Liver

The human liver is arranged into lobules that are indistinct. In the center of a liver lobule is a central vein that drains out to hepatic veins. The edges of the lobules are defined by portal triads, each of which contains a small hepatic artery branch, a portal vein branch, and a small bile duct. There are also some scattered lymphocytes and connective tissue in the triads. A fine network of reticulin fibers supports the lobule. The hepatocytes of the lobule are arranged into three dimensional branching plates (two dimensional cords as seen in histologic sections).

The hepatocytes bordering the triad make up the limiting plate. The cords are generally two cells thick, but branching, with a small bile canaliculus running between them to coalesce into bile ducts. Running between the cords are sinusoids supplied with portal blood that drain to the central vein of the lobule. The sinusoids are lined by endothelial cells. There are also scattered Kupffer cells in the sinusoids; these cells are part of the mononuclear phagocyte system composed of fixed macrophages.

The hepatocytes are complex, metabolically active cells with numerous functions: lipoprotein metabolism, gluconeogenesis, glycogenolysis, detoxification of wastes, excretion of bile, etc. The hepatocytes store abundant glycogen.

Biliary Tract

The larger bile ducts in the hepatic triads coalesce to intrahepatic bile ducts that become the right and left hepatic ducts that fuse at the undersurface of the liver to become the common bile duct. About halfway down the common bile duct, the cystic duct branches off to the gallbladder. The intrahepatic ducts, cystic duct, and the common bile duct are lined by a tall columnar epithelium.

The gallbladder stores bile excreted from the liver. The columnar mucosa is arranged in folds over the lamina propria, allowing expansion. Beneath the lamina propria is a muscularis, and surrounding the gallbladder is a connective tissue layer and serosa. The gallbladder mucosa transports out sodium in the bile, passively followed by chloride and water. Thus, bile excreted by the liver and stored in the gallbladder becomes more concentrated. The muscularis of the gallbladder, contracts under the influence of the hormone cholecystokinin excreted by enteroendocrine cells of the small intestine.

Pancreas

The pancreas consists of two separate sets of tissue. The exocrine pancreas comprises over 99% of the mass and is composed of the acinar cells that secrete into a duct system that drains into the pancreatic duct terminating at the ampulla of Vater. The endocrine pancreas consists of scattered islets of Langerhans that have cells secreting hormones into the bloodstream. The exocrine acini are arranged into lobules. The acini connect to indistinct intercalated ducts, lined by low cuboidal epithelium, which drain into intralobular ducts that coalesce to interlobular ducts that drain into the main pancreatic duct. Thin connective tissue septae with arterioles and venules separate the lobules. Admixed with the lobules toward the periphery of the pancreas are variable numbers of steatocytes. The pancreatic acini are composed of cells that have zymogen granules. Products of the exocrine pancreas include secretions containing bicarbonate and enzymes including lipase, amylase, trypsin and chymotrypsin.