Digestive System Associated Glands

Liver: Histology

1. Is the primary metabolic organ in the body and the largest gland

4. It serves as an interface between the GI (extracts nutrients) and the blood (which distributes nutrients)

5. The liver is in the optimal position in the circulatory chain for accumulating and metabolizing nutrients and elminating / neutralizing toxins

6. 2. 2. synthesis and secretion of bile

   3. Metabolic functions:

      5. Urea formation (from ammonium ions). When break apart proteins, ammonia is byproduct.

Liver gross anatomy:

5. Weighs about 3 lbs

6. A short transverse fissue (portal hepatis) partially separates the L and R lobes posteriorly and is the site of entry / exit of blood vessels, lymphatics, nerves, and ducts.

3. The liver has a dual blood supply, receiving blood from both arterial and venous sources.

4. 75% of the blood supply of the liver comes from the hepatic portal vein from capillaries of the GI, spleen, pancreas.

9. 25% of the blood is derived from the L and R hepatic arteries, branches of the celiac trunk of the abdominal aorta.

   1. The hepaic arteries give rise to smaller distributing arterioles and inlet arterioles

12. 1. Sublobular veins and collecting veins

    2. Eventually form the right and left hepatic veins that exit the liver

3. 1. This contains the major vessels of the liver: hepatic artery, hepatic portal vein, bile duct, lymph vessels.

   2. The Triad: incoming hepatic arteriole, incoming hepatic portal venule, bile duct (can see very well due to large cuboidal cells in wall)

Hepatocytes of the lobule are arranged in plates or cords separated by sinusoids.

1. the center of each lobule is pierced by central vein which drains blood form the sinusoids.

1. 1. Defined by hexagonal shaped parenchymal region and bounded by portal tracts at each corner of the hexagon, with a central vein in its center.

   2. Blood flows from perimeter of lobule, run through sinusoids, drain into central veins. The excretory product of the liver (bile) flows in opposite direction- it flows outward toward the bile duct portion of triad. Blood goes from periphery to center, bile goes from center to periphery.

2. 2. 2. A triangular region of hepatocytes with a central vein at the apices of the triangle and the portal triad (with accompanying bile duct) at the center

      3. Bile flows from the periphery to the center of portal lobule into the interlobular bile duct.

3. 4. The meridian of the diamond is traversed by pre-terminal branches of a distributing arteriole which supplies blood to the sinusoids.

   5. Can divide the hepatocytes into 3 zones:

1. receive excellent supply of oxygenated blood and nutrients

2. minimally exposed to metabolic waste

1. receives poorly oxygenated blood

2. high exposure to metabolic waste

3. The sinusoids form a continuous vascular channel within the lobule which ultimately empties in the central vein at the center of the lobule.

4. Hepatocytes are not in direct contact with the sinusoids but are separated from them by a thin perisinusoidal space = space of Disse:

   5. Perisinusoidal cells of Ito

      1. Irregular cells with long cytoplasmic processes

1. the endothelial cells are discontinuous and without a continuous basal lamina.

2. the membrane of the endothelial cells is riddled with clusters of small holes = sieve plates.

2. Derived form mononuclear phagocytic line.

3. These cells phagocytose damaged red blood cells and plasma debris.

1. 3. The cells are large polygonal shaped cells which are packed together to form thin plates / cords of the liver parenchyma.

   4. They exhibit variation in structure depending on their position within the liver lobule. Smooth ER would be greater in zone 3 for detox etc.

2. 4. prominent golgi (for molecular packing)

   5. numerous peroxisomes, lysosomes, endosomes.

   7. 3. Methylation, conjugation, and oxidation are the most common methods for drug inactivation and detoxification performed by the hepatocyte SER.

      4. The cytochrome P-450 system is the major enzyme system in the liver involved in drug metabolism.

      6. 1. When give drug orally, it gets into vasculature by GI absorption.

         2. That blood which carries the drug will first go to the liver. This is unfortunate since the liver is most capable organ for destroying the drug.

1. 2. Those that border the sinusoids = sinusoidal domain

2. