Liver, Gallbladder, and Pancreas Microanatomy Notes PDF

Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 1 of 38 MICROANATOMY OF THE LIVER, GALLBLADDER, AND PANCREAS Lecture Learning Objectives: 1. Describe the microanatomy and functi...

Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 1 of 38 MICROANATOMY OF THE LIVER, GALLBLADDER, AND PANCREAS Lecture Learning Objectives: 1. Describe the microanatomy and functions of the liver. Describe the flow of blood through the liver. Describe the 3 lobular organizations (anatomic lobule, portal lobule, liver acinus) imposed on the histological arrangement of tissues in the liver and the functions they show best. Describe the histological organization of the hepatic cords and sinusoids, including the space of Disse, and identify them at the LM (H&E) and TEM levels. Describe the histological organization of the portal canal and identify the components at the LM level. Describe the hepatocyte at the LM (H&E) and TEM levels, including its relationship to the space of Disse and its formation of the bile canaliculus. Describe the role of the hepatocyte and its organelles in the production of plasma proteins, detoxification of lipid-soluble drugs and steroids, fat metabolism, storage and release of glycogen, and alcohol metabolism Describe bile production and the role of the hepatocyte in the process. Describe the morphology and function of the liver sinusoid (and its endothelium) at the LM and TEM levels, including its relationship to the Kupffer cell and hepatic stellate cell (lipocyte). Describe the flow of bile through the liver. Describe the production and flow of lymph through the liver. 2. Describe the microanatomy and function of the gallbladder. Describe the components of the layers of the gallbladder wall. Describe the function of the gallbladder and the cellular mechanism underlying this function. Describe the signals that cause the release of bile from the gallbladder. Compare and contrast the structure of the gallbladder and small intestine at the LM level. Describe the biliary system and the enterohepatic circulation. 3. Describe the microanatomy and functions of the pancreas. Describe the overall organization of the pancreas. For the exocrine pancreas, describe the secretory acini and duct system, the products produced, and their signals for release. For the endocrine pancreas, describe the secretory cell organization, types of cells, their relative frequency, and their function. Identify an islet of Langerhans at the LM level. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 2 of 38 Lecture Content Outline Liver: Basic concepts I. Liver A. Structure of the liver B. Lobular organization C. Hepatocytes D. Functions of hepatocytes E. Hepatic sinusoids F. Bile flow G. Lymphatic flow II. Gallbladder A. Characteristics B. Layers of the gallbladder wall C. Functions of the gallbladder D. Distinguishing gallbladder from small intestine E. Biliary system and the enterohepatic circulation II. Pancreas A. Organization B. Exocrine pancreas C. Endocrine pancreas (islets of Langerhans) MICROANATOMY OF THE LIVER, GALLBLADDER, AND PANCREAS LIVER: BASIC CONCEPTS A. DIGESTIVE SYSTEM: ASSOCIATED ORGANS 1. Extramural organs: liver and pancreas 2. Storage organ: gallbladder B. FUNCTIONS OF LIVER (see Section ID for details) 1. Has both endocrine and exocrine functions a. Endocrine functions: synthesis and release of products into blood, e.g., plasma proteins b. Exocrine function: production of bile 2. Other functions, e.g., detoxification of blood, storage of iron, etc. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 3 of 38 I. LIVER A. STRUCTURE OF THE LIVER (Figure 1) 1. Largest gland, incompletely divided into four lobes. 2. Parenchyma (Figure 2): Organized plates of hepatocytes (main liver cell Figure 1. Organization of liver. Modified from Graphic 15-2 in type), separated by rd Color Atlas of Histology (Gartner & Hiatt, 3 ed., 2000). sinusoids (discontinuous capillaries). 3. Stroma a. Fibrous stroma (Figure 2): Dense irregular connective tissue (Type Figure 2. LM of liver parenchyma and fibrous stroma. I collagen) forms a thin Capsule on left. From Meyer (1970). capsule, surrounds vessels and ducts, and provides some separation into lobules. b. Reticular stroma (Figure 3): reticular fibers provide supporting network for hepatocytes and Figure 3. LM section of liver stained with silver for reticular fibers. Note staining around vessels and hexagonal appearance of lobule. From Meyer (1970). sinusoids. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 4 of 38 4. Hilus: site of blood vessel entry and bile duct exit (Figures 1 and 4). 5. Blood supply (Figures 1 and 4) a. Portal vein i. Carries 75% of the Figure 4. Blood flow through the liver. Modified from Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). afferent blood. ii. Blood is rich in nutrients (as well as in potential toxins), but relatively poor in oxygen, coming from capillary beds of the gastrointestinal tract. iii. Part of the venous portal system connecting the gastrointestinal tract and liver. b. Hepatic artery i. Carries remaining 25% of afferent blood. ii. Blood is rich in oxygen, coming from the left ventricle of the heart. 6. Blood flow (Figure 4) Portal vein and hepatic artery → Interlobular branches → Terminal branches → Sinusoids → Central vein → Sublobular veins → Hepatic veins → Inferior vena cava Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 5 of 38 7. Bile flow a. As noted already, the liver also produces bile, which represents its exocrine function. b. Bile flow will be described in detail in Section IF. For now note that bile is produced by the major cells in the liver (hepatocytes) and flows out of the liver via a duct system that follows the vascular tree, although bile flows in the opposite direction as blood, i.e., toward the hilus (Figure 4). Figure 5. Schematic of three ways to interpret liver structure and function (3 lobule types). Modified from Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). B. LOBULAR ORGANIZATION - three ways to conceptualize and interpret liver structure and function (Figure 5). 1. Anatomic or classic lobule a. Microscopically, the liver is organized into roughly hexagonal lobules containing a radial array of plates of Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 6 of 38 hepatocytes (Figures 1, 5, and 6). i. The hexagonal lobule, based on microscopic structure, is called an “anatomic” lobule or “classic” lobule (Figures 5 and 7). ii. Each anatomic (classic) lobule is centered around a central vein and bordered at each corner by a portal canal (portal triad + connective tissue). (Figures 5, 6, and 7) b. Portal canals (portal triad + connective tissue) i. Found at corners of lobule (Figures 6 and 7). ii. Portal triad consists of interlobular branches of portal vein, hepatic artery, and bile duct (Fig. 8). iii. Portal triad is surrounded by connective tissue of the portal canal. Figure 6. LM section of human liver showing Figure 7. Schematic of a classic lobule. Modified from Human Histology: boundaries of lobule. Usually they are not this A Microfiche Atlas (Erlandsen & Magney, 1985). clearly defined. See Figure 3 for another example. Figure 18.4b from Histology: A Text and th Atlas (Ross & Pawlina, 6 ed., 2011). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 7 of 38 c. Terminal branches of portal veins and hepatic arteries run between lobules (sides of hexagon, Figures 7 and 8) and empty blood into sinusoids, where the Figure 8. LM of portal triad, showing branch of portal vein (center, large vessel), branch of hepatic artery (just left of vein), and branches of bile duct (far left). Triad is surrounded blood is mixed by connective tissue of portal canal. Terminal branch of portal vein.visible in upper left corner. From Meyer (1970). (Figures 7 and 9). d. Sinusoids run between plates of liver cells or hepatocytes (Figure 9), carrying blood to central vein Figure 9. LM of hepatocytes lined by sinusoids. Center: (terminal hepatic Kupffer cell within sinusoid. From Meyer (1970). venule) (Figures 5, 7, 10). e. Bile flows in opposite direction of blood: through intercellular Figure 10. LM of central vein in liver. From Human Histology: canaliculi to canals A Microfiche Atlas (Erlandsen & Magney, 1985). of Hering to interlobular bile ducts (Figure 7, see Section IF). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 8 of 38 2. Portal lobule (Figures 5 and 11). a. Triangle-shaped lobule that centers on a bile ductule and emphasizes the exocrine function of the liver. b. Contains parts of 3 adjacent anatomic Figure 11. Schematic of a portal lobule. Modified from Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). lobules. c. Lobule includes any hepatocytes that supply bile to a particular bile ductule in the portal canal. 3. Liver acinus (functional lobule; Rappaport's lobule) (Figures 5 and 12). a. A small, roughly diamond-shaped mass of unencapsulated hepatocytes that emphasizes the Figure 12. Top: Schematic of a liver acinus (functional lobule). From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). Bottom: Zones of liver delivery of blood to acinus. Modified from Figure 18.6 from Histology: A Text th and Atlas (Ross & Pawlina, 6 ed., 2011). hepatocytes. b. Organized around an axis containing the terminal branches of the portal vein and hepatic artery (Figure 12). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 9 of 38 c. Lobule includes any hepatocytes that are supplied with blood by a particular set of terminal branches running between two classic lobules. d. The liver acinus is important for understanding the exposure of hepatocytes to oxygen as well as blood-borne toxins, e.g., this lobule is used in toxicology. e. The liver acinus can be divided into three zones (Figure 12, bottom): i. Zone 1: cells in the center of this lobule (zone 1) are the first to receive oxygen, nutrients, and toxins. ii. Zone 2: intermediate zone. iii. Zone 3: cells adjacent to central veins, are the last to be affected by incoming toxins (lower levels), but are also exposed to relatively low oxygen levels (hypoxia). C. HEPATOCYTES 1. Large, polyhedral cells with round, centrally located nuclei (Figure 9). 