Histology of GI Glands PDF, UPCM 2024-2025

OS 206: ABDOMEN AND PELVIS HISTOLOGY OF GI GLANDS UPCM 2029 | Dr. Kenny Seng, MD, FAFN | LU3 A.Y. 2024-2025 OUTLINE PAROTID GLAND...

OS 206: ABDOMEN AND PELVIS HISTOLOGY OF GI GLANDS UPCM 2029 | Dr. Kenny Seng, MD, FAFN | LU3 A.Y. 2024-2025 OUTLINE PAROTID GLAND ​ Branched acinar glands with exclusively serous acini I.​ Salivary Glands III.​ Pancreas ○​ Darkly stained in H&E due to ribosomes (basophilic) A.​ Major Salivary Glands A.​ Exocrine Portion ​ Serous cells are protein-secreting cells B.​ Cells of the Salivary B.​ Endocrine Portion ​ Secrete watery serous saliva abundant in α-amylase Glands IV.​ Gallbladder ○​ Initiates hydrolysis of carbohydrates C.​ Ductal System A.​ Gross Anatomy and ○​ Breakdown of proline-rich proteins with antimicrobial and other D.​ Nerve Supply Functions protective properties [Mescher, 2018] II.​ Liver B.​ Histologic Features ​ Contain large amounts of adipose tissue within the parenchyma A.​ Basic Organizational C.​ Bile: Role, Route, and ○​ A salivary gland containing adipose cells is usually automatically Unit Composition the parotid gland B.​ Liver Histology ○​ Do NOT confuse with pale-staining mucous cells (Figure 3) C.​ Hepatic Blood Flow ​ Adipose droplets become empty spaces once fats dissolve during slide preparation I.​ SALIVARY GLANDS A.​ MAJOR SALIVARY GLANDS ​ There are three bilateral pairs of major salivary glands: ○​ Parotid gland ○​ Submandibular gland ○​ Sublingual gland ​ Have acini as secretory units of the duct system, opening into the oral cavity via branching ducts ​ Secrete about 90% of total saliva volume ​ Stimulated mechanically or chemically (NOT voluntary) Figure 3. Mucous cells (L) and Fat Cell Space (R) Figure 1. Major Salivary Glands Table 2. Characteristics of Major Salivary Glands Figure 4. Acinar cells of the parotid gland. Striations of a duct (SD), septum (CT), and numerous serous acini (A). [Mescher, 2018] PAROTID SUBMANDIBULAR SUBLINGUAL Cell Type Completely Mixed (serous and Mostly mucous serous mucous) Location Each cheek near Underneath the Underneath the the ear ramus of the tongue mandible Size Largest Intermediate Smallest Specific Duct Stensen’s duct Wharton’s duct Duct of Rivinus Openings of Buccal cavity Floor of buccal Floor of buccal the duct across the cavity at the sides cavity at the maxillary second of lingual frenulum sides of lingual Figure 5. Parotid gland (low-power). Lobules (L), Septa (S), Excretory ducts (E), molar frenulum Serous Cells (SC). [Young, et al. (2013)] ​ Another indicator of parotid glands is the presence of their striated Amount of 10-25% of total 60-75% of total 5% of total saliva excretory ducts to concentrate the saliva saliva saliva production saliva production production SUBMANDIBULAR GLAND production ​ Branched tubuloacinar glands with mixed serous and mucinous ​ Each salivary gland is encased in a connective tissue capsule acini (Figure 6) ○​ Main indicator of major salivary glands ○​ Also contains serous demilunes ○​ Minor salivary glands do NOT have a capsule [Pawlina and Ross (2016)] ​ Secretes amylase, proline-rich proteins, and lysozyme ​ Connective tissue septum partitions the gland parenchyma into ○​ Lysozymes promote hydrolysis of bacterial cell wall lobes and further subdivides it into lobules ○​ Each lobule has its own duct, which eventually joins other ducts to drain into the main duct Figure 6. Submandibular gland. Serous acini (A), “serous demilunes” (S), Mucous cells (M) ​ In Figure 6, well-stained serous acini, “serous demilunes”, and pale-staining mucous cells grouped as tubules Figure 2. Connective Tissue Septum like “Venice” Trans 08 TG19: Mercado, G., Mercado, P., Merino, Montenegro, Morales, Nakpil, Narvasa TH: Narvasa 1 of 11  ○​ The crescent-shaped “serous demilunes” arise at least in part artifactually ​ Due to disproportionate swelling of the adjacent mucous cells during slide preparation ○​ Small intralobular ducts drain each lobule SUBLINGUAL GLAND ) Figure 9. Serous cell in the parotid gland MUCOUS CELLS ​ Light-staining columnar-shaped cells with basally-oriented and flat nuclei (Figure 10) ○​ Due to high quantities of carbohydrates in contrast to serous Figure 7. Sublingual gland. cells Mucous cells (M), Intralobular Ducts (ID), Striated Muscle (SM) ​ Contains Mucin ​ Smallest of the major glands ○​ Mucin is mostly carbohydrates, hence have more prominent ​ Tubuloacinar glands predominated by mucous cells (Figure 7) Golgi compared to serous cells ○​ Stain very poorly with H&E, in contrast to the serous acini shown ​ Light-staining due to high quantities of carbohydrates in in the parotid contrast to serous cells B.​ CELLS OF THE SALIVARY GLANDS ​ Less prominent (conspicuous) RER, mitochondria ​ Often organized as cylindrical tubules rather than acini ​ The salivary secretory unit consists of a terminal branched ○​ Center of where apices point to is the duct tubulo-acinar structure composed of three secretory cell types ​ Produces hydrophilic mucin, which provides lubricating properties ○​ Serous in saliva ○​ Mucous ○​ Seromucous (mixture of both types) ​ Main function of these cells is the formation and secretion of saliva ○​ Saliva is secreted for buffering and evaporative cooling Buffering if within normal pH (6.5-6.9) ○​ Modified saliva starts from striated and terminal ducts, resulting in a hypotonic end product Table 2. Summary of serous and mucous glands differences TYPE SEROUS MUCOUS Thin, watery solution Thick, viscous Secretion Type with proteins secretion with mucin Round, more central Flattened nuclei Nuclei Position nuclei (compared to against basement mucous cells) membrane Figure 10. Mucous Cells Condensed Chromatin Dispersed Chromatin SEROMUCOUS CELLS chromatin ​ Tubuloacinar secretory units with both serous and mucous Dark staining due to Light staining due to secretion Staining with H&E presence of presence of mucin ​ Periodic acid-Schiff stain (PAS): preferentially stains zymogens carbohydrates magenta red color Duct Size Large, striated ducts Small, striated ducts ○​ Mucinous acini will take a magenta red color, while serous cells maintain their original color (Figure 12) Solubilizing dry food, Lubricating oral maintaining oral Function cavity, making food hygiene, initiating slippery starch digestion Predominantly in Predominantly in Location Parotid Sublingual SEROUS CELLS ​ Dark-staining pyramidal cells surrounding a duct basally-oriented round nuclei located at the periphery of each acinus ​ Have all the features of a cell specialized for the synthesis, storage, and secretion of protein ○​ Main protein component: Amylase ○​ Rough endoplasmic reticulum for protein translation (ribosomal sites → cisternae; N- and O-linked glycosylation) ○​ Packaged in the golgi apparatus (carbohydrate moieties are Figure 11. Mixed glands added) ○​ Producing secretory granules → exocytosis to the lumen ○​ Endocytosis of the granule membrane Figure 12. Mixed glands differentiated with PAS SEROUS DEMILUNES ​ Proximal mucous cells covered with distal cap cells with a half-moon appearance ​ Considered as mucinous acini with serous acini sort of wrapped around the periphery of the acinus ​ Usually seen in mixed glands (i.e., submandibular glands) Figure 8. Parotid gland showing the striated duct, serous cells and adipocytes OS 206 Histology of GI Glands 2 of 11  Figure 13. Serous Demilunes MYOEPITHELIAL CELLS ​ 1-3 myoepithelial cells in each salivary and piece body with 4-8 Figure 16. Comparison between the components of the major salivary glands processes Note the ratio of cells in the acinus and length of salivary ducts ​ Desmosomes between myoepithelial cells and secretory cells ​ Myofilaments frequently aggregated to form dark bodies along the FORMATION & SECRETION OF SALIVA course of the process ​ There are 2 main stages to the secretion of saliva: ​ Myoepithelial cells of the intercalated ducts are more ○​ Formation of primary saliva spindle-shaped and with fewer processes (very similar to that of ​ Formed by the serous and mucous cells smooth muscle cells) ​ Involves intercalated ducts ​ Functions as support secretory cells that contract and widen the ○​ Modification of primary saliva to modified saliva diameter of the intercalated ducts ​ Involves striated and terminal ducts ​ The end product is hypotonic (less sodium) C.​ DUCTAL SYSTEM ​ The salivary ductal system is continuous with the lumen of the salivary acinus ​ Flow of secretion: acinar cell → intercalated duct → striated duct → intralobular duct → interlobular duct → lobular duct → main duct → oral cavity ○​ The intercalated duct is difficult to see because it has a flattened single-cell layer of simple squamous epithelium that does not take up stain ○​ The striated duct is easy to identify because of its cuboidal epithelium and striations ○​ The succeeding ducts are located in the connective tissue septum Figure 17. Cells and ducts found in the salivary gland Doc mentioned that there’s no need to differentiate the intralobular and interlobular ducts. As long as they are in the connective tissue septations, they are already considered excretory ducts. Figure 18. Schematic diagram of the exocrine salivary gland INTERCALATED DUCTS ​ Most prominent in glands with watery serous secretion (parotid) ​ Small diameter; right next to acinus ​ Lined by simple squamous or low cuboidal cells Figure 14. Simplified illustration of the ductal system ○​ Centrally located nucleus ○​ Well-developed RER, Golgi apparatus ○​ Occasionally secretory granules ○​ Few microvilli ​ Hard to locate ○​ Usually collapsed and does NOT have any supporting structures to keep it patent during slide preparation ○​ Pale staining compared to surrounding serous cells Figure 15. Flow of secretion through the ductal system ​ As you move farther away from the acinus, more layers are added to the epithelium of the duct itself ​ As you proceed through each individual ductal system, the epithelium grows taller ○​ From cuboidal eventually being columnar and subsequently more layers are added, it being pseudostratified before eventually becoming stratified columnar Figure 19. Histology of different ducts Note how striated ducts are more prominent than intercalated ducts OS 206 Histology of GI Glands 3 of 11  Figure 20. Collapsed intercalated duct surrounded by serous cells STRIATED DUCTS ​ Cuboidal cells with a centrally located nucleus ○​ Lined by columnar cells ○​ Eosinophilic cytoplasm ​ Have very prominent striations near the basement membrane Figure 24. Electrolyte levels at different acinus distances Note how sodium levels drop and potassium levels increase as the secretions ○​ Striations are indentations of the cytoplasmic membrane with approach the intralobular duct many mitochondria present between the folds ○​ Interdigitated mitochondria activates the Na-K-ATPase pump CONNECTIVE TISSUE in order to make saliva hypotonic ​ Surrounds main secretory ducts compared to striated ducts ​ Modify the secretion from the acini into a hypotonic solution ○​ Will help you identify that what you are looking at is actually an ○​ Hypotonic solution → low Na and Cl, high K excretory duct and not a striated duct ​ Composition ○​ Fibroblasts ○​ Inflammatory cells ○​ Mast cells ○​ Adipose cells ○​ Extracellular matrix: glycoproteins, proteoglycans ○​ Collagen and oxytalan fibers ○​ Blood supply Figure 21. Cross section of a striated duct Figure 25. Connective tissue surrounding an excretory duct Figure 22. Longitudinal section of a striated duct Figure 26. Micrograph of the submandibular gland D.​ NERVE SUPPLY ​ No direct nerve supply ​ Saliva production is NOT voluntary ○​ Controlled by autonomic nervous system ○​ E.g. When taking an exam (stressed), the mouth is dry VS. when sitting in front of food (hungry), the mouth is salivating ​ No direct inhibitory innervation ​ Parasympathetic and sympathetic impulse, but parasympathetic is more prevalent ○​ Parasympathetic impulses can occur in isolation, evoke most of the fluid to be excreted, cause exocytosis, induce contraction of myoepithelial cells, cause vasodilation Figure 23. Cross section of a parotid gland showing a striated duct [A: acini, AL: acini lumen, ID: intercalated duct, PC: plasma cells, StD: striated duct] DUCTAL MODIFICATION ​ Controlled by the autonomic nervous system ○​ Modifies the amount and quality of the saliva secreted ​ Occurs in striated and terminal ducts ​ Saliva is modified via reabsorption and secretion of electrolytes ○​ Final salivary product is hypotonic Figure 27. Nerve supply of the parotid gland OS 206 Histology of GI Glands 4 of 11  II.​ LIVER ​ Heaviest gland in the body ​ Processes drugs by making them water-soluble ​ Has endocrine and exocrine function Table 3. Exocrine and Endocrine Portions of the Liver Exocrine Endocrine ​ Synthesizes and secretes ​ Synthesizes and secretes bile numerous plasma proteins via system of ducts into the bloodstream ○​ Essential for digestion in the ○​ Albumin, fibrinogen, intestine prothrombin, lipoproteins, etc. FUNCTIONS ​ Production and secretion of bile ○​ Liver secretes approximately 1 L of bile a day ​ Storage of carbohydrate (glycogen) and lipids (triglycerides) Figure 30. Basic Organizational Unit of the Liver ​ Protein synthesis: fibrinogen, prothrombin ​ Anticoagulant production: heparin Table 4. Difference between Pig and Human Liver ​ Bile pigments from the breakdown of hemoglobin: bilirubin, Pig Liver Human Liver biliverdin ​ Gives the golden brown colored stool (normal) ​ Less prominent septations ○​ Responsible for the brown pigment of stool ​ More prominent septations ​ Portal triads are not always ○​ Babies with biliary atresia: ​ It's easier to identify the found at every corner ​ Deficient biliary tree hexagonal plate ​ NOT always 6 sided (but ​ Bile can NOT make it to the intestines definitely polygonal) ​ Can still defecate since the GI tract is intact ​ The stool is pale in color (Acholic stools) Figure 28. Anterior View of the Liver Figure 31. Pig Liver Figure 32. Pig (Left) vs Human (Right) Liver Figure 29. Posterior View of the Liver CLASSICAL HEPATIC LOBULE A.​ BASIC ORGANIZATIONAL UNIT ​ Basic functional unit of the liver COMPOSITION ○​ Analogous to bone osteon ​ Hexagonal-shaped plate with: ​ Hepatocytes: major functional cells of the liver ○​ Central vein ○​ Metabolic, secretory, and endocrine functions ○​ Portal triad located at the corner ​ Produce and secrete bile (excretory product and digestive ○​ Sinusoids: blood vessels that surround hepatocytes secretion) and plasma proteins ​ Drains to the central vein ​ Bile canaliculi: ducts between hepatocytes that collect bile ○​ Hepatocytes: lined up like rays of sun coming out of the central ○​ Exit as common hepatic duct (from liver) + cystic duct (from vein gallbladder) → common bile duct ​ Bathe in oxygenated and deoxygenated blood ​ Hepatic sinusoids (capillary system of the liver): highly ​ Products of hepatocytes will be conveyed into the central vein permeable blood capillaries receiving and mixing: for drainage out of the liver ○​ Oxygenated blood from hepatic artery ○​ Flow is from corner to the center ○​ Deoxygenated nutrient-rich blood from hepatic portal vein PORTAL TRIAD ​ Hepatic artery: branch of celiac trunk responsible for delivering oxygenated blood to the liver ​ Hepatic portal vein: carries partially deoxygenated blood and carbohydrates, amino acids, lipids, and nucleic acids from the GI tract to the liver ​ Bile duct: carries bile out of the