Digestive system III: , , And Liver Introduction - The embryology , gross morphology , and histology of the normal human liver ”the single largest organ in the human body ”are described in this chapter . - In many instances, immunohistologic studies of liver tissue have the potential to yield more information than electron microscopy. Embryology - The liver arises as the hepatic diverticulum from the endodermal lining of the most distal portion of the foregut during the 3 to 5 week of gestation. - In embryos 4 to 5 mm in length. - The hepatic diverticulum differentiates cranially into proliferating hepatic cords and caudally into the gallbladder and extrahepatic bile ducts. - The anastomosing cords of hepatic epithelial cells grow into the mesenchyme of the septum transversum. - As the hepatic cords extend outward during the 5 week of gestation, they are interpenetrated by the inwardly growing capillary plexus, which arises from the vitelline veins in the outer margins of the septum transversum and forms the primitive hepatic sinusoids. - Scattered mesenchymal cells lie between the endothelial wall of the sinusoids and the hepatic cords and form the hepatic stroma, as well as the capsule. - Hematopoietic tissue and Kupffer cells also derive from splanchnic mesenchyme, begins during the 6 week. - By 9 weeks' gestation accounts for approximately 10% of the total weight of the embryo. - The bile canaliculi appear in the 10-mm embryo (9weeks) as intercellular spaces between immature (hepatoblasts). - The epithelium of the intrahepatic bile ducts arises from the proximal part of the primitive hepatic cords. - the epithelial layer in direct contact with the mesenchyme around the portal vein transforms into “type cells. - The canals of Hering, which connect bile canaliculi to bile ducts, include both typical hepatocytes and bile duct cells. - bile formation by the 11 week. - Excretion of bile into the duodenum by the 16 week. * The liver accounts for approximately 5% of the newborn 's body weight. * Apoptosis - Apoptosis occurs at all stages during fetal growth and development of ductal plates and hepatoblasts. - There is a good correlation between the proliferative and the apoptotic activities in the ductal plate, depending on the remodeling process. - Apoptosis, is induced by transforming growth factor-²1

Ductal plate ( arrows ) developing around the portal vein mesenchyme in the liver of a 10- week-old embryo. There is extramedullary hematopoiesis in the sinusoids. Gross Morphology Liver is the largest gland in the body, weighing about 1500 gr , it has endocrine and exocrine functions It is relatively larger in infancy, due to a large left lobe. - The liver resides in the abdominal right upper quadrant . - It measures about 10 cm in vertical span, 12 to 15 cm in thickness , and 15 to 20 cm in its greatest transverse diameter. - It is divided by deep grooves into two large lobes ”the right (lateral to falciform ligament) and left (medial to falciform ligament) ”and two smaller lobes ”the caudate and quadrate lobes. - Functionally, the division into eight segments. . Each segment is served by its own vascular pedicle of arterial and portal venous blood supply and branch of the biliary tree. - The superior , anterior, and lateral surfaces of the liver are smooth and almost completely covered by peritoneum. - A thin layer of fibrous connective tissue, the Glisson's capsule, surrounds the liver and extends into the parenchyma to form extensions that support arterial and biliary structures, except at where blood and lymph vessels and bile ducts enter or exit the gland - Liver has a lobular organization - Classical lobule in which hepatocytes arranged as an hexagon - Receives portal blood (75%) from small intestine via portal vein and oxygenated blood from hepatic arteries (25%). - Anteriorly, the falciform and round ligaments, connect the liver to the abdominal wall. - Inferior vena cava, to which two to four hepatic veins connect. - The fossa of the gallbladder and the round ligament separate the quadrate lobe from the right and left liver lobes, respectively. Anatomic structure of the liver. This diagram shows the gross view of the diaphragmatic and visceral surfaces of the liver, with labeled anatomic landmarks found on both surfaces. The enlarged cross- sectional area of the liver (bottom) shows the general microscopic organization of the liver into lobules. Note the presence of hepatic portal triads at the periphery of each lobule, with the terminal hepatic venule (central vein) in the center of the lobule. Glisson' capsule The major functions of the liver may be summarised as follows: metabolism - Oxidising triglycerides to produce energy - Synthesis of plasma lipoproteins - Synthesis of cholesterol and phospholipid Carbohydrate metabolism. - Converting carbohydrates and proteins into fatty acids and triglyceride. - Regulation of blood glucose concentration by .glycogenesis, glycogenolysis and gluconeogenesis Protein metabolism. - Synthesis of plasma proteins, including albumin and clotting factors (e.g. prothrombin and fibrinogen), transporter proteins (e.g. haptoglobin, transferrin, and hemopexin), globulins. - Synthesis of non-essential amino acids. Detoxification of metabolic waste products (e.g. deamination of amino acids and production of urea). Phagocytosis - Kupffer cells phagocytize worn-out and dying red and white blood cells, some bacteria Storage - Storage of glycogen, vitamins (A,D & K) , iron, copper, folic acid. Intermediary metabolism - Detoxification of various drugs and toxins (e.g. alcohol). Secretion - Synthesis and secretion of bile ( 600-1000 mL/day.) . Bile salts (bilirubin), cholesterol, , fat-soluble hormones, lecithin. - Neutralizes and dilutes stomach acid. - Bile salts emulsify fats. . Most are reabsorbed in the ileum. CLINICAL POINT - Because of its strategic location, large mass, and fragile capsule (that provides relatively little protection to the organ), the liver is prone to many injuries. - Its complex vascular supply and enormous blood reserve also make it vulnerable to bleeding with extensive blood loss (exsanguination) into the abdominal cavity. - Blunt and penetrating traumas are the most common causes of subcapsular hepatic hematomas localized collections of extravasated blood contained by Glisson capsule that may subsequently rupture and be life threatening. - Subcapsular hematomas may also occur as a complication of preeclampsia in pregnancy (a leading cause of maternal death) or in some parasitic infestations (e.g., amebiasis, schistosomiasis) of the liver, which cause hemorrhage, liver necrosis, , and subsequent . - Although most patients are managed conservatively, some require urgent surgical intervention. Blood Supply to the Liver

- To appreciate the myriad functions of the liver introduced previously, one must first understand its unique blood supply and how blood is distributed to the hepatocytes.

The liver has a dual blood supply consisting of: - a venous (portal) supply via the hepatic portal vein provides deoxygenated blood Nutrients, drugs, toxins, microbes and - an arterial supply via the hepatic artery. - Both vessels enter the liver at a hilum or porta hepatis, the same site at which the , carrying the bile secreted by the liver, and the lymphatic vessels leave the liver. - Bile flows in a direction opposite to that of the blood. Blood supply to the liver: the portal triad Structural Organization of the Liver The structural components of the liver include: - parenchyma, consisting of organized plates of hepatocytes, which in the adult are normally one cell thick and are separated by sinusoidal capillaries. . In young individuals up to 6 years of age, the liver cells are arranged in plates two cells thick. - connective tissue stroma that is continuous with the fibrous capsule of Glisson. - Blood vessels, nerves, lymphatic vessels, and bile ducts travel within the connective tissue stroma. - sinusoidal capillaries (sinusoids), the vascular channels between the plates of hepatocytes. - perisinusoidal spaces (spaces of Disse), which lie between the sinusoidal endothelium and the hepatocytes. - the organization of these structural elements to understand the major functions of the liver. Liver Lobules There are three ways to describe the structure of the liver in terms of a functional unit: - The classic lobule, - The portal lobule, and - The liver acinus. Classic hepatic lobules

- Longitudinal axis of each classical lobule is occupied by central vein - Hepatocytes radiate from central vein and separating from each other by vascular spaces known as hepatic sinusoids - Limiting membrane pierced by inlet venules and inlet arterioles to join hepatic sinusoids - Blood from periphery flows slowly through sinusoids into central vein Diagram of a classic liver lobule Photomicrographs of pig and human Cut transversely hepatic lobules are polygonal units showing plates of epithelial cells called hepatocytes radiating from a central venule (C). In all cases peripheral connective tissue of portal areas with microvasculature and small bile duct (D) branches can be seen and in humans as in other mammals these are present at the boundaries between two or more hepatic lobules. The vessels near the bile ducts branches are a venule (V) off the portal vein and an arteriole (A) off the hepatic artery. Both X150. H&E. Structure and function of the liver portal lobule The portal lobule emphasizes the exocrine functions of the liver. - Portal lobule is a triangular region that portal area is located in its center and central veins form apices of the triangle. - The major exocrine function of the liver is bile secretion. - the morphologic axis of the portal lobule is the interlobular bile duct of the portal triad of the classic lobule. - Its outer margins are imaginary lines drawn between the three central veins that are closest to that portal triad. - These lines define a roughly triangular block of tissue that includes those portions of three classic lobules that secrete the bile that drains into its axial bile duct. portal lobule Liver architecture (b) Pig, H&E (LP) (c) Human, H&E (LP) Histology of the portal tract and central vein

- Portal area (triad) is where three classical lobules are in contact with each other, more CT elements present, houses branches of hepatic artery, tributaries of portal vein, , and lymph vessels. - Portal areas are isolated from liver parenchyma by modified hepatocytes which form the limiting plate. - Space of Möll separates the limiting plate from the CT elements of portal area

Portal tract H&E (MP) Structure and function of the liver acinus The liver acinus is the structural unit that provides the best correlation between blood perfusion, metabolic activity, and liver pathology. - Hepatic acinus (acinus of Rappaport) is diamond-shaped, a distributing artery located in center of acinus and represents the smaller functional unit of the hepatic parenchyma. .The short axis of the acinus is defined by the terminal branches of the portal triad that lie along the border between two classic lobules. . The long axis of the acinus is a line drawn between the two central veins closest to the short axis. - Three regions of parenchyma surrounding the artery arranged in three concentric elliptical zones surrounding the short axis : . Zone 1 is closest to the short axis. . Zone 3 is farthest from the short axis and closest to the terminal hepatic vein (central vein). . Zone 2 lies between zones 1 and 3 but has no sharp boundaries. - As a result of the sinusoidal blood flow, the oxygen gradient, metabolic activity of the hepatocytes, and distribution of hepatic enzymes vary across the three zones. - The distribution of liver damage resulting from ischemia and exposure to toxic substances can be explained using this zonal interpretation. - Cells in zone 1 are the first to receive oxygen, nutrients, and toxins from the sinusoidal blood and the first to show morphologic changes after bile duct occlusion (bile stasis). - These cells are also the last to die if circulation is impaired and the first to regenerate. - On the other hand, cells in zone 3 are the first to show ischemic necrosis (centrilobular necrosis) in situations of reduced perfusion and the first to show fat accumulation. - They are the last to respond to toxic substances and bile stasis. - Normal variations in enzyme activity, the number and size of cytoplasmic organelles, and the size of cytoplasmic glycogen deposits are also seen between zones 1 and 3.

The heterogenecity 0f hepatocytes from the perilobular to the centrolobular regions Photomicrograph of centrilobular necrosis in human liver. Concepts of structure-function relationships in liver. CLINICAL POINT - A broad panel of laboratory tests known collectively as liver function tests is used clinically for diagnosis of liver disorders. - They also assess disease severity, prognosis, and treatment outcome. - Serum aspartate aminotransferase (AST) and alanine transaminase (ALT) cytosolic enzymes released into bloodstream in response to hepatocyte injury are sensitive indicators of liver damage. - The highest elevations occur in patients with acute viral hepatitis and toxin- induced hepatic necrosis. - The AST-to-ALT ratio may also be useful in distinguishing different causes of . - A high ratio suggests advanced alcoholic ; . lower values are seen in those with viral hepatitis. . Whereas chronic disorders, such as , lead to decreased serum levels of albumin (a protein synthesized exclusively in the liver), bile duct obstruction and intrahepatic cholestasis cause elevations in alkaline phosphatase (an enzyme present in the biliary duct system). Blood Vessels of the Parenchyma The blood vessels that occupy the portal canals are called interlobular vessels. which branch into, - distributing vessels located at the periphery of the lobule, which send - inlet vessels to the - sinusoids. . In the sinusoids, the blood flows centripetally toward the - central vein. . is a thin-walled vessel. . The endothelial lining of the central vein is surrounded by small amounts of spirally arranged connective tissue fibers. The central vein empties into a - sublobular vein, which converge to form larger - hepatic veins that empty into the - inferior vena cava. The structure of the hepatic artery is like that of other arteries. - In addition to providing arterial blood directly to the sinusoids, the hepatic artery provides arterial blood to the connective tissue and other structures in the larger portal canals. Diagram of the flow of blood and bile in the liver. Structure of hepatic sinusoids Hepatic sinusoids are lined with a thin discontinuous endothelium. - The space between the anastomosing plates of hepatocytes are occupied by hepatic sinusoids. - Sinosuidal lining cells are fenestrated are not in contact with each other . The discontinuous sinusoidal endothelium has a discontinuous basal lamina that is absent over large areas. . The discontinuity of the endothelium is evident in two ways: a. Large fenestrae, without diaphragms, are present within the endothelial cells. b. Large gaps are present between neighboring endothelial cells. - They prevent direct contact between blood and hepatocytes - Resident macrophages (stellate sinusoidal macrophage known as Kupffer cells) associated with lining cells. - The space that separates sinusoidal lining cells from hepatocytes is called (space of Disse). - Microvilli of hepatocytes occupy much of the space. - Type III that is present in space support lining cells(no basal lamina) Central vein, liver, human, H&E ×500; inset ×800, Hepatic sinusoids, liver, rat, glutaraldehyde osmium fixation, toluidine blue ×900. Scanning electron micrograph of a vascular corrosion cast illustrating the complex network of hepatic sinusoids in a mouse liver Open fenestrae are evident in the endothelial cell cytoplasm. Note the space of Disse between the sinusoidal wall and the hepatocytes.

ligation Structure and function of Kupffer cells Kupffer cells belong to the mononuclear phagocytotic system. - Like other members of the mononuclear phagocytotic system, Kupffer cells are - derived from monocytes. - Kupffer cells do not form junctions with neighboring endothelial cells. - Kupffer cells suggests that they may be involved in the final breakdown of some damaged or senile red blood cells that reach the liver from the spleen. - Some of the ferritin iron may be converted to hemosiderin granules and stored in the cells. . This function is greatly increased after splenectomy when it is then essential for red blood cell disposal. Liver Histology

47 Hepatic sinusoids & Kupffer cells

CLINICAL POINT - In a normal liver, Kupffer cells break down senescent red blood cells (RBCs) via phagocytosis and lysosomal degradation. - The resulting iron-containing pigment (hemosiderin) is stored in Kupffer cells so it can then be made available for production of new hemoglobin, the O2- transporting component of RBCs. - In fibrotic liver disease, Kupffer cells produce cytokines that stimulate nearby hepatic stellate cells to produce collagen and other components of the extracellular matrix (ECM). - Under some conditions, activation of Kupffer cells, which have capacity to undergo mitosis, may play a role in pathogenesis of other disorders, such as ethanol-induced liver injury common in chronic alcoholism. Pit cells

- Pit cells exist in the liver sinusoids and often adhere to endothelial cells, although they incidentally contact Kupffer cells (Kc). - pit cells can penetrate the fenestrae of the Ec, and enter the space of Disse and can directly contact the microvilli of hepatocytes . . The frequency of pit cells in liver tissue is about an average of 1 pit cell per 10 Kupffer cells. - NK cells promote hepatic tolerance. . It has been reported that NK cells can inhibit dendritic cell (DC) activation by producing the suppressive factors transforming growth factor-beta (TGF-β) and IL-10 in an NKG2A-dependent manner. - NK cells play a role in liver regeneration. . Suppressing NK cell activity using the immunosuppressive drug FK506 or depleting NK cells using anti-asialo GM1 treatment promotes liver regeneration - Pit cells remain in the liver about 2 weeks and are dependent on Kupffer cells. - Pit cells also proliferate locally, when stimulated with interleukin-2. Transmission electron micrograph of a pit cell in a rat hepatic sinusoidal lumen (L). The pit cell shows polarity with an eccentric nucleus. The cytoplasm is abundant and contains characteristic electr on-dense granules and other organelles lying mainly on one side of the nucleus. The cell contacts an endothelial cell (E) and a portion of a (K) with a positive peroxidase reaction product in the rough endoplasmic reticulum. Bar = 1 μm. (from Hepatology,1988; 8: 46-52, with permission) One hour after the injection of colon carcinoma cells (CC531) into syngeneic Wagrain rats, a pit cell (left) and a Kupffer cell (lower right) were found attached to a tumor cell (center). Pit cells kill these tumor cells in vitro by secreting perforin and granzyme, causing the apoptosis of the tumor cells. Note that the pit cell granules are assembled at the side facing the tumor cell. The pit cell has not degranulated. It is thought that this secretion triggers membrane changes in the tumor cells, which are recognized by the Kupffer cell and initiate the phagocytic reaction. The first stage in phagocytosis is determined by the attachment of the effector cell to the target cell, which is clearly depicted here. The combination of apoptotic induction (by the pit cell) and phagocytosis (by the Kupffer cell) will kill this tumor cell. The single red blood cell in the sinusoid measures approximately 7 μm. Structure of the space of Disse Perisinusoidal Space (Space of Disse) The perisinusoidal space is the site of exchange of materials between blood and liver cells. - The perisinusoidal space (space of Disse) lies between the basal surfaces of hepatocytes and the basal surfaces of endothelial cells and Kupffer cells. - Fat storing cells ( known as Ito cells or Stellate cells) may present in Disse space and store vitamin A. - Pit cells which are natural killer cells also may be seen in space of Disse.

- In the fetal liver, the space between blood vessels and hepatocytes contains islands of blood-forming cells. - In cases of chronic anemia in the adult, blood-forming cells may again appear in the perisinusoidal space. Electron micrograph showing the perisinusoidal space (of Disse). The perisinusoidal space (D) is located between the hepatocytes (H) and the sinusoid. A gap (large arrow) separates the endothelial cells (En) that line the sinusoid. Such gaps allow easy passage of small substances between the sinusoid and the perisinusoidal space. Numerous microvilli extend from the hepatocytes into the perisinusoidal space. These processes are long and frequently branch (small arrow). A red blood cell (RBC) is within the sinusoid. 18,000.

CLINICAL POINT - Cirrhosis of the liver is the end stage of chronic liver disease caused usually by alcohol abuse, biliary obstruction, or viral hepatitis. - Excessive deposition of connective tissue stroma produces abnormal fibrous septa made of collagen fiber bundles, which link portal tracts to each other and to hepatic veins. - Persistent liver cell necrosis leads to nodules of regenerating hepatocytes encircled by fibrosis. - This morphologic pattern advances to marked disruption in microscopic architecture of the entire liver. - Disease progression causes distortion of the vascular supply, , reduced hepatocyte function, and liver failure. Structure and function of hepatic stellate cells The hepatic stellate cells (Ito cells) store vitamin A; - however, in pathologic conditions, they differentiate into and synthesize collagen. - The other cell type found in the perisinusoidal space is the (commonly called an Ito cell). . These cells of mesenchymal origin are the primary storage site for hepatic - vitamin A in the form of retinyl esters within cytoplasmic lipid droplets. - Vitamin A is released from the hepatic stellate cell as retinol (alcohol form) bound to retinolbinding protein (RBP). - For many years, fish liver oils (e.g., cod liver oil) were medically and economically important nutritional sources of vitamin A. -In certain pathologic conditions, such as chronic inflammation or liver cirrhosis, hepatic stellate cells lose their lipid and vitamin A storage capability and differentiate into cells with characteristics of myofibroblasts. . These cells appear to play a significant role in hepatic fibrogenesis; . they synthesize and deposit type I and type III collagen within the perisinusoidal space, resulting in liver fibrosis. - This collagen is continuous with the connective tissue of the portal space and the connective tissue surrounding the central vein. - An increased amount of perisinusoidal fibrous stroma is an early sign of liver response to toxic substances. - The cytoplasm of hepatic stellate cells contains contractile elements, such as desmin and smooth muscle -actin filaments. - During cell contraction, they increase the vascular resistance within the sinusoids by constricting the vascular channels, leading to portal hypertension. - In addition, hepatic stellate cells play a role in remodeling the extracellular matrix during recovery from liver injury. Electron micrograph showing the perisinusoidalspace (of Disse). Cellular Elements of the Hepatic Sinusoid. Hepatic stellate cells (blue cell) are key perisinusoidal cells that activate to myofibroblasts in liver injury Periportal vitamin A-rich stellate cell which contains a number of lipid droplets and has indented nucleus. Bar = 5 micrometers.

CLINICAL POINT - Many acute and chronic diseases of liver activate transformation of quiescent stellate cells into -like cells that engage in the inflammatory fibrotic response by undergoing mitosis and synthesizing and secreting increased amounts of ECM consisting of collagen (types I, III, and IV), laminin, fibronectin, and proteoglycans. - Activated stellate cells may play a role in pathogenesis of portal hypertension, cirrhosis of the liver, and fibrotic capsule formation around tumors in hepatocellular carcinoma. - Intrahepatic cholestasis is a pathologic state of reduced bile formation or flow. . It leads to jaundice, a yellowing of the skin and sclera of the eyes, because of excess circulating bilirubin. . Plug-like deposits of this bile pigment in dilated canaliculi, hepatocytes, and intrahepatic bile ducts are a histologic hallmark. . By electron microscopy, microvilli of canaliculi are fewer or look blunted. - Cholestasis may be due to an ion pump or permeability defect in the canalicular membrane or in contractile of canaliculi and bile ducts. - Elevated levels of serum alkaline phosphatase, an enzyme in canaliculi and ductal epithelium, are diagnostic. Lymphatic Pathway

Hepatic lymph originates in the periportal space. -The periportal space (space of Mall), is described between the stroma of the portal canal and the outermost hepatocytes. - From this collecting site, the fluid then enters; - lymphatic capillaries that travel with the other components of the portal triad. - About 80% of the hepatic lymph follows this pathway and drains into the thoracic duct, forming the major portion of the thoracic duct lymph. periportal space (of Mall)

cholangiocytes (CH) surrounded by a complete basal lamina (BL). The narrow space (asterisks) into which microvilli of hepatocytes project is the periportal space (of Mall), 6,000. Ultrastructure and function of Hepatocytes Hepatocytes make up the anastomosing cell plates of the liver lobule. - Hepatocytes - Hepatocytes are polygonal 20-30 µm diameter, forming anastomosing plates of one to two thickness cells. - They constitute about 80% of the cell population of the liver. - Hepatocyte nuclei are large and spherical and occupy the center of the cell. . Many cells in the adult liver are binucleate; - The hepatocyte cytoplasm is generally acidophilic. - Specific cytoplasmic components may be identified by routine and special staining procedures, including: . Basophilic regions that represent rough endoplasmic reticulum (rER) and free ribosomes; . Numerous mitochondria; (800 to 1,000 mitochondria per cell); . Multiple small Golgi complexes; . Large numbers of peroxisomes; . Deposits of glycogen; . Lipid droplets of various sizes; . lipofuscin pigment within lysosomes; - The hepatocyte is polyhedral; . for convenience, it is described as having six surfaces, although there may be more. - A schematic section of a cuboidal hepatocyte is shown in. . Two of its surfaces face the perisinusoidal domains (basal surface), have many microvilli protrude to perisinusoidal space (of Disse), endocrine secretion of hepatocytes release here. . The plasma membrane of two surfaces faces a neighboring hepatocyte and a . . Assuming that the cell is cuboidal, the remaining, two surfaces, would also face neighboring cells and bile canaliculi (lateral and apical surfaces), microvilli, Na-K ATPase and gap junctions are common characteristics of lateral domain. - Hepatocytes are relatively long-lived for cells associated with the digestive system; . their average lifespan is about 5 months. . In addition, liver cells are capable of considerable regeneration when liver substance is lost to hepatotoxic processes, disease, or surgery. Hepatocytesc Hepatocytesc

This image shows the close proximity between the blood in the sinusoids and hepatocytes. Endothelial cells line the sinusoids. These endothelial cells form a discontinuous endothelium that has wide gaps between cells and lacks a basement membrane and is therefore very permeable. Hepatocytes secrete bile into canaliculi that are defined by junctions between hepatocytes. Bile flows through theses narrow tubes toward the bile duct. Also visible is a Kupffer cell. Kupffer cells are the resident macrophages of the liver and are typically found within the lumen of the sinusoids. Hepatocytes Hepatocyte Hepatocyte ultrastructure and major functions.

Hepatocytes

Peroxisomes are numerous in hepatocytes. - Hepatocytes have as many as 200 to 300 peroxisomes per cell. - They are in diameter from 0.2 to 1.0 μm . - Are a major site of oxygen use . - They contain a large amount of oxidase that generates toxic hydrogen peroxide, H2O2. - The enzyme catalase, also residing within peroxisomes. . In fact, about one half of the ethanol that is ingested is converted to acetaldehyde by enzymes contained in liver peroxisomes and, - D-amino acid oxidase, as well as alcohol dehydrogenase, are found in peroxisomes. - Peroxisomes are involved in breakdown of fatty acids (- oxidation) as well as gluconeogenesis and metabolism of purines. Hepatocytes sER can be extensive in hepatocytes. - The sER extensive but varies with metabolic activity. - The sER contains enzymes involved in degradation and conjugation of toxins and drugs as well as enzymes responsible for synthesizing cholesterol and the lipid portion of lipoproteins.

- The sER undergoes hypertrophy after administration of alcohol, drugs (i.e., phenobarbital, anabolic steroids, and progesterone), and certain chemotherapeutic agents used to treat cancer. - Stimulation of the sER by ethanol enhances its ability to detoxify other drugs, certain carcinogens, and some pesticides. - On the other hand, metabolism by the sER can actually increase the hepatocyte-damaging effects of some toxic compounds, such as carbon tetrachloride (CCl4) and 3,4-benzpyrene. Hepatocytes The large Golgi apparatus in hepatocytes consists of as many as 50 Golgi units. - the Golgi apparatus As many as 50 Golgi units, each consisting of three to five closely stacked cisternae, plus many large and small vesicles, are found in hepatocytes secretion of bile. - Golgi cisternae and vesicles near the sinusoidal surfaces of the cell , however, contain electron-dense granules 25 to 80 nm in diameter that are believed to be VLDL and other lipoprotein precursors. Hepatocytes Lysosomes concentrated near the bile canaliculus correspond to the peribiliary dense bodies seen in histologic sections. - Hepatocyte lysosomes are so heterogeneous. - In addition to normal lysosomal enzymes, TEM reveals other components: . Pigment granules (lipofuscin). . Partially digested cytoplasmic organelles. . Myelin figures. - Hepatocyte lysosomes may also be a normal storage site for iron (as a ferreting complex) . Ultrastructure of hepatocytes, perisinusoidal space, and bile canaliculi. This electron microscope shows two hepatocytes separated by their cell membranes. A junctional complex is located near the apical portion of these cells. Above this junction is the bile canaliculus into which the cells secrete bile from their apical surface. Also available in these cells are numerous mitochondria. Hepatocyte

(A) Binucleated hepatocyte. (B and C) Portions of hepatocytes showing a well-developed endoplasmic reticulum, peroxisomes (p), cristae-rich mitochondria (m), and nuclear pores (arrows). (D–G) Periportal hepatocytes showing altered mitochondria (m). In the inset of (E) an autophagosome containing an altered mitochondrion is shown. Dashed boxes indicate free-ribosome-rich cytoplasmic areas. Note the lack of glycogen stores both in pericentral and periportal hepatocytes. l, lipid droplet; ly, lysosomes; p, peroxisomes; r, free ribosomes. Magnifications: (A, B, D and E) bars=1 mm; (C, F and G) bars=0.5 mm CLINICAL POINT - Excessive ethanol consumption is toxic to the liver and may cause morphologic changes and clinical symptoms including liver cell damage, extensive fibrosis, and inflammation. - Hepatocytes in the alcoholic liver accumulate large amounts of fat and often become distended beyond recognition. - By electron microscopy, mitochondria appear grossly enlarged, with a bizarre shape, and smooth endoplasmic reticulum is distended. - In severe cases, inclusions known as Mallory bodies are scattered in the cytoplasm of damaged hepatocytes. - They are composed of aggregates of intermediate (cytokeratin) filaments of the cytoskeleton. - These ultrastructural changes parallel functional alterations in cell oxidation and metabolism. - The most common genetic liver disease in infancy and childhood is the autosomal recessive a1-antitrypsin deficiency. - It is marked by abnormally low levels of this serum protease inhibitor, a glycoprotein usually produced by hepatocytes. - A defect in migration of secretory protein from the RER to the Golgi complex results in accumulation of mutant protein in dilated RER cisternae. - A distinctive feature— faintly eosinophilic, periodic acid Schiff–positive cytoplasmic inclusions, 1-10 mm in diameter—leads to severe damage to hepatocytes. - This disorder causes hepatitis in newborns and often causes cirrhosis, for which liver transplantation may be indicated. Biliary Tree - The biliary tree is the three-dimensional system of channels of increasing diameter that bile flows through from the hepatocytes to the gallbladder and then to the intestine. - In the adult human liver, there are more than 2 kilometers of interconnecting bile ductules and ducts of different sizes and shapes. Biliary Tree

The biliary tree is lined by cholangiocytes which monitor bile flow and regulate its content. - Cholangiocytes are epithelial cells that line the biliary tree. - Each cholangiocyte contains primary cilium that sense changes in lumenal flow resulting in alterations of cholangiocytes secretion. Biliary Tree Intrahepatic bile ductules. Scanning electron micrograph of the luminal surface of the bile duct. Structure and function of bile canaliculi Biliary Tree

The bile canaliculus is a small canal formed by apposed grooves in the surface of adjacent hepatocytes. - The smallest branches of the biliary tree are the bile canaliculi into which the hepatocytes secrete bile. - They form a complete loop around four sides of the idealized six-sided hepatocytes. - They are approximately 0.5 μm in luminal diameter. - Microvilli of the two adjacent hepatocytes extend into the canalicular lumen. - Adenosine triphosphatase (ATPase) and other alkaline phosphatases can be localized on the plasma membranes of the canaliculi, suggesting that bile secretion into this space is an active process. - Near the portal canal but still within the lobule, bile canaliculi transform into the short canals of Hering. Bile canaliculi (a) Enzyme histochemical staining for ATPase (HP) (b) Iron haematoxylin (HP)

Electron micrograph showing portions of three adjacent hepatocytes and the intervening bile canaliculi Biliary Tree A characteristic feature of the canal of Hering is its lining made of two types of cells, hepatocytes and cholangiocytes. - The canal of Hering is a channel partially lined by hepatocytes and partially by cuboidal shaped cholangiocytes. - Functionally, as demonstrated by videomicroscopy, the canal of Hering exhibits contractile activity that assists with unidirectional bile flow toward the portal canal.

- Because the canal of Hering represents the smallest and most proximal tributary of the biliary tree containing cholangiocytes, it often is involved in the same diseases that affect small bile ducts. - Functional disturbance in contractile activity as well as injury or destruction of the canals of Hering may contribute to intrahepatic cholestasis (obstruction of the bile flow). Biliary Tree Canal of Hering serves as a reservoir of liver progenitor cells. - Due to their location at the crucial interface between hepatocytes and cholangiocytes, it has been proposed that the hepatic stem cells’ niche is present either in the canals of Hering or in their vicinity. - This hypothesis was supported by the appearance of liver cell precursors near the canals of Hering in most of pathologic conditions characterized by extensive damage to hepatocytes. . these cells express dual markers of both biliary and hepatocyte antigens - Therefore, it has been concluded that the canal of Hering consists of or harbors specific hepatic stem cells. - Laboratory studies suggest that in the future, hepatic stem cells may ultimately have therapeutic use in the treatment of liver diseases. Canals of Hering

Canals of Hering and the intrahepatic ductile. a. Photomicrograph showing an area near a portal canal. Arrows indicate regions where bile canaliculi are draining into canals of Hering. Note that the canal of Hering is partially lined by hepatocytes and partially by cholangiocytes. It drains into intrahepatic bile ductule surrounded by hepatocytes, in contrast to the interlobular bile duct, which is embedded in the connective tissue of the portal canal. The terminal branch of a portal vein (lower right) accompanied by a small bile ductule are evident. 800. The canal of Hering, lined by two cuboidal, cholangiocyte-like cells with scant cytoplasmic organelles and by hepatocytes with abundant mitochondria. It is likely that these small cells are functional both as cholangiocytes, involved with bile flow and processing, and as facultative hepatic progenitor cells Model of the hepatic stem cell niche in the canals of Hering. Label-retaining cells (black nuclei) represent putative stem cells and are located in the canals of Hering as keratin-expressing cells. Also, cholangiocytes of intralobular bile ducts, small hepatocytes, and less-characterized null cells retain the BrdU label after acetaminophen treatment of mice (50). Coexpression of keratin 19 and albumin in small hepatocytes at the interface of cholangiocytes and hepatocytes in the canals of Hering indicate that hepatocytes are generated at this site by stem/progenitor cells (116). A continuous production of hepatocytes within the portal field is supported by SOX9 fate-mapping analysis (54). Small cholangiocytes lining the canals of Hering and ductules may represent cholangiocyte precursors, which potentially contribute to the cholangiocyte population of larger bile ducts (116) and could explain the presence of label-retaining cholangiocytes at this site (50). Putative liver stem cell niche in the Canals of Hering. Liver progenitor cells (LPCs) reside in close proximity to hepatocytes and cholangiocytes, and likely receive paracrine signals from neighboring adult cells during injury (top image). Our study explored heterotypic interactions between hepatocytes and LPCs, and tested a hypothesis that ethanol injury pushes LPC to choose cholangiocytic fate instead of the default hepatic fate (bottom image). GF, growth factor. b. Electron micrograph showing an intrahepatic bile ductule. The ductule collects bile from the canals of Hering. It is close to the hepatocytes, but the actual connection between bile canaliculi and the intrahepatic ductule is not evident in this plane of section. The ductule is composed of cholangiocytes (CH) surrounded by a complete basal lamina (BL). The narrow space (asterisks) into which microvilli of hepatocytes project is the periportal space (of Mall), not the perisinusoidal space (of Disse). 6,000. Biliary Tree Intrahepatic bile ductules carry bile to hepatic ducts. - The ductules have a diameter of about 1.0 to 1.5 μm and carry bile to - the interlobular bile ducts that form part of the portal triad. - These ducts range from 15 to 40 μm in diameter and are lined by cholangiocytes. - As the bile ducts get larger, they gradually acquire a dense connective tissue investment containing numerous elastic fibers. . Smooth muscle cells appear in this connective tissue as the ducts approach the hilum. - Interlobular ducts join to form the right and left hepatic ducts, - which in turn join at the hilum to form the . - In some individuals, the ducts of Luschka are located in the connective tissue between the liver and the gallbladder, . near the neck of the gallbladder. . These ducts connect with the , not with the lumen of the gallbladder.

Biliary Tree Extrahepatic bile ducts carry the bile to the gallbladder and duodenum. - The common hepatic duct is about 3 cm long and is . lined with tall columnar epithelial cells . . All of the layers of the alimentary canal are represented in the duct, except the muscularis mucosae. - The cystic duct connects the common hepatic duct to the gallbladder and carries bile both into and out of the gallbladder. - the fused duct is called the common bile duct and extends for about 7 cm to the wall of the . - duodenum at the ampulla of Vater. . A thickening of the muscularis externa of the duodenum at the ampulla constitutes . the , which surrounds the openings of both the common bile duct and the and acts as a valve to regulate the flow of bile and pancreatic juice into the duodenum. Mucous glands are present around the common bile duct at the papilla and drain into recesses between the papillary fronds Occasional strands of smooth muscle are demonstrated in the upper portion of the common bile duct reacting with antibodies directed against desmin (immunoperoxidase technique). The adult human liver secretes, on average, about 1 L of bile/ day. - The bile fulfills two major functions. - It is involved in the absorption of fat and is used by the liver as a vehicle for excretion of cholesterol, bilirubin, iron, and copper. - About 90% of the bile salts. - Cholesterol and the phospholipid lecithin, as well as most of the - Electrolytes and water delivered to the gut with the bile, are also reabsorbed and recycled.

- Bilirubin glucuronide, the detoxified end product of hemoglobin breakdown, is not recycled. . It is excreted with the feces and gives them their color. - Failure to absorb bilirubin or failure to conjugate it or secrete glucuronide can produce jaundice. Bile flow from the liver is regulated by hormonal and neural control. - Cholecystokinin (CCK released when acidic, fatty chyme enters intestines), gastrin, and motilin increase bile flow . - Steroid hormones (i.e., estrogen during pregnancy) decrease bile secretion from the liver. - Parasympathetic stimulation increases bile flow by prompting contraction of the gallbladder and relaxation of the sphincter of Oddi. Composition of Bile The liver has both sympathetic and parasympathetic innervation. - Sympathetic fibers innervate blood vessels, and increased stimulation in this system causes an increase of vascular resistance, decreased hepatic blood volume, and a rapid increase of serum levels of glucose. - The parasympathetic fibers are assumed to innervate the large ducts (those that contain smooth muscle in their walls) and possibly blood vessels; . their stimulation promotes glucose uptake and utilization. . The cell bodies of parasympathetic neurons are often present near the port hepatis. The gallbladder Gallbladder - The gallbladder is a pear-shaped, distensible sac with a volume of about 50 mL in humans. . The gallbladder is a blind pouch that leads, via a neck, to the cystic duct. . It is attached to the visceral surface of the liver. - The gallbladder concentrates and stores bile. . The gallbladder can store and remove about 90% of the water from the incoming bile, which results in an increase of bile salts, cholesterol, and bilirubin concentrations up to 10-fold. Biliary tract and gallbladder.