2. Cells are often binucleate and polyploid. 3. Cells border on sinusoids with intervening space of Disse into which microvilli project (Figures 13 and 14). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 10 of 38 Figure 13. Diagram of hepatocytes and their relationship to sinusoids and other cells found th in the liver. Figure 18.11 from Histology: A Text and Atlas (Ross & Pawlina, 6 ed., 2011). 4. Cells also border neighboring hepatocytes with bile canaliculi (Figures 13 and 14) a. Channels that are formed by the plasma membranes of adjacent hepatocytes and transport bile. b. Sealed off by junctional complexes Figure 14. TEM of hepatocytes and surrounding liver with zonula (hepatic) sinusoids in the liver. H: hepatocyte, D: space of Disse, BC: bile canaliculus, S: sinusoidal endothelial cell, L: occludens (Figure 13; lipid droplets, E: erythrocytes in sinusoids. Figure 15.12a in th Wheater’s Functional Histology (Young, et al., 6 ed., see Section IF). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 11 of 38 5. Hepatocytes are characterized by well- developed rER, sER, Golgi, lysosomes, and peroxisomes (Figure 13). They also usually contain glycogen inclusions Figure 15. TEM of hepatocyte. Note bile canaliculi at 3 and (Figures 13 and 15). 10 o’clock. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). 6. Hepatocytes can regenerate, and the liver has the capacity to regenerate after injury and to adjust its size to match its host. D. FUNCTIONS OF HEPATOCYTES 1. Synthesis of plasma proteins (in rER), including albumin, fibrinogen, and coagulation factors. They are released into the space of Disse and from there flow into the sinusoid. 2. Detoxification of lipid-soluble drugs and steroids in smooth ER by transforming nonpolar compounds into polar metabolites, facilitating their elimination by the kidneys. 3. Fat metabolism: This will be covered in more detail in the Lipoproteins lecture. a. Chylomicrons (see Microanatomy of the Intestines and Digestion and Absorption lectures) undergo lipolysis in the circulation, thereby delivering fatty acids to tissues. b. Chylomicron remnants are then taken up by the hepatocytes and metabolized. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 12 of 38 c. Resultant fatty acids are esterified to triglycerides by sER. i. Some triglycerides are stored in fat droplets. ii. Most are coated with a mixture of proteins, cholesterol, and phospholipid and are released into the plasma as VLDL (very low density lipid) particles. 4. Glycogen synthesis, storage, and release (sER and cytosol). 5. Storage of iron as ferritin or hemosiderin granules. 6. Metabolism of alcohol (ethanol): This will be covered in more detail in the Metabolic Energy Balance lecture. a. Metabolism of alcohol usually occurs through the alcohol dehydrogenase (ADH) pathway, producing acetaldehyde and then acetate. b. However, chronic intake of alcohol activates the microsomal ethanol-oxidizing system (MEOS). i. The MEOS produces acetaldehyde and an excess of oxygen radicals. ii. Reactive oxygen damages hepatocyte plasma membranes. 7. Participate in metabolism of vitamins A, D, and E. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 13 of 38 8. Conjugation of bilirubin (Figure 16) a. Hepatocytes also carry out glucuronide conjugation of bilirubin, which is a water insoluble breakdown product of the heme portion of hemoglobin. This unconjugated bilirubin is sometimes called “indirect bilirubin”. Figure 16. Schematic showing production of bilirubin. Modified from Figure 8.38 in Physiology (Costanzo, 6th ed., 2018). b. Cells of the reticuloendothelial system, e.g., macrophages and Kupffer cells, degrade hemoglobin, yielding water- insoluble bilirubin (indirect bilirubin), which is carried in the blood bound to albumin. c. The liver extracts bilirubin from blood and conjugates it with glucuronic acid in the SER to form bilirubin glucuronide (conjugated bilirubin), which is secreted into bile and excreted in urine. i. Reaction is catalyzed by UDP glucuronosyl- transferase. ii. Bilirubin glucuronide (conjugated bilirubin) is water soluble and is sometimes called “direct bilirubin”. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 14 of 38 9. Production of bile (Figure 17): This will be covered in more detail in the Sterol Biosynthesis lecture. a. Synthesis of bile components i. Hepatocytes synthesize cholesterol, phospholipids, Figure 17. Diagram of hepatocytes and sinusoids, and bile salts showing the formation of bile. Modifed from Figure 18.11 from Histology: A Text and Atlas (Ross & Pawlina, th 6 ed., 2011). in the smooth ER. The critical property of bile salts is that they are amphipathic (both hydrophilic and hydrophobic). Bile salts solubilize or emulsify dietary lipids. ii. Unused bile salts that were reabsorbed by enterocytes in the ileum and returned to the liver via the portal vein (see Microanatomy of the Microanatomy of the Intestines and Digestion and Absorption lectures) are also added to bile. iii. Bilirubin glucuronide is also secreted in bile and accounts for bile’s yellow color. iv. The constituents of bile are bile salts (50%), bile pigments such as bilirubin (2%), cholesterol (4%), and phospholipids (40%). Bile also contains electrolytes, any metabolites of drugs and heavy metals, and water. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 15 of 38 b. Hepatocytes release bile into bile canaliculi. c. Canalicular plasma membrane contains specific ATP- dependent transporters that mediate bile secretion. d. Bile secretion is continuous, with the rate dependent primarily on blood flow to the liver. E. HEPATIC SINUSOIDS (Figures 9 and 13) 1. Discontinuous capillaries that lack a continuous basal lamina 2. Two cell types form the lining of the sinusoid a. Discontinuous endothelium (endothelial cells with large gaps) (Figures 18, 20, and 21). b. Kupffer cells i. These resident phagocytic cells are sinusoidal macrophages, derived from monocytes (Figures 9, 18, 19, and 20). ii. They may span the sinusoidal lumen (Figures 19 and 20). iii. They function in the breakdown of aged RBCs (Figure 20, lower right). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 16 of 38 Figure 18. TEM of liver sinusoid, showing endothelial cell at bottom center and Kupffer cell at left. Space of Disse lies between these cells and hepatocytes. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). Figure 19. TEM of Kupffer cell in liver sinusoid. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). Figure 21. Low (top) and high (bottom) magnification TEM images of liver sinusioid. S: Sinusoid lining cells (endothelial cells), D: space of Disse, H: hepatocytes, E: erythrocytes, Mv: microvilli. Figure 15.12 in Wheater’s th Functional Histology (Young et al., 6 ed., 2014) Figure 20. SEM of liver sinusoid, showing sinusoidal endothelium (S) with discontinuities and microvilli (Mv) of hepatocytes (Hc). KC: Kupffer cell. Modified from Tissues and Organs (Kessel and Kardon, 1979). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 17 of 38 3. Space of Disse a. Perisinusoidal space that surrounds the sinusoid. b. Space of Disse lies between plasma membrane of hepatocytes and basal surfaces of endothelial cells and Kupffer cells (Figures 13, 18, 19, and 21). c. Space of Disse contains microvillous projections of hepatocytes and some reticular fibers (Figure 21). d. Hepatic stellate cells (lipocytes, cells of Ito) are located within the space of Disse (Figure 22). i. Normally store lipids and Figure 22. TEM of hepatic stellate cell (lipocyte) in liver. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). vitamin A, and produce reticular fibers (type III collagen). ii. During liver cirrhosis (fibrosis), stellate cells undergo trans-differentiation to myofibroblast-like cells and secrete large amounts of type I collagen and extracellular matrix into the space of Disse. Continued on next page. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 18 of 38 F. BILE FLOW 1. Bile is produced and secreted by hepatocytes into bile canaliculi: plasma membrane-bound intercellular channels sealed by tight junctions (Figures 23 and 24). 2. Bile flows through the canaliculi, which course between hepatocytes within the lobule (Figures 23, 24, 25, 26, and 27). Figure 23. TEM of hepatocytes showing hepatic sinusoids (HS) and bile canaliculi (arrows). Note two nuclei (*) in hepatocyte at center. Modified from 3. Near the outer edge of the Netter’s Essential Histology, p. 319 (Ovalle and st Nahirney, 1 ed., 2008). lobule, the bile is collected into intralobular bile ductules (canals of Hering), lined by simple low cuboidal epithelium (Figure 26). 4. The bile is then collected into interlobular bile ductules within Figure 24. TEM of bile canaliculus between two the portal canal (Figure 26). hepatocytes. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). They are lined by simple cuboidal epithelium. 5. As they collect bile from multiple ductules and their size increases, the bile ducts become lined by columnar epithelium, and they are surrounded by increasing amounts of connective tissue. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 19 of 38 Figure 27. LM of bile canaliculi stained for ATPase. From Meyer (1970). Figure 25. LM of hepatocytes showing bile canaliculi (arrowheads and arrows). Figure 18.14 from Histology: th A Text and Atlas (Ross & Pawlina, 6 ed., 2011). Figure 28 LM of portal canal with large portal vein (top center) and branches of hepatic artery and bile ductule in lower right. Bottom center: longitudinal cut through canal of Hering. Right center: triangular lymphatic vessel. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). Figure 29. LM of intrahepatic bile duct wall. From Meyer (1970). Figure 26. SEM of liver, showing bile canaliculi (BC). Mv: microvilli, Hc: hepatocytes, RF: reticular fibers, EL: endothelium, S: sinusoid. From Tissues and Organs (Kessel and Kardon, 1979). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 20 of 38 6. These ducts are drained by larger intrahepatic bile ducts. a. They are lined by columnar epithelium and surrounded by an increasing amount of dense fibroelastic connective tissue (Figure 29). b. As the hilus is approached, some smooth muscle appears in the connective tissue as well. 7. Bile is then collected into the right and left hepatic ducts (Figure 30). 8. Finally it flows into the common hepatic duct, formed at the hilum (porta) (Figure 30). Wall of common hepatic duct contains four layers (mucosa, submucosa, muscularis, and Figure 30. Relationship of liver, gall bladder, pancreas, adventitia). and duodenum. Modified from Figure 18.15 in Histology: h A Text and Atlas (Ross & Pawlina, 6 ed., 2011). G. LYMPHATIC FLOW 1. Most lymph forms from the space of Disse. 2. Lymph drains from the space of Disse to the periportal space of Mall, which lies between the portal connective tissue and the hepatocytes at the border of the portal canal. 3. Lymph then drains to lymphatic vessels within portal canals (Figure 28). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 21 of 38 4. Lymph then flows to larger vessels that parallel the bile passageways and leaves the liver at the hilus. 5. Note: As noted earlier, most of the material in the space of Disse goes into the lumen of the sinusoid, but some fluid percolates out into the periportal space of Mall and exits the liver as lymph. II. GALLBLADDER A. CHARACTERISTICS 1. Distensible bag connected to common bile duct via cystic duct (Figure 30). Figure 31. Low mag LM of gall bladder mucosa. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). 2. It can concentrate and store up to 50 ml of bile. 3. Histologic appearance a. Filled gallbladder has a smooth inner surface. b. Empty gallbladder is thrown into numerous folds or rugae (Figures 31 and 32). Figure 32. Gall bladder wall, showing mucosa and muscularis externa. Note Rokitansky- Aschoff crypts (sinuses). Modified from Figure 18.18 from Histology: A Text and Atlas (Ross et th al., 6 ed., 2011). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 22 of 38 B. LAYERS OF THE GALLBLADDER WALL (Figure 32) 1. Mucosa (Figures 31 and 32) a. Simple columnar epithelium with apical microvilli and junctional complexes (Figure 33). i. Microvilli not as well developed as Figure 33. High magnification LM of gall bladder those on enterocytes. epithelium and lamina propria. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). ii. Cells have lateral interdigitations with transport ATPase in membranes for pumping of ions (Na+, Cl-, HCO3-) and water absorption. b. Surface epithelium may be invaginated as deeply as the muscular layer forming diverticulae or Rokitansky- Aschoff crypts (Figures 31 and 32). c. Lamina propria: Loose connective tissue that is richly vascularized with fenestrated capillaries (Figure 33). 2. No submucosa. 3. Muscularis externa: multiple layers of smooth muscle, separated by networks of elastic fibers (Figure 32). 4. Serosa or adventitia: broad layer of connective tissue containing blood vessels, nerves, and lymphatics. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 23 of 38 C. FUNCTIONS OF THE GALLBLADDER 1. Stores bile. 2. Concentrates bile by absorption of water from lumen. a. This is transepithelial transport of an isotonic fluid from the lumen to the vasculature. i. Ions, e.g., Na+, are pumped into intercellular lateral spaces and water follows. ii. This is the same mechanism used by enterocytes in the large intestines (see Microanatomy of the Intestines lecture). b. Intercellular spaces become distended in the active gallbladder (Figure 34). Figure 34. TEM of gall bladder epithelium before (left) and during (right) active fluid transport. Note distended lateral intercellular spaces on right. From Human D. DISTINGUISHING Histology: A Microfiche Atlas (Erlandsen & Magney, 1985). GALLBLADDER FROM SMALL INTESTINE 1. Histologically, gallbladder is often confused with small intestine, since the highly folded mucosa of the gallbladder can make it look like villi are present. 2. The first major difference is in the lamina propria (compare Figures 32 and 35). a. Lamina propria of gallbladder: No mucosal glands Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 24 of 38 b. Lamina propria of small intestines: Mucosal glands are evident deep Figure 35 (Figure 2 in Microanatomy of the Intestines lecture). Layers of the wall of the small intestine (jejunum here). Note large submucosal folds (plicae circulares, one labeled PC) and finger-like in lamina mucosal projections, the villi (V). Muc: mucosa; SubM: submucosa; ME: muscularis externa; L: central lacteal (lymphatic vessel). Note propria. crypts of Lieberkuhn in mucosa, just above submucosa. Plate 60 th from Ross & Pawlina, 6 ed. (2011). These are intestinal glands (crypts of Lieberkühn). 