liver Figure 33. Classical Hepatic Lobule OS 206 Histology of GI Glands 5 of 11  Figure 38. Details of the Acinus Figure 34. Another Classical Hepatic Lobule Table 5. Details of the Acinus PORTAL LOBULE Classic Lobule Portal Lobule Portal Acinus ​ Portal triad is at the center ​ Center: Central ​ Center: Portal ​ Triangular in shape vein triad ​ Center: Hepatic ​ Central veins at the corners ​ Shape: Hexagonal ​ Shape: Triangular artery ​ Essentially for the flow of bile out of the liver ​ Corners: Portal ​ Corners: Central ​ Divided into zones PORTAL/HEPATIC ACINUS triad veins ​ Flow of oxygenated ​ Center is the hepatic artery (portal triad) ​ Flow from corner to ​ Flow of bile out of blood ​ Short axis: branches of portal triad between 2 classic lobules center the liver ​ Long axis: between 2 central veins ​ In patients with congestive heart failure, Zone 3 takes most of the ​ Relates to the amount of oxygen that is present within the hepatic damage because oxygen is not well-distributed lobule; how oxygenated blood makes its way to the central vein ○​ Farther from the portal triad, closer to the central vein: lesser oxygen ​ During ischemia, hepatocytes closer to the central vein are first affected ○​ Acinus is divided into zones ​ Zone 1 is closer to the portal triad ○​ The closer to the portal triad, the more it receives oxygen ​ Zone 1 has the highest oxygen since its closest to the portal triad ​ More glycolysis in Zone 3 as adaptation to lower oxygen Figure 39. Liver of Patients with congestive heart failure Figure 35. Diagram of Portal Acini B.​ LIVER HISTOLOGY ​ Hepatocytes: Polygonal epithelial cells that form branching, irregular plates; ○​ The major functioning cell of the liver ○​ Functions for carbohydrate, lipid, and protein metabolism as well as for detoxification ○​ Rich in mitochondria and Golgi bodies ○​ Arranged in cords oriented towards the central vein Figure 36. Zones of the Acinus Figure 40. Liver Cells (Hepatocytes) ​ Sinusoids: Blood vessels right next to hepatocytes ○​ Capillary system that contains blood from the Hepatic Portal Vein and Hepatic Artery which drains into the central vein Figure 37. Details of Hepatic Acinus OS 206 Histology of GI Glands 6 of 11  Figure 41. Hepatocytes with Sinusoids ​ Space of Disse: Space between the sinusoids and hepatocytes ○​ Contain the plasma ○​ Perisinusoidal space between basal surfaces of the endothelial and the surfaces of the hepatocytes ​ Endothelium: the porous lining of the hepatic sinusoids ​ Bile Canaliculi: ductal system that collects bile from the hepatocytes ○​ Found between the hepatocytes ○​ Come together to form the bile duct ○​ Not seen next to the sinusoids → bile is NOT mixed with blood Figure 45. Portal Triad (PV - Portal Vein, HA - Hepatic Artery, BD - Bile Duct) ​ Reticular Fiber Meshwork: holds the entire structure together Figure 42. Bile Canaliculi (Black arrow) ​ Hexagonal Plate: cords of hepatocytes radiating out of the central vein Figure 46. Reticular Fiber Meshwork ​ Kupffer Cells: resident macrophages (phagocytic cells) of the liver; cleans up the nutrients not absorbed by the Space of Disse Figure 47. Kupffer Cells ​ The liver also serves as a site of hematopoiesis during early fetal life when the skeletal muscle has not matured yet (ie. no bone marrow yet) Figure 43. Cords of hepatocytes radiating out of the central vein Figure 48. Hematopoiesis in the Liver Other histological features of the liver: ​ Space of Mall: periportal space ​ Canals of Herring: larger biliary canaliculi within the lobule just before the portal triad ​ Ito cell: hepatic stellate cell for storage of vitamin A; also has an important role in liver cirrhosis (transforms into fibroblasts that deposit collagen, hardening the liver) Figure 44. Hexagonal Plate ​ Portal Triad: three major tubes that supply the liver; found at the corners of the hexagonal plate ○​ Portal Vein: carries blood from the abdomen to the liver; thin-walled and biggest ○​ Hepatic Artery: delivers oxygenated blood to the liver; smooth muscle-walled with flattened nuclei ○​ Bile Duct: carries bile from the liver into the gallbladder; simple cuboidal epithelium OS 206 Histology of GI Glands 7 of 11  Figure 52. Liver Vasculature III.