Histology Chapter 15 Liver & Pancreas PDF

Histology – Chapter 15 Liver & Pancreas Materials Edited, added and improved by Prof. Wael Abdel-Rahman Hassan Based on: Wheater's Functional Histology: A Text and Colour Atlas, By: Barbara Young, John W. Heat & Resour...

Histology – Chapter 15 Liver & Pancreas Materials Edited, added and improved by Prof. Wael Abdel-Rahman Hassan Based on: Wheater's Functional Histology: A Text and Colour Atlas, By: Barbara Young, John W. Heat & Resources at ELSEVIER Chapter 15. Liver & Pancreas Liver and biliary system The liver is a solid organ composed of tightly packed, pink-staining plates of hepatocytes. The outer surface of the liver is covered by a collagenous tissue called Glisson's capsule over which is a layer of mesothelial cells from the peritoneum. The structural integrity of the liver is maintained by a delicate meshwork of extracellular matrix in the form of a fine meshwork of reticulin fibres continuous with Glisson's capsule. The hepatic lobule  The hepatic lobule This is the structural unit of the liver. It is hexagonal in shape centered on a terminal hepatic venule called centrilobular venule (or central vein). The portal tracts (or portal triads) are positioned at the angles of the hepatic lobule and contains three main structures: a terminal branch of the hepatic portal vein which, a terminal branch of the hepatic artery, and a bile ductules (which drains the bile canaliculus located within each plate of hepatocytes ). Lymphatics are also present in the portal tracts. The bile ductules merge to form larger, more centrally located trabecular ducts which drain via intrahepatic ducts into the right and left hepatic ducts, the common hepatic duct and then to the duodenum via the common bile duct. The hepatocytes are seen as anastomosing plates or double layered rows radiating from the central vein. In between these rows are the hepatic sinusoids receiving blood from both the hepatic portal and hepatic arterial systems. The sinusoids are lined by flat a discontinuous, fenestrated endothelium, which has no basement membrane and which is separated from the hepatocytes by a narrow space (space of Disse), which drains into the lymphatics of the portal tracts. Scattered among the endothelial cells are large plump phagocytic cells with ovoid nuclei known as Kupffer cells which form part of the monocyte-macrophage defence system. Hepatocytes are large polyhedral cells with round nuclei with peripherally dispersed chromatin and prominent nucleoli. The nuclei vary greatly in size. Binucleate cells are also common in normal liver. The layer of hepatocytes immediately bordering the portal tract is known as the limiting plate. The blood from the portal vein and hepatic artery branches flows away from the portal tract to the adjacent central veins. In some species, such as the pig the lobule is outlined by bands of fibrous tissue. In humans and most other species, no such clear structural definition exists, although lobules can be roughly outlined as an hexagonal array of portal tracts arranged around a terminal hepatic venule. The hepatic acinus is a more physiologically useful model of liver. The acinus is a roughly berry-shaped unit of liver parenchyma centered on a portal tract. The gall bladder It is a muscular sac lined by a simple columnar epithelium; it has a capacity of about 100 mL in humans.  Pancreas The exocrine pancreas The exocrine pancreas, which forms the bulk of the gland, secretes an enzyme-rich alkaline fluid into the duodenum via the pancreatic duct. The pancreas is a lobulated gland covered by a thin collagenous capsule which extends as delicate septa between the lobules. Each pancreatic acinus is made up of an irregular cluster of pyramid-shaped secretory acinar cells, the apices of which surround a minute central lumen which drain into intercalated ducts. The acinus contain two types of cells:  The acinus contain two types of cells: 1- The acinar cells are typical protein-secreting, pyramidal-shaped cells. The nuclei are basally located and surrounded by basophilic cytoplasm with rough endoplasmic reticulum; the apices of the cells are packed with eosinophilic secretory granules containing proenzymes. 2- The centroacinar cells sometimes seen in the centers of the acini as one or more nuclei of with pale nuclei and sparse pale-stained cytoplasm; these represent the terminal lining cells of intercalated ducts. The intercalated ducts drain into intralobular ducts. The intercalated ducts are lined by simple low cuboidal epithelium, which becomes stratified cuboidal in the larger ducts.  The endocrine pancreas The endocrine pancreas is formed of “Islets of Langerhans” scattered throughout the exocrine glandular tissue. The islets vary in size and are most numerous in the tail of the pancreas. The islets are composed of groups of up to 3000 secretory cells supported by a fine collagenous network containing numerous fenestrated capillaries. A delicate capsule surrounds each islet. The endocrine cells are small with a pale stained granular cytoplasm in contrast to the large cells of the surrounding exocrine pancreatic acini. The islets contain a variety of cell types each responsible for secretion of one type of polypeptide hormone: 1- The main secretory products of the endocrine pancreas are insulin (from beta cells) and glucagons (from alpha cells), polypeptide hormones which play an important role in carbohydrate metabolism. 2- Somatostatin (from delta cells) has a wide variety of effects on gastrointestinal function and may inhibit insulin and glucagon secretion, 3- The enterochromaffin (EC) cell, secretes different peptides including motilin, serotonin and substance P. 4- Other peptides: vasoactive intestinal peptide (VIP), pancreatic polypeptide (PP). The immunoperoxidase technique (immunohistochemistry) is able to differentiate between these cells through detecting specific intercellular products.