3. The second major difference is in the epithelium (compare Figures 33 and 36). a. Gallbladder epithelium i. No goblet cells Figure 36 (Figure 9 in Microanatomy of the Intestines lecture). LM of epithelium of two neighboring villi. Note ii. Microvilli not pale-staining goblet cells and striated border of microvilli on enterocytes. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). well developed b. Small intestinal epithelium i. Contains goblet cells ii. Has a distinct striated border (well-developed microvilli) Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 25 of 38 E. BILIARY SYSTEM AND THE ENTEROHEPATIC CIRCULATION 1. The components of the biliary system are the liver, gallbladder, bile duct, duodenum, ileum, and portal circulation (Figure 37). 2. The enterohepatic circulation refers to the circulation of bile between the intestines and the liver. a. The hepatocytes Figure 37. Schematic of the liver, large ducts, and continuously gallbladder of the biliary system, showing connection to the duodenum via the ampulla (of Vater). Figure 46-4B from rd Medical Physiology (Boron & Boulpaep, 3 ed., 2017). synthesize and secrete the constituents of bile into bile canaliculi and then into the intrahepatic bile ducts (Figure 38, Step 1). Figure 38. Schematic showing secretion and enterohepatic circulation of bile salts Light blue arrows show the path of bile flow; yellow arrows show the movement of ions and water. CCK, Cholecystokinin.. Figure 8.12 in Physiology (Costanzo, 6th ed., 2018). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 26 of 38 b. Bile flows out of the liver through the bile ducts and is delivered to the duodenum or stored in the gallbladder, where the bile is concentrated (Figure 38, Step 2). c. When chyme reaches the small intestine, CCK is secreted from enteroendocrine cells (I cells) in the duodenum. CCK has two separate but coordinated actions on the biliary system causing stored bile to flow from the gallbladder into the lumen of the duodenum (Figure 38, Step 3): i. CCK stimulates contraction of the gallbladder. ii. CCK causes relaxation of the sphincter of Oddi. d. Fate of bile salts (bile acids) i. In the small intestine, the bile salts emulsify and solubilize dietary lipids. ii. Unused bile salts are reabsorbed by enterocytes in the ileum and enter the portal vein (portal circulation) (Figure 38, Step 4). iii. The recovered bile salts enter the liver via the portal vein (Figure 38, Step 5), and hepatocytes take up the bile salts from the blood in the liver sinusoids. iv. The bile salts are reconjugated within the hepatocyte, and they are then secreted with fresh bile into the bile canaliculi. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 27 of 38 e. Fate of conjugated bilirubin (bilirubin glucuronide) (Figure 39) i. As noted earlier, bilirubin, a yellow- colored byproduct of hemoglobin metabolism, is the major bile pigment. Figure 39. Schematic showing production and fate of bilirubin. Figure 8.38 in Physiology (Costanzo, 6th ed., ii. Bilirubin 2018). travels in blood bound to albumin. iii. Hepatocytes take up the bilirubin and form water soluble conjugated bilirubin (bilirubin glucuronide, direct bilirubin) within the smooth ER. iv. The conjugated bilirubin is secreted as part of bile into a bile canaliculus and eventually exits the liver via the common hepatic duct. v. Bilirubin glucuronide enters the duodenum as a component of bile and is converted back to bilirubin, which is then converted to urobilinogen by the action of intestinal bacteria. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 28 of 38 vi. A portion of the urobilinogen is recirculated to the liver, a portion is excreted in the urine, and a portion is oxidized to urobilin and stercobilin, the compounds that give feces its dark color. f. More details about the types of bile salts (acids) and the enterohepatic circulation will be provided in the Sterol Biosynthesis lecture. III. PANCREAS (Figure 37) A. ORGANIZATION 1. Large organ without a distinct capsule. Delicate connective tissue divides lobules. 2. Carries out both exocrine and endocrine functions. a. Exocrine function carried out by acinar cells. Figure 37. Schematic of large ducts and gallbladder of the biliary system, showing connection to the duodenum via the ampulla (of Vater). Modified from Figure 18.15 in Histology: A Text and Atlas (Ross & b. Endocrine function h Pawlina, 6 ed., 2011). performed by cells in islets of Langerhans. 3. Exocrine pancreas constitutes approximately 90% of the pancreas. The rest of the pancreatic tissue is the endocrine pancreas (2%), blood vessels, connective tissue (stroma), and interstitial fluid (Figure 40). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 29 of 38 Figure 40. LM of pancreas. Note pale-staining islet of Figure 41. LM of pancreas, showing islet of Langerhans on right among darker staining acini of Langerhans at right. Darkly stained cells are acinar exocrine pancreas. From Meyer (1970). cells of exocrine pancreas. From Meyer (1970). B. EXOCRINE PANCREAS 1. Compound tubuloalveolar exocrine gland with serous alveoli (Figure 41). 2. The exocrine pancreas secretes approximately 1 L of fluid per day into the lumen of the duodenum. 3. The secretion consists of: a. An aqueous component that is high in HCO3− i. Along with secretion from Brunner’s glands, HCO3- functions to neutralize the H+ in the chyme delivered to the duodenum from the pyloric stomach (antrum). ii. Produced by intercalated ducts in exocrine pancreas. b. An enzymatic component i. Mixture of active enzymes and inactive proenzymes (zymogens). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 30 of 38 ii. The enzymatic component functions to digest carbohydrates, proteins, and lipids into absorbable molecules (see Digestion and Absorption lecture). iii. Produced by pancreatic acinar cells. Figure 42. LM (H&E stain) showing pancreatic acinar cells Figure 43. TEM of pancreatic acinar cells. 4: RER, 5: with basophilic basal staining and acidophilic apical mitochondria, 6: Golgi, 7: prezymogen granules, 8: staining. Virtual slide OkSt006. zymogen granules, 11: acinar lumen. From Histology: A Text and Atlas (Rhodin, 1974). 4. Pancreatic acinar cells (Figures 42 and 43) a. Pyramidal cells that form a spherical acinus surrounding a lumen. b. Cellular characteristics (Figures 42 and 43) i. Distinct basophilia in basal (outer) cytoplasm (Figure 42), due to presence of abundant rER in region (Figure 43). ii. In apical cytoplasm there are many acidophilic zymogen granules, each containing a mixture of different pancreatic enzyme and proenzymes (zymogens) (Figures 42 and 43). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 31 of 38 c. Pancreatic enzymes i. When activated, pancreatic enzymes can hydrolyze most food substances. ii. The pancreatic enzymes are synthesized on the rough endoplasmic reticulum of the acinar cells. iii. The enzymes are stored in the zymogen granules, which contain a mixture of different proenzymes, until a stimulus (e.g., parasympathetic activity or CCK) triggers their secretion. iv. Pancreatic amylase and lipases are secreted as active enzymes. v. Five major pancreatic proteases are secreted as inactive precursors (zymogens): trypsinogen, chymotrypsinogen, procarboxypeptidase A, procarboxypeptidase B, and proelastase. vi. Trypsinogen is activated by enterokinase in the glycocalyx of enterocytes to trypsin which, in turn, activates the other zymogen (inactive) enzymes (see Microanatomy of the Intestines and Digestion and Absorption lectures). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 32 of 38 Figure 44. Duct system of exocrine pancreas. In some texts, large intercalated ducts are referred to as “intralobular collecting ducts”. We will call all intralobular ducts “intercalated ducts”. Note there are no striated ducts in the pancreas. Modified from Figure 16-9A from st Atlas of Histology (Cui, 1 ed., 2011). 5. Duct system (Figure 44) a. Intercalated ducts drain the acini (Figures 44 and 45). i. Intercalated ducts have a low simple cuboidal epithelium. Figure 45. LM of pancreatic acinus with centroacinar cells and intercalated duct. Figure 18.20a from th Histology: A Text and Atlas (Ross & Pawlina, 6 ed., 2011). ii. They extend into acini as centroacinar cells, which stain lightly (Figures 44, 45, 46, 47). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 33 of 38 iii. Intercalated ducts produce a bicarbonate-rich watery secretion. Figure 46. TEM of centroacinar cells (5) in Figure 47. Section through pancreas showing pancreatic acini, intercalated pancreatic acinus. , 1: acinar lumen, 3: ducts, and endocrine pancreas (islets of Langerhans). Centroacinar cells acinar cells, 4: zymogen granules. From often appear as lightly stained cells in the center of the acinus. Modified st Histology: A Text and Atlas (Rhodin, 1974). from Figure 16-9A from Atlas of Histology (Cui, et al., 1 ed., 2011). b. The small intercalated ducts deliver the secretion to larger intercalated ducts. i. They have simple cuboidal epithelium and are located within the lobule (Figure 47). ii. In some texts, these large intercalated ducts are called “intralobular collecting ducts”. iii. For our purposes, all intralobular ducts in the pancreas will be called “intercalated ducts”. c. Interlobular ducts collect product from the intralobular ducts. They have simple columnar epithelium and are located in connective tissue septae (Figure 48). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 34 of 38 d. The interlobular ducts merge and deliver product to the main duct. 6. Control of exocrine pancreas (Figures 49 and 50). Figure 48. LM showing pancreatic acini, intercalated duct (upper left), and interlobular ducts surrounded by connective tissue (center). Virtual slide OkSt006. a. Acinar cells are stimulated to secrete proenzymes by: i. Cholecystokinin (CCK) from enteroendocrine cells (I cells) in duodenum. ii. Acetylcholine from Figure 49. Control of exocrine pancreas. Modified from Fig 17- parasympathetic nd 9 in Histology and Cell Biology (Kierszenbaum, 2 ed., 2007). post-ganglionic nerves (Figure 49). Parasympathetic preganglionic vagal nerves activate parasympathetic post-ganglionic neurons in the pancreatic ganglia. b. Duct cells are stimulated by secretin released from enteroendocrine cells (S cells) in duodenum to increase bicarbonate release into the ductal lumen. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 35 of 38 c. Release of CCK and secretin triggered by entrance of acidic chime into duodenum. d. Recall that CCK also stimulates contraction of the gallbladder to release bile (Figure 50) and causes relaxation of the sphincter of Oddi. Figure 50. Control of gallbladder and pancreas by enteroendocrine cells in intestines. Modified from Fig 16- 10 in Histology and Cell Biology st (Kierszenbaum, 1 ed., 2002). C. ENDOCRINE PANCREAS (ISLETS OF LANGERHANS) 1. Structure of endocrine pancreas a. Clumps or islands of endocrine cells, distributed among the acini of the exocrine pancreas. b. Clumps (islets of Langerhans) surrounded by delicate connective tissue and supplied by fenestrated capillaries. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 36 of 38 2. In H&E-stained sections, islets of Langerhans appear as pale-staining (light pink) clumps among darker staining acini (Figures 40, 41, and 47). 3. Cell types: Distinguished by special stains at the LM level (Figure 51) and by granule morphology at EM level (Figure 52). Hormone products can be identified by immunocytochemical techniques. Figure 51. LM of aldehyde fuchsin-stained pancreas. Note specific staining of insulin within β- a. Beta (β) cells: 60-70% of cells in islet of Langerhans. From Human Histology: A Microfiche Atlas (Erlandsen & Magney, cell population of islets; concentrated in the center of the islets. i. Secrete the hormone insulin, which is required for the uptake of Figure 52. TEM of islet of Langerhans. A: α-cell glucose by muscle (produces glucagon), B: β-cell (produces insulin), δ-cell (produces somatostatin), PP: PP-cell (produces and adipose cells. pancreatic polypeptide). Modified from Human Histology: A Microfiche Atlas (Erlandsen & Magney, ii. Release of insulin is triggered by action of glucose on β cells. iii. Insulin also stimulates glycogen synthesis in liver hepatocytes. iv. Diabetes results from a destruction of beta cells (Type 1) or a deficiency in insulin receptor mechanisms (Type 2). Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 37 of 38 b. Alpha cells: 15% - 20% of cells i. Secrete the hormone.glucagon. ii. Glucagon release is stimulated by low blood glucose. iii. Glucagon stimulates the metabolic breakdown of glycogen in the liver. c. Delta (δ or D) cells: 5% - 10% of cells i. Secrete somatostatin, which inhibits release of other pancreatic hormones. ii. Also secrete some gastrin. d. PP cells (F cells): 3% - 5% of cells; produce pancreatic polypeptide, which self-regulates pancreatic secretions. References: Costanzo, L.S. Physiology, 6th ed., Elsevier: Philadelphia, 2018, Ch. 8. Cui, D., Daley, W., Fratkin, J.D., Haines, D.E., and Lynch, J.C. Atlas of Histology: With Functional and Clinical Correlations, 1st ed., Lippincott, Williams and Wilkins: Baltimore, 2011, Ch. 16. Erlandsen, S.L. and Magney, J.E., Human Histology: A Microfiche Atlas, University of Minnesota Press: Minneapolis, 1985. Gartner, L.P. and Hiatt, J.L., Color Atlas of Histology, 3rd ed., Lippincott Williams & Wilkins: Philadelphia, 2000, Ch. 15. Microanatomy of the Liver, Gallbladder, and Pancreas Rod D. Braun Page 38 of 38 Kessel, R.G. and Kardon, R.H., Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979. Kierszenbaum, A.L., Histology and Cell Biology: An Introduction to Pathology, 1st ed., Mosby: St. Louis, 2002, Ch. 16. Kierszenbaum, A.L., Histology and Cell Biology: An Introduction to Pathology, 2nd ed., Mosby: St. Louis, 2007, Ch. 16, 17, and 19. Meyer, D.L., Unpublished histology slide atlas, Department of Anatomy, Wayne State University School of Medicine, Detroit, 1970. Ovalle, W.K. and Nahirney, P.C., Netter’s Essential Histology, 1st ed., Saunders Elsevier: Philadelphia, 2008. Rhodin, J.A.G., Histology: A Text and Atlas, Oxford University Press: New York, 1974. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 6th ed., Lippincott Williams & Wilkins: Philadelphia, 2011, Ch. 18. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 7th ed., Lippincott, Williams, & Wilkins, WoltersKluwer Health: Philadelphia, 2016, Ch. 18. (major source) Young and Heath, Wheater’s Functional Histology: A Text and Colour Atlas, 4th ed., Churchill Livingstone: Edinburgh, 2000, Ch. 15. Young, B., O’Dowd, G., and Woodford, P. Wheater’s Functional Histology, 6th ed., Elsevier- Churchill-Livingstone: Philadelphia, 2014, Ch. 15. RDB: 10/3/2022