Diagram showing the relationship of hepatic, pancreatic, and gallbladder ducts. The gallbladder is a blind pouch joined to a single cystic duct in which numerous mucosal folds form the spiral valve (of Heister). The cystic duct joins with the common hepatic duct, and together they form the common bile duct that leads into the duodenum. At the entry to the duodenum, the common bile duct is joined by the main pancreatic duct to form the hepatopancreatic ampulla (of Vater), and together they enter the second part of the duodenum. Sphincters can be found at the distal part of these ducts. The sphincters of the common bile duct (of Boyden), the main pancreatic duct, and the hepatopancreatic ampulla (of Oddi) control the flow of bile and pancreatic secretion into the duodenum. When the common bile duct sphincter contracts, bile cannot enter the duodenum; it backs up and flows into the gallbladder, where it is concentrated and stored. Structure and function of the gallbladder mucosa Gallbladder The wall of the gallbladder lacks a muscularis mucosae and submucosa. - Mucosa is lined by simple columnar epithelium with two type of- cells: . more clear cells and infrequent brush cells. . Cells have microvilli, Na –K ATPase pump - The mucosae is highly folded. - lamina propria is composed of loose CT, in the neck region lamina propria houses simple tubuloalveolar mucus glands - Smooth muscle layer (muscularis externa) is composed of thin obliquely oriented fibers with perimuscular connective tissue (numerous collagen and elastic fibers among the bundles of smooth muscle cells.) - thick layer of dense connective tissue (adventitia). .This layer contains large blood vessels, an extensive lymphatic network, and the autonomic nerves that innervate the muscularis externa and the blood vessels (cell bodies of parasympathetic neurons are found in the wall of the cystic duct). - The unattached surface is covered by a Serosal. - In addition, deep diverticula of the mucosa, called Rokitansky Aschoff sinuses, sometimes extend through the muscularis externa. - They are thought to presage pathologic changes and develop as the result of hyperplasia (excessive growth of cells) and herniation of epithelial cells through the muscularis externa. - Also, bacteria may accumulate in these sinuses, causing chronic inflammation that is a risk factor for the formation of gallstones. Gallbladder (a) H&E (LP) (b) H&E (MP) (c) H&E (LP) Gallbladder, human, H&E ×45, Mucosa, gallbladder, human, H&E ×325, Mucosa, gallbladder, human, H&E ×550 And Mucosa, gallbladder, human, H&E ×550. RAS= Rokitansky-Aschoff sinuses. Ganglion cells are readily seen in the connective tissue layers of the gallbladder Gallbladder Mucosa of the gallbladder has several characteristic features. - The empty or partially filled gallbladder has numerous deep mucosal folds. - The mucosal surface consists of - simple columnar epithelium. with: . Numerous, apical microvilli . Apical junctional complexes. . Localized concentrations of mitochondria in the apical and basal cytoplasm . Complex lateral plications. - Localization of Na/K–activated ATPase on their lateral plasma membranes and secretory vesicles filled with glycoproteins in their apical cytoplasm. - The lamina propria rich in fenestrated capillaries and small venules, but there are no lymphatic vessels in this layer. - Mucin-secreting glands are sometimes present in the lamina propria in the normal human gallbladder, especially near the neck of the organ, but they are more commonly found in inflamed . . Cells that appear identical to enteroendocrine cells of the intestine are also found in these glands.

Electron micrographs of gallbladder epithelium a. The tall columnar cells display features typical of absorptive cells, with microvilli on their apical surface, an apical junctional complex separating the lumen of the gallbladder from the lateral intercellular space, and numerous mitochondria in the apical portion of the cell. 3,000. b. During active fluid transport, salt is pumped from the cytoplasm into the intercellular space, and water follows the salt. Both salt and water then diff use into the cell from the lumen. As this process continues, the intercellular space becomes greatly distended (arrows). Fluid moves from the engorged intercellular space (arrows) across the basal lamina into the underlying connective tissue (CT) and then into blood vessels. The increase in size of the lateral intercellular space during active fluid transport is evident with the light microscope. 3,000. Mucous glands are present only in the neck of the normal gallbladder Photomicrograph of the Rokitansky-Aschoff sinuses in the wall of the gallbladder. CLINICAL POINT - The presence of stones in the gallbladder or extrahepatic biliary ducts is known as cholelithiasis. - Gallstones are made of various components of bile. - They are often solid deposits of cholesterol or calcium salts. - Most patients have no symptoms, but gallstones lodged in bile ducts between - the liver and intestine can block bile flow and cause jaundice (or icterus) and severe abdominal pain known as biliary colic. - Gallstones may also cause gallbladder inflammation or infection known as cholecystitis, marked by mucosal inflammation with abnormal thickening of the muscularis layer. - The most common gallstone treatment method is laparoscopic surgery. - Although relatively uncommon, primary tumors of the gallbladder are usually adenocarcinomas. - Occurring more frequently in men than in women, risk factors are chronic cholecystitis and cholelithiasis. - Usually affecting the fundus and neck of the organ, such neoplasms tend to metastasize rapidly to adjacent organs, so patients usually have advanced disease at time of diagnosis; . therefore early detection and treatment are important. - Diagnosis is via transabdominal ultrasonography, endoscopic retrograde cholangiopancreatography, and magnetic resonance imaging; . liver function tests show elevations of serum alkaline phosphatase and bilirubin. . Surgical treatment (cholecystectomy and resection of part of adjacent liver and lymph nodes) is curative for tumors involving the mucosa and submucosa. Pancreas Overview of the pancreas Gross

- In adults, the pancreas usually measures 15 to 20 cm in length and weighs 85 to 120 g. - It is slightly larger in men than in women. - The pancreas weighs 2 to 3 g in the newborn and reaches 7 g at 1 year of age. - The weight of the gland gradually decreases after 40 years of age to a mean of 70 g in the ninth decade of life. - The pancreas is composed of four anatomic regions: . the head, neck, body, and tail. - The head is an lies in the C-shaped curve of the duodenum. - The centrally located body of the pancreas crosses the midline of the human body, - the tail extends toward the hilum of the spleen.

- - The pancreatic duct (of Wirsung) extends through the length of the gland and empties into the duodenum at the hepatopancreatic ampulla (of Vater) . The hepatopancreatic sphincter (of Oddi) surrounds the ampulla and not only regulates the flow of bile and pancreatic juice into the duodenum but also prevents reflux of intestinal contents into the pancreatic duct. . In some individuals, an accessory pancreatic duct ( of Santorini ). - A thin layer of loose connective tissue forms a capsule around the gland. - From this capsule, septa extend into the gland, dividing it into ill-defined lobules. - Within the lobules, a stroma surrounds the parenchymal units. - Between the lobules, larger amounts of connective tissue surround the larger ducts, blood vessels, and nerves. - Moreover, in the connective tissue surrounding the pancreatic duct, there are small mucous glands that empty into the duct. Gross appearance of the normal pancreas. The bulbous head (left) is connected to the neck, body, and tail, which merge imperceptibly. The parenchyma consists of distinctly lobulated pink-tan fleshy tissue. The pancreatic duct (opened longitudinally) is thin and smooth throughout its course. Schematic diagram of the pancreas showing the pattern of the major ducts and their tributaries. In this example the accessory duct is patent at the duodenum through the minor papilla. Anatomic variations in the paths of the pancreatic and bile ducts at the ampulla. A long common channel (the prototypical ampulla) is only present in some individuals. In others, the ducts fuse within only a few millimeters of the duodenum, resulting in a short common channel, or the two ducts enter separately. Embryology of pancreatic development

- During the fourth to fifth weeks of gestation, the pancreas forms from the endoderm of the distal embryonic foregut as dorsal and ventral buds. . The dorsal bud forms opposite the hepatic diverticulum, . The ventral bud, which may be bilobed, forms adjacent to the hepatic diverticulum. . The duct from the ventral pancreas is closely apposed to the common bile duct. . With the rotation of the duodenum during the sixth week, . The ventral pancreas with the common bile duct migrates circumferentially to the right around the posterior aspect of the duodenum to lie posterior and inferior to the dorsal pancreas. . The two portions generally fuse during the seventh week. . The dorsal portion makes up the superior head as well as the entire neck, body, and tail of the adult gland. . The ampulla of Vater develops during the eighth week. Embryology of pancreatic development. a 30 days after fertilization. The ventral and dorsal buds develop on opposite sides of the primordial foregut. b The ventral pancreatic bud develops from the hepatic diverticulum arising from the duodenum, and the dorsal pancreatic bud arises separately from the dorsal aspect of the duodenum. c As the duodenum rotates to the right assuming the C shape, the ventral pancreatic bud continues its rotation before fusing with the dorsal pancreatic bud. d The dorsal pancreatic bud forms the body and tail of the pancreas and also gives rise to the accessory pancreatic duct which empties into the minor duodenal papilla of Santorini. Permission to use this figure was obtained from Hugh A. Tilson, Ph.D., Editor in Chief of Environmental Health Sciences on March 01, 2011 10 week old fetus-dorsal pouch: individual or clustered insulin (A), glucagon (B), pancreatic polypeptide (C) and somatostatin (D) cells stained with specific hormone antibodies in primary tubules or their buds Structure and function of Pancreas The pancreas The pancreas is an exocrine and endocrine gland. - Unlike the liver, in which the exocrine and secretory (endocrine) functions reside in the same cell, the dual functions of the pancreas are relegated to two structurally distinct components. - Thin capsule with septa between lobules - The exocrine component; . acini - (99% of the cells in pancreas) a. mixture of enzymes called "pancreatic juice" - The endocrine component, (of Langerhans) (1% of all cells) . Synthesizes and secretes the hormones insulin, glucagon, pancreatic polypeptide (PP) and somatostatin into the blood. - These hormones regulate glucose, lipid, and protein metabolism in the body. - Centroacinar cells occupy the lumen of the acini, these cells are beginning of the duct system - Intercalated ducts, Intralobular ducts, and interlobular ducts

Pancreas H&E (LP) Exocrine Pancreas The exocrine pancreas is a serous gland. - The exocrine pancreas resembles the parotid gland. - The secretory units are acinar or tubuloacinar in shape. - Periacinar connective tissue is minimal. - Pancreatic acini are unique among glandular acini; . the initial duct that leads from the acinus, - Centroacinar cells and intercalated ducts manufacture a serous, bicarbonate-rich alkaline fluid - The duct cells located inside the acinus are referred to as centroacinar cells. - The acinar cells are characterized by distinct basophilia in the basal cytoplasm and by acidophilic zymogen granules in the apical cytoplasm. - The acinar cells (Serous cells) are truncated pyramidal in shape basophilic,with a round nucleus that is located basally, RER, GA, and secretory granules - Acidophilic Zymogen granules contain: trypsinogen, chymotrypsinogen, carboxypeptidase, ribonuclease, DNAase, lipase, elastase, amylase - Secretion controlled by secretin and CCK from duodenum and vagus nerve. - Acinar cells are joined by junctional complexes at their apical poles, thus forming an isolated lumen into which small microvilli extend from the apical surfaces. Pancreatic acini Electron micrograph of the pancreatic acinus and intercalated duct Pancreatic Acinus of Exocrine Pancreas Exocrine Pancreas Zymogen granules contain a variety of digestive enzymes in an inactive form. - Pancreatic enzymes the proenzymes, contained in pancreatic zymogen granules are: • Proteolytic endopeptidases (trypsinogen, chymotrypsinogen) and proteolytic exopeptidases (procarboxypeptidase, proaminopeptidase) digest proteins by cleaving their internal peptide bonds (endopeptidases) or by cleaving amino acids from the carboxyl or amino end of the peptide. • Amylolytic enzymes (-amylase) . • Lipases digest lipids by cleaving ester bonds of triglycerides, producing free fatty acids. • Nucleolytic enzymes (deoxyribonuclease and ribonuclease) digest nucleic acids, producing mononucleotides. - The pancreatic digestive enzymes Initially, the enterokinases in the glycocalyx of the microvilli of the intestinal absorptive cells converts trypsinogen to trypsin, a potent proteolyti enzyme. Pancreatic acinar cell ultrastructure. Duct System of the Exocrine Pancreas Duct System of the Exocrine Pancreas

Centroacinar cells are intercalated duct cells located in the acinus. - Centroacinar cells are continuous with the cells of the short intercalated duct that lies outside the acinus. - The intercalated ducts are short and drain into - intralobular collecting ducts. That - drains into the larger interlobular ducts, which are . lined with a low columnar epithelium in which enteroendocrine cells and occasional goblet cells may be found, that drain directly into - the main pancreatic duct, which runs the length of the gland parallel to its long axis. - A second large duct, the accessory pancreatic duct, arises in the head of the pancreas.

Pancreatic acinus and its duct system Duct System of the Exocrine Pancreas

The intercalated ducts add bicarbonate and water to the exocrine secretion. - The pancreas secretes about 1 L of fluid per day, about equal to the initial volume of the hepatic bile secretion. - Whereas bile is concentrated in the gallbladder, the entire volume of the pancreatic secretion is delivered to the duodenum. - Although the acini secrete a small volume of protein-rich fluid, - The intercalated duct cells secrete a large volume of fluid rich in: . sodium and bicarbonate. -The bicarbonate serves to . neutralize the acidity of the chyme that enters the duodenum from the stomach and to establish . The optimal pH for the activity of the major pancreatic enzymes. The ductal system within the lobules consists of innumerable intercalated ducts that fuse to form intralobular ducts Pancreatic Acinar Cells and smallest Ducts - High Power Intralobular ducts come together to form interlobular ducts. B. The interlobular ducts are surrounded by a variably thick rim of dense fibrous tissue and carry the pancreatic secretions to the major ducts, receiving tributaries of small interlobular ducts as they pass through the connective tissue septa of the gland. The cells of the intralobular and smaller interlobular ducts contain mucin in the apical cytoplasm that stains positively with Alcian blue/PAS. B. Staining with mucicarmine shows a similar distribution of mucin. The duct of Wirsung at the papilla of Vater is lined by a single layer of tall columnar cells with occasional interspersed goblet cells. Accessory pancreatic ducts and acini are also observed. Some of the small ducts around the duct of Wirsung at the major papilla contain a few cells that are immunoreactive for chromogranin A (immunoperoxidase technique) As the main pancreatic duct enters the ampulla of Vater, the ductal epithelium forms broad, The duct of Santorini at the minor papilla contains papillary fronds and is surrounded by muscle bundles The duct of Santorini at the minor papilla is lined by tall columnar cells with interspersed goblet cells. Accessory pancreatic ducts pierce the large smooth muscle bundles to empty into the lumen of the common bile duct Pancreatic exocrine secretion is under hormonal and neural control. -Two hormones secreted by the enteroendocrine cells of the duodenum: . Secretin and cholecystokinin (CCK). - (The entry of the acidic chyme into the duodenum stimulates the release of these hormones into the blood) - Secretin is a polypeptide hormone (27 amino acid residues) that stimulates the duct cells to secrete a large volume of fluid with a high HCO3 concentration. - CCK is a polypeptide hormone (33 amino acid residues) that causes the acinar cells to secrete their proenzymes. - Autonomic innervation. . Sympathetic nerve fibers are involved in regulation of pancreatic blood flow. . Parasympathetic fibers stimulate activity of acinar as well as centroacinar cells. Control of Pancreatic Secretion

24-167 The site of action of hormones and neuropeptides mediating the secretory response of acinar and ductal cells. Small autonomic ganglia are located within lobules of acinar tissue. Endocrine Pancreas Endocrine Pancreas

The endocrine pancreas is a diffuse organ that secretes hormones that regulate blood glucose levels. - The islets of Langerhans, cell groupings of varying size. - It is estimated that 1 million to 3 million islets constitute about 1% to 2% of the volume of the pancreas but are most numerous in the tail. - Individual islets may contain only a few cells or many hundreds of cells that are arranged in short, irregular cords that are profusely invested with a network of fenestrated capillaries. - The definitive endocrine cells of the islets develop between 9 and 12 weeks of gestation. - The islets of Langerhans appear as clusters of pale-staining cells. Photomicrograph of the pancreas Endocrine Pancreas

- After Zenkerformol fixation and staining by the Mallory-Azan method, it is possible to identify three principal cell types designated A (alpha), B (beta), and D (delta) cells. - With this method, the A cells stain red, the B cells stain brownish orange, and the D cells stain blue. - About 5% of the cells appear to be unstained after this procedure. - TEM allows identification of the principal cell types by the size and density of their secretory granules. Diagram of an islet of Langerhans stained by the Mallory-Azan method.