​ PANCREAS ​ Lies posterior to the greater curvature of the stomach ​ Head of the pancreas ○​ Embraced by the C-shaped curve of the duodenum ​ Because pancreatic secretions empty into the duodenum Figure 49. Space of Disse and Ito Cell ○​ Has an uncinate process ​ Hook-shaped, inferior portion of the head ​ The superior mesenteric artery (SMA) is on its left ​ Neck of the pancreas ○​ Overlies the SMA and the origin of the portal vein (SMV + splenic vein) ​ Body of the pancreas ○​ Lies to the left of both SMA and SMV ○​ At the level of L1-L2 vertebrae ​ Tail of the pancreas ○​ Closely related to the hilum of the spleen Figure 50. Other Histological Features of the Liver C.​ HEPATIC BLOOD FLOW ​ The liver receives a dual blood supply: hepatic portal vein (HPV) and hepatic artery ○​ Hepatic Artery: carries oxygenated, nutrient-poor blood ○​ Hepatic Portal Vein: carries deoxygenated blood with newly absorbed nutrients and possibly drugs, microbes or toxins absorbed from the GI Tract Figure 53. Pancreas and related structures ​ Pancreas is a mixed endocrine and exocrine gland ○​ Exocrine ​ 99% of cells are acini ​ Synthesizes and secretes pancreatic juices (mixture of fluid and digestive enzymes) via a system of ducts into the duodenum Figure 51. Hepatic Blood Flow ○​ Endocrine ​ The blood flows from either the hepatic portal vein (HPV) or the ​ 1% of cells are pancreatic islets (islets of Langerhans) hepatic vein into the liver sinusoids and into the central vein. ​ Synthesizes and secretes hormones (e.g. insulin, glucagon, ○​ The hepatocytes get bathed with both oxygenated and somatostatin, pancreatic polypeptides) into the bloodstream deoxygenated blood and their products will be conveyed into the ​ Pancreatic juices central vein for drainage. ○​ 1200 -1500mL secreted daily ​ The liver is then drained by the hepatic vein and would later on ○​ Alkaline in nature flow back to the heart through the inferior vena cava. ○​ Composition: ​ Mostly water ​ Sodium bicarbonate: buffers acidic stomach chyme ​ Enzymes: ​Pancreatic amylase: breaks down carbohydrates ​Proteolytic enzymes: breaks down proteins ○​ Trypsin (secreted as trypsinogen) ○​ Chymotrypsin (secreted as chymotrypsinogen) ○​ Carboxypeptidase (secreted as procarboxypeptidase) ○​ Elastase (secreted as proelastase) ​Pancreatic lipase: breaks down lipids ​Ribonuclease and Deoxyribonuclease: breaks down RNA and DNA OS 206 Histology of GI Glands 8 of 11  ​ Found in fibroconnective tissue septa ○​ Main ducts ​ More and more of the cellular layer added to it is surrounded by the connective tissue ​ Striated ducts are NOT present in the pancreas (only in the parotid) ○​ secretions do not need to be hypotonic ​ Centroacinar cells and Islets of Langerhans are only present in the pancreas (NOT in the parotid) Figure 54. Exocrine and endocrine portions of the pancreas (diagram) Figure 58. Pancreatic duct with connective tissue ​ Pancreatic juices secreted via the main pancreatic duct and accessory duct into the duodenum: ○​ Main pancreatic duct (Wirsung) Figure 55. Exocrine and endocrine portions of the pancreas (H&E stain) ​ Begins at the tail, runs to the right along the entire pancreas A.​ EXOCRINE PORTION ​ Joins the common bile duct (comprised of hepatic + cystic ​ Resembles the parotid gland ducts) to form the hepatopancreatic ampulla (Ampulla of Vater) ○​ because it also makes serous secretions ​Enters the descending part of the duodenum at the greater ​ Composed of serous cells with a few modifications specific to the duodenal papilla pancreas ○​ Accessory pancreatic duct (Santorini) ​ Compound acinous gland ​ Begins at the lower portion of the head ○​ Each acinus consists of 5-8 pyramidal cells that sit on a basal ​Drains a small portion of the head and body lamina and surround a central lumen ​ Enters the descending part of the duodenum at the lesser ​ Acinar cells duodenal papilla (about 2 cm above the greater papilla) ○​ Basally located nuclei and rough endoplasmic reticulum ○​ Supranuclear Golgi zone ○​ Apically located zymogenic granules (facing the lumen) ​ Contain digestive enzymes ​ Centroacinar cells ○​ Specific to pancreas ○​ Pale-staining cell ○​ Line the lumen of the acinus ○​ Secrete large amounts of bicarbonate ​ Bicarbonate added to pancreatic juices (makes it alkaline) Figure 59. Entry of the main pancreatic duct into the duodenum (L); X-ray of the biliary tree (R) B.​ ENDOCRINE PORTION ​ More nuclei and pale-staining ​ Highly vascularized because hormones are sent into the blood Figure 56. Acinar and centroacinar cells ​ Pathway ○​ Intercalated ducts ​ Lined with simple squamous epithelium ​ Collapsed Figure 60. Vascularization (capillaries) in the pancreas ​ Connected to the lumen of the acinus ​ Islets of Langerhans ○​ Masses of richly vascularized endocrine cells scattered throughout the pancreas ○​ Separated from surrounding acinar cells by thin capsule of reticulated fibers ○​ 3 cell types in the islets distinguished via special stains Figure 57. Exocrine cells with intercalated duct (H&E stain) ○​ Intralobular ducts ​ Lined with simple cuboidal epithelium ​ Size on the order of magnitude of acini ○​ Interlobular ducts ​ Lined with simple columnar epithelium OS 206 Histology of GI Glands 9 of 11  Figure 61. Exocrine and endocrine portions of the pancreas (H&E staining) Figure 63. Gallbladder, liver, duodenum, and relevant ducts Table 6. CELL TYPES IN THE ENDOCRINE PANCREAS CELL TYPE SECRETION EFFECT raise blood glucose Alpha cells (20%) glucAgon level lower blood glucose Beta cells (75%) insulin level inhibit both glucagon Delta cells (5%) somatostatin and insulin secretion Figure 64. Gallbladder with important ducts and vasculature Figure 62. Immunostaining of insulin-reactive cells (Beta cells) ​ Hormonal control of pancreatic secretions: ○​ Secretin ​ Stimulates duct cells to secrete large volume of fluid with high bicarbonate concentration + low to no enzymes ○​ Cholecystokinin (CCK) ​ Causes acinar cells to secrete proenzymes Figure 65. Gallbladder in relation to the duodenum IV.​ GALLBLADDER PORTAL LOBULE ​ Found inferior to the liver at the right upper quadrant of the ​ Contraction of smooth muscle fibers eject contents of abdomen gallbladder into cystic duct ○​ Junction of the right 9th costal cartilage and lateral border of the ​ Location for storage and concentration of bile produced by the rectus abdominis liver until it is needed in the small intestine A.​ GROSS ANATOMY AND FUNCTIONS ○​ Repository for bile that is produced in the liver ​ Similar to urine made by the kidney stored in the urinary ​ Pear-shaped, distensible sac, with about 30-50mL capacity bladder ​ Attached to the posteroinferior surface of the liver ​ Absorbs water and ions to concentrate bile up to ten-fold ○​ Junction of the right 9th costal cartilage and lateral border of the rectus abdominis B.​ HISTOLOGIC FEATURES ​ Attached to the posteroinferior surface of the liver ​ Mucosa (Epithelium + Lamina propria) ○​ Lies in a fossa between the right and left lobe of the liver ​ Mucosal folds Rokintansky-Aschoff sinuses ​ Parts: ○​ Unique to gallbladder ○​ Fundus ​ Muscularis externa ​ The rounded blind end located at the tip of the 9th costal ​ Serosa/Adventitia (dense CT + visceral mesothelium) cartilage at the midclavicular line and contacts the transverse ​ No lymphoid collections like in Peyer’s patches etc. colon ​ Pouch that is seen protruding underneath the surface of the 3 LAYERS liver ○​ Body 1. Mucosa ​ Rests on the upper part of the duodenum and transverse ​ lined with tall columnar epithelium and underlying basal colon lamina & lamina propria ○​ Neck →​The gallbladder has no muscularis mucosa or submucosal ​ Narrow part that gives rise to the cystic duct with spiral layer (unlike the GI tract) valves (Heister’s valves) ​ Mucosa highly folded and irregular ​ Valves prevent premature secretion of bile into the →​Different from small intestine which has villi with a more intestines regular appearance ​ Ejection of contents of the gall bladder into the cystic duct →​Gallbladder villi are unsymmetrical/haphazard (vs. intestines’ requires contraction of smooth muscle fibers of the regularly spaced out villi) and mucosa has no goblet cells gallbladder walls because they do NOT secrete mucus ​ Cholecystokinin relaxes the valves →​No glandular structures and pits (unlike the stomach) ​ Cystic artery: important structure to take note of when performing ​ Parts: a cholecystectomy →​Epithelium ▪​ Tall columnar epithelium with microvilli (absorptive epithelium) ▪​ Epithelium lining the biliary system does NOT contain mucus-producing cells →​Rokintansky-Aschoff sinuses OS 206 Histology of GI Glands 10 of 11  ▪​ Infoldings in mucosa that reach the muscle layer 3. Adventitia or Serosa (muscularis externa) ​ Dense connective tissue layer covered with visceral −​ Diverticula of the mucosa that are associated with mesothelium epithelial layer cholecystitis →​Adventitia - where it is attached to the liver ▪​ Not exposed to the visceral peritoneum ▪​ Where it is stuck on the interior surface →​Serosa - where it is free in the peritoneum C.​ BILE: ROLE, ROUTE, AND COMPOSITION ​ Hepatocytes secrete 800-1000mL of bile daily ​ Mostly water, bile salts, cholesterol, lecithin, bile pigments and several ions ​ Partially excretory product/ partially digestive secretion ​ Bilirubin – principal bile pigment ○​ Derived from heme of recycled RBCs ○​ Breakdown product stercobilin gives feces brown color ​ Bile salts play role in emulsification ○​ Also aid in absorption of lipids following digestion ADDITIONAL INFORMATION [2025 Trans] Cholecystokinin (CCK) Figure 63. Tall columnar epithelium and underlying basal lamina & lamina → Produced by the enteroendocrine cells of the intestine propria; w/ basally located nuclei and fine microvilli border → Controls the contraction of the GI → Causes the relaxation of the sphincter of Oddi 2. Muscularis Externa (Smooth Muscle Layer) → Stimulated by presence of ingested fats in the small ​ Layers of smooth muscle with irregular orientation intestine ​ Found right after epithelium ​ Random orientation of smooth muscle cells (fibers) ROUTE OF BILE FILLING ​ Muscularis propria NOT like the rest of GIT smooth muscle organisation, which is divided into muscularis mucosa (inside ​ Hepatocyte → bile canaliculi → bile ductules → bile ducts → right submucosa), muscularis externa (circular and longitudinal) and left hepatic ducts → common hepatic duct → cystic duct (storage) → common bile duct → pancreatic juice → descending portion of duodenum ○​ Since the sphincters of Boyden and Oddi do NOT relax, the bile makes its way into the gallbladder easily to be stored. ○​ NOT all bile makes it into the intestine ​ When someone has a gallstone, it caused obstructions because the space for bile to pass through to fill the gallbladder is so small ○​ This causes gallbladder distention ○​ When the gallbladder contracts it is painful and causes colicky abdominal pain Figure x. Route of bile until filling of the gallbladder V.​ REFERENCES AND CITATION ​ UPCM 2028 Trans. (February 15, 2024). Histology of GI Glands ​ UPCM 2025 Trans. (2021). Histology of GI Glands Figure 64. Muscularis externa of the gallbladder OS 206 Histology of GI Glands 11 of 11