DIGESTIVE SYSTEM III continued

5. The liver cells or hepatocytes are arranged in an interconnecting network of plates that are one or two cells thick.

a. The sinusoids lie between these plates allowing the blood to percolate between adjacent plates. b. As blood passes through the sinusoids, nutrients, toxins, other wastes are transferred to the hepatocytes where they are processed. c. Eventually the blood that passes through the lobule tissues empties into the central collecting vein and is returned to the large hepatic veins that exit the liver. d. From there the blood will empty into the inferior vena cava and be returned to the heart.

5. The fact that the blood enters the lobule from its periphery, passes through the tissues and empties into a central vein means that the blood composition and the composition of the hepatocytes will show a gradient. a. There will be higher concentrations of both nutrients and systemic tissue wastes in more peripheral hepatocytes of the lobule and a lower concentration in more central hepatocytes. b. Differences such as these are responsible in part for the fact that the physiology of the peripheral hepatocytes is different than that of the central hepatocytes.

D. Liver cells - hepatocytes - functional cells of the liver 1. As seen with the light microscope. a. Polyhedral, six or more surfaces. b  m in "diameter” c. One or 2 nuclei. d. Cytoplasm is eosinophilic when stained with H&E. This is due to large numbers of mitochondria and some smooth ER. e. Surface of each cell is adjacent to blood sinusoid walls with a space between the liver cell and the endothelial cells - called space of Disse.

2. As see with the electron microscope - ultrastructure a. Central nucleus b. Numerous mitochondria c. Abundant rough and smooth ER S.C. Kempf 2/14/00 Figure in web notes

S.C. Kempf 2/14/00 d. Perinuclear golgi bodies, lipid droplets, glycogen granules, lysosomes, peroxisomes e. Sometimes autophagosomes are present f. The plasmalemma of hepatocytes forms microvilli that extend into the space of Disse. g. Reticular supportive fibers are present on the hepatocyte side of endothelial cells.

S.C. Kempf 2/14/00 h. Where adjacent hepatocytes abutt, there are small canals (bile canaliculi) that are formed from invaginations of the plasmalemma of adjacent cells. i. Microvilli extend into these canals. j. Bile acids are secreted into the canaliculi. k. synthesized blood proteins are exocytosed into the space of Disse

k. The canaliculi form an anastomosing network that empties into the bile ductules (Hering's canals). l. The bile ductules empty into the the interlobular bile ducts of the triads and eventually these connect to the hepatic ducts that empty into the common bile duct.

E. Another common cell type found associated with the hepatocytes and the sinusoids are the Kupfer cells. 1. Specialized macrophages (phagocytes) that are derived from monocytes. 2. Phagocytose worn out erythrocytes, bacteria and particulate matter. F. The other common cell type in the liver is the endothelial cells that line the sinusoids.

3. Functions of hepatocytes a. Protein synthesis * Albumin, prothrombin, fibrinogen which are components of blood plasma * Interesting fact is that these proteins do not accumulate in the cytoplasm of hepatocytes in groups of vesicles. Rather, the vesicles are exocytosed into the blood as synthesis occurs. b. Bile acid synthesis * Only 10% of bile acids released from the common bile duct at any given time were actively synthesized and released into the bile canaliculi. * The other 90% are recycled from the intestine as components of the blood. These are absorbed by hepatocytes and re-introduced into the bile canaliculi.

c. Storage of metabolites. * Lipid and carbohydrate in form of glycogen are stored in hepatocytes. * This is reserve energy for use between meals. * Also vitamin storage. d. Conversion of lipids and amino acids to glucose. e. Amino acid deamination that results in production of glucose and lipid (from non-nitrogenous parts) and urea (from the removed amine group). f. Detoxification & inactivation - various drugs and toxic substances can be inactivated by hepatocytes

E. Gall bladder 1. This is a storage organ for bile secretions of the liver. 2. The gall bladder also concentrates bile by reabsorption of water. 3. Connects to common bile duct via cystic duct.

4. The wall of the gall bladder shows some similarity to the structure of the digestive tract in general, but lacks a submucosa. a. has mucosa composed of columnar epithelium and lamina propria. b. Muscularis externa c. well developed serosa or adventitia depending on location however, d. no muscularis mucosae e. lacks a submucosa f. smooth muscle layer (muscularis externa) - inner layer is longitudinal, outer layer circular

5. Mucus secreting tubuloacinar glands can be found near the connection of the gall bladder with the cystic duct. a. These are formed as epithelial invaginations into the lamina propria, - sort of like crypts of Lieberkuhn. 6. The smooth muscle (muscularis externa) surrounding the mucosa contracts to expel stored bile into the intestine via the common bile duct. a. This contraction is caused by the hormone cholecystokinin that is produced by enteroendocrine cells in the crypts of the small intestine. 7. Gall bladder contents are expressed into a dialated portion of the duodenum called the ampulla of Vater. 8. Flow of bile out of the common bile duct is controlled by the sphincter of Oddi, a ring of smooth muscle that surrounds the common bile duct where it connects to the ampulla of Vater. ampulla of Vater