Endocrine Pancreas Islet cells, other than B cells, are counterparts of the enteroendocrine cells of the gastrointestinal mucosa. Each cell type can be correlated with a specific hormone, and each has a specific location in the islet: - B cells constitute about 70% located in its central portion. . They secrete insulin. . B cells contain secretory granules about 300 nm with a dense polyhedral core(crystallized insulin). - A cells constitute about 15% to 20% located peripherally in the islets. . They secrete glucagon. . A cells contain secretory granules about 250 nm that are more uniform in size and more densely. - D cells constitute about 5% to 10% located peripherally in the islets. . D cells secrete somatostatin, which is contained in secretory granules (300 to 350 nm) and contain material of low to medium electron density. - The minor islet cells constitute about 5% of the islet tissue and may be equivalent to the pale cells seen after Mallory- Azan staining. - PP cells secrete pancreatic polypeptide. - Some cells may secrete more than one hormone in addition to glucagon in the A-cell cytoplasm, these include gastric inhibitory peptide (GIP), CCK, and adrenocorticotropic hormone (ACTH)- endorphin.

- Although there is no clear morphologic evidence for the presence of G cells (gastrin cells) in the islets, gastrin may also be secreted by one or more of the islet cells. - Certain pancreatic islet cell tumors secrete large amounts of gastrin, thereby producing excessive acid secretion in the stomach (Zollinger- Ellison syndrome). Photomicrographs of islets of Langerhans The distribution of the different peptide-producing cells within the islets of Langerhans. Beta cells (labeled for insulin) are the most numerous (A), and are situated in the central regions of the islet. Alpha cells (labeled for glucagon) are generally arranged around the periphery (B). Delta cells (labeled for somatostatin) (C) and PP cells (labeled for pancreatic polypeptide) (D) are much less numerous and do not display an obvious pattern of arrangement. Islet of Langerhans in human adult pancreas. This immunofluorescence image shows the islet of Langerhans and distribution of glucagon-secreted A cells (green) and insulin-secreted B cells (red) in the adult pancreas. Cells were counterstained with 4,6- diamidino-2- phenylindole (DAPI) stain that reacts with nuclear DNA and exhibits blue fluorescence over the nuclei. Note that B cells comprise most of the islet cells and A cells are scattered throughout the islets. 280. (From Scharfmann R, Xiao X, Heimberg H, Mallet J, Ravassard P. Beta cells within single human islets originate from multiple progenitors. PLoS ONE 2008;2:e3559.) Ultrastructural appearance of islets of Langerhans. The islets are circumscribed and separated from the adjacent acinar cells (left). The peptide granules are randomly distributed within the cytoplasm. Lipid inclusions (ceroid bodies) are found within the β cells. Ultrastructural appearance of islet cell granules. A. Alpha cell granules are round and contain an eccentric electron-dense core within a less dense peripheral region. There is a thin halo beneath the limiting membrane. B. Beta cell granules are polymorphous and contain crystalline cores with a wide halo beneath the limiting membrane. C. Delta cell granules are round with a moderately dense core surrounded by a very thin halo. D. The granules of PP cells are smaller and have homogenous hyperdense cores. Adult pancreatic islet cells: glucagon cell (bottom), insulin cell (upper right), somatostatin cell (left) are seen. Note the fenestrated capillary vessel at upper lef Extrainsular endocrine cells within the ducts and between the acini are only detectable with immunohistochemical labeling for chromogranin. A group of cells below the lining epithelium of the duct of Santorini at the minor papilla is immunoreactive for chromogranin A (immunoperoxidase technique) Characteristics of Pancreatic Hormones Functions of Pancreatic Hormones Insulin, decreases blood glucose levels. Its principal effects are on the liver, skeletal muscle, and adipose tissue. - Insulin has multiple individual actions in each of these tissues. - In general, insulin stimulates: • uptake of glucose. • storage of glucose by activation of glycogen synthase and subsequent glycogen synthesis. • phosphorylation and use of glucose by promoting its glycolysis. - Absence or inadequate amounts of insulin lead to elevated blood glucose levels and the presence of glucose in the urine, a condition known as diabetes mellitus. - Reduced expression of insulin and insulin growth factors in the central nervous system (CNS) has recently been linked to Alzheimer’s disease. . Insulin stimulates glycerol synthesis and inhibits lipase activity in adipose cells. - Circulating insulin also increases the amount of amino acids taken up by cells and inhibits protein catabolism. Functions of Pancreatic Hormones Glucagon, secreted in amounts second only to insulin, increases blood glucose levels. - Glucagon stimulates release of glucose into the bloodstream and stimulates gluconeogenesis and glycogenolysis (breakdown of glycogen) in the liver. - Glucagon also stimulates proteolysis to promote gluconeogenesis, mobilizes fats from adipose cells, and stimulates hepatic lipase.

Somatostatin inhibits insulin and glucagon secretion. pancreatic polypeptide stimulates gastric chief cells, inhibits bile secretion and intestinal motility, inhibits pancreatic enzymes and HCO3 secretion. Principal Cell Types in Pancreatic Islets

Minor Cell Types in Pancreatic Islets Regulation of Islet Activity

- A blood glucose level above the normal 70 mg/100 mL (70 mg/dL) stimulates release of insulin. - Some amino acids and increased blood fatty acid levels also stimulate insulin release. - CCK and glucagon, released in the islet by the A cells, act as paracrine secretions to stimulate B cell secretion of insulin. - Glucagon is also released in response to low levels of fatty acids in the blood. - Insulin inhibits release of glucagon by A cells. - The islets have both sympathetic and parasympathetic innervation. . About 10% of the islet cells have nerve endings directly on their plasma membrane. Well-developed gap junctions are located between islet cells. . Ionic events triggered by synaptic transmitters at the nerve endings are carried from cell to cell across these junctions. - Autonomic nerves may have direct effects on hormone secretion by A and B cells. . Parasympathetic (cholinergic) stimulation increases secretion of both insulin and glucagon; . sympathetic (adrenergic) stimulation increases glucagon release but inhibits insulin release. The islets of Langerhans contain alpha, beta, and delta cells that produce glucagon, insulin, and somatostatin, respectively. A fourth type of islet cell, the F (or PP) cell, is located at the periphery of the islets and secretes pancreatic polypeptide. These hormones regulate one another’s secretion through paracrine cell-cell interactions Feedback regulation of pancreatic islets. Note that glucose has opposite effects on insulin and glucagon secretion, and override of negative feedback is achieved by parasympathetic and sympathetic input as well as by the feed-forward input of glucagon-like peptide 1 The blood supply to the pancreas provides a cascading perfusion of the islets and acini. - Several arterioles enter the periphery of the islets and branch into fenestrated capillaries. . In humans, the capillaries first perfuse the A and D cells, peripherally, before the blood reaches the B cells, centrally. . Large efferent capillaries leave the islet and branch into the capillary networks that surround the acini of the exocrine pancreas. - This cascading flow resembles the portal systems of other endocrine glands (pituitary, adrenal). - Secretions of the islet cells have regulatory effects on the acinar cells: . Insulin, the vasoactive intestinal peptide (VIP), and CCK stimulate exocrine secretion. . Glucagon, pancreatic polypeptide (PP), and somatostatin inhibit exocrine secretion.