○​ Large number of lymphoid nodules in mucosa and submucosa ​ Digestive enzymes are NOT produced by the cells of the large ​ Germinal centers (pale center) in these nodules are sites of intestine B-lymphocyte production ​ Villi and plicae circulares are NOT present in colon ○​ Primary follicles ​ Homogenous ○​ Secondary follicles ​ With a pale center ​ Active centers since they have been exposed to antigens Figure 43. Mucosa of the Large Intestine [UPCM 2026 Trans] SUBMUCOSA ​ Contains connective tissue cells and fibers, various blood vessels, and nerves ​ Meissner/Submucosal Plexus ○​ Innervates goblet cells and muscularis mucosae Figure 47. Appendix OS 206 Histology of GI Tract 9 of 14  IV.​ TRANSITION ZONES C.​ RECTO-ANAL JUNCTION ​ Abrupt transition from simple columnar epithelium of the rectum A.​ ESOPHAGO-GASTRIC JUNCTION to stratified squamous epithelium of the anus ​ Also known as gastroesophageal junction ○​ At first, the epithelium of the anus is not keratinized, but within a ​ Abrupt transition from stratified squamous epithelium of the few millimeters it becomes keratinized[Histology Guide] esophagus to simple columnar glandular mucosa of the stomach ○​ The epithelium type in the anus is due to the regular trauma it ​ The muscularis mucosae (MM) is continuous across the junction experiences during defecation ​ The underlying submucosa (SM) and muscularis propria/externa (MP) continue beneath the mucosal junction Table 6. ESOPHAGOGASTRIC JUNCTION FEATURES Table 4. ESOPHAGOGASTRIC JUNCTION FEATURES STRUCTURE RECTUM ANUS STRUCTURE ESOPHAGUS STOMACH Epithelium Simple columnar Stratified squamous epithelium epithelium Epithelium Stratified squamous Simple columnar non-keratinizing epithelium Goblet cells Within mucosa None epithelium Glands Esophageal glands Gastric glands within submucosa within mucosa Figure 51. Recto-anal junction[2028 Trans] From UPCM 2028 Trans: Figure 48. Esophago-gastric junction ​ Branched tubular circumanal glands open at the recto-anal junction into small pits at the distal ends of the columns of Morgagni Figure 49. Esophago-gastric junction[2025 Trans] B.​ GASTRODUODENAL JUNCTION ​ Both lined by simple columnar epithelium; differentiated under the microscope by other structures Figure 52. Recto-anal junction ​ Pyloric sphincter ○​ Large smooth muscle in the junction separating the pylorus of V.​ REFERENCES the stomach and the duodenum of the intestine Alexander, W. (2003). Dorland’s Illustrated Medical Dictionary. W B Table 5. GASTRODUODENAL JUNCTION FEATURES Saunders Company Histology Guide. (2024). Recto-Anal Junction. STRUCTURE STOMACH DUODENUM https://histologyguide.com/slideview/MHS-258-recto-anal-junction/ Epithelium Simple columnar epithelium 14-slide-1.htmL Junqueira, L. C., & Carneiro, J. (2018). Basic histology: Text & atlas (14th ed.). McGraw-Hill Education. Glands Pyloric glands Brunner’s glands University of Oklahoma Health Sciences Center. (n.d.). Lower within mucosa within submucosa gastrointestinal system. Retrieved February 5, 2025, from https://www.ouhsc.edu/histology/text%20sections/lower%20gi.html Prominent mucosal Gastric pits Villi, intestinal UPCM Trans 2028 structures crypts UPCM Trans 2027 Figure 50. Gastro-duodenal junction OS 206 Histology of GI Tract 10 of 14  APPENDIX Figure 53. Layers of the digestive tract Figure 54. Layers of esophagus. OS 206 Histology of GI Tract 11 of 14  Figure 55. Layers of stomach (Mucosa, Submucosa, Muscularis Externa, Serosa).[diFiore] Table 7..PRINCIPAL ENTEROENDOCRINE CELLS IN THE GI TRACT[Junqueira] MAJOR ACTION CELL TYPE MAJOR LOCATION HORMONE PRODUCED PROMOTES INHIBITS D cells Pylorus, duodenum, and Somatostatin secretion from other pancreatic islets DNES cells nearby EC cells Stomach, small and large Serotonin and substance Increased gut motility intestines P G cells Pylorus Gastrin Gastric acid secretion I cells Small intestine Cholecystokinin (CCK) Pancreatic enzyme Gastric acid secretion secretion, gallbladder contraction K cells Duodenum and jejunum GGastric inhibitory Gastric acid secretion polypeptide (GIP) L cells Ileum and colon Glucagon-like peptide Insulin secretion Gastric acid secretion (GLP-1) Sense of hunger L cells Ileum and colon Peptide YY H2O and electrolyte Gastric acid secretion absorption in large intestine Mo cells Small intestine Motilin Increased gut motility N cells Ileum Neurotensin Gastric acid secretion S cells Small intestine Secretin Pancreatic and biliary Gastric acid secretion bicarbonate and water Stomach emptying secretion OS 206 Histology of GI Tract 12 of 14 Table 8. SUMMARY DISTINGUISHING DIGESTIVE TRACT FEATURES[Junqueira] MUSCULARIS (INNER MUCOSA (EPITHELIUM, CIRCULAR AND OUTER REGIONS AND SUBMUCOSA (WITH LAMINA PROPRIA, LONGITUDINAL LAYERS ADVENTITIA/SEROSA SUBDIVISIONS SUBMUCOSAL PLEXUSES) MUSCULARIS MUCOSA) WITH MYENTERIC PLEXUSES Esophagus (upper, Nonkeratinized stratified Small esophageal glands Both layers striated Adventitia, except at middle, lower) squamous epithelium; (mainly mucous) muscle in upper region; lower end with serosa cardiac glands at lower both layers smooth end muscle in lower region; smooth and striated muscle fascicles mingled in middle region Stomach (cardia, Surface mucous cells and No distinguishing features Three indistinct layers of Serosa fundus, body, pylorus) gastric pits leading to smooth muscle (inner gastric glands with oblique, middle circular, parietal and chief cells, (in and outer longitudinal) the fundus and body) or to mucous cardiac glands and pyloric glands Small intestine Plicae circulares; villi, with Duodenal (Brunner) No distinguishing features Mainly serosa (duodenum, enterocytes and goblet glands (entirely mucous); jejunum, ileum) cells, and crypts/glands possible extensions of with Paneth cells and Peyer patches in ileum stem cells; Peyer patches in ileum Large intestine Intestinal glands with No distinguishing features Outer longitudinal layer Mainly serosa, with (cecum, colon, goblet cells and separated into three adventitia at rectum rectum) absorptive cells bands, the teniae coli Anal canal Stratified squamous Venous sinuses Inner circular layer Adventitia epithelium; longitudinal thickened as internal anal columns sphincter Esophagus (upper, Nonkeratinized stratified Small esophageal glands Both layers striated Adventitia, except at middle, lower) squamous epithelium; (mainly mucous) muscle in upper region; lower end with serosa cardiac glands at lower both layers smooth end muscle in lower region; smooth and striated muscle fascicles mingled in middle region Stomach (cardia, Surface mucous cells and No distinguishing features Three indistinct layers of Serosa fundus, body, pylorus) gastric pits leading to smooth muscle (inner gastric glands with oblique, middle circular, parietal and chief cells, (in and outer longitudinal) the fundus and body) or to mucous cardiac glands and pyloric glands Small intestine Plicae circulares; villi, with Duodenal (Brunner) No distinguishing features Mainly serosa (duodenum, enterocytes and goblet glands (entirely mucous); jejunum, ileum) cells, and crypts/glands possible extensions of with Paneth cells and Peyer patches in ileum stem cells; Peyer patches in ileum Large intestine Intestinal glands with No distinguishing features Outer longitudinal layer Mainly serosa, with (cecum, colon, goblet cells and separated into three adventitia at rectum rectum) absorptive cells bands, the teniae coli Anal canal Stratified squamous Venous sinuses Inner circular layer Adventitia epithelium; longitudinal thickened as internal anal columns sphincter Table 9. DIFFUSE NEUROENDOCRINE SYSTEM CELLS OF THE GASTROINTESTINAL TRACTJunqueira] OS 206 Histology of GI Tract 13 of 14  CELL LOCATION HORMONE PRODUCED FUNCTION A Stomach, small intestine Glucagon Elevates blood glucose D Stomach, intestines Somatostatin Inhibits hormone release by DNES cells EC Stomach, intestines Serotonin, substance P Increases peristaltic movement enterochromaffin cell ECL Stomach Histamine Stimulates HCl secretion enterochromaffin-like cell G Stomach, small intestine Gastrin Stimulates HCl secretion;gastric motility gastrin-producing cell GL Stomach, intestines Glicentin Elevates blood glucose glicentin-producing cell Stimulates release of pancreatic enzymes I Small intestine Cholecystokinin and contraction of gallbladder K Small intestine Gastric inhibitory peptide Inhibits HCl secretion Mo Small intestine Motilin Increases intestinal peristalsis motilin-producing cell N Small intestine Neurotensin Decreases intestinal peristalsis neurotensin-producing cell PP Stimulates enzyme release by chief cells; pancreatic Stomach, large intestine Pancreatic polypeptide inhibits release of pancreatic enzymes polypeptide-producing cell S Small intestine Secretin Stimulates release of pancreatic buffer VIP Increases peristalsis of intestines; vasoactive intestinal Stomach, intestines Vasoactive intestinal peptide stimulates elimination of water and ions peptide-producing cell OS 206 Histology of GI Tract 14 of 14