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Organs Associated with the Digestive Tract SUMMARY OF KEY POINTS Salivary Glands

  • Salivary glands have secretory units of either protein-secreting serous cells, usually organized in round or oval acini, or of mucin-secreting mucous cells in elongated tubules.

  • Parotid glands have only serous acini; sublingual glands are mixed but have primarily mucous tubules, some with serous demilunes; submandibular glands are also mixed but have mainly serous acini.

  • Salivary secretory units are drained by simple cuboidal intercalated ducts, which merge as simple columnar striated ducts, which merge further as the larger interlobular or excretory ducts.

  • Cells of striated ducts have mitochondria-lined, basolateral membrane folds specialized for electrolyte reabsorption from the secretion; excretory ducts are unusual in having stratified cuboidal or columnar cells.

  • Pancreatic islets of endocrine cells are embedded in exocrine serous acinar tissue, which comprises most of the pancreas and in which the cells secrete hydrolytic digestive enzymes for delivery to the duodenum.

  • Each pancreatic acinar cell is pyramidal, with secretory (zymogen) granules in the narrow apical end and Golgi complexes, much rough ER, and a large nucleus at the basal end.

  • Intercalated ducts draining pancreatic acini, including their initial centroacinar cells that insert into the acinar lumen, secrete bicarbonate ions (HCO3) to neutralize chyme entering the duodenum from the stomach.

  • Liver hepatocytes are large epithelial cells with large central nuclei (polyploid and often binucleated), much smooth and rough ER, and many small Golgi complexes.

  • Hepatocytes have many functions, including endocrine (plasma protein secretion), exocrine (bile secretion), glucose storage (glycogen granules), and detoxification (using SER and peroxisomes).

  • In the liver, hepatocytes are organized into irregular plates to form polygonal hepatic lobules in which the hepatocyte plates radiate toward a small central vein.

  • Each hepatic lobule is surrounded by sparse connective tissue that is more abundant in the portal areas at the corners.

  • Portal areas or tracts contain a small lymphatic and the portal triad: a portal venule branch from the portal vein, a hepatic arteriole branch of the hepatic artery, and a bile ductule branch of the biliary tree.

  • In the lobules the portal venule and hepatic arteriole both branch into irregular sinusoids between the hepatic plates where the nutrient-rich and O2-rich blood mixes, flows past hepatocytes, and drains to the central vein.

  • The endothelium of the hepatic sinusoids is discontinuous and fenestrated; between it and the hepatocytes is the perisinusoidal space (of Disse) where exchange occurs between the hepatocytes and blood plasma.

  • The sinusoidal endothelium includes many specialized stellate macrophages or Kupffer cells, which recognize and remove effete erythrocytes, releasing iron and bilirubin for uptake by hepatocytes.

  • Also present in the perisinusoidal spaces are hepatic stellate cells (or Ito cells) containing many small lipid droplets for storage of vitamin A and other fat-soluble vitamins.

  • Between adherent hepatocytes in the hepatic plates are grooves called bile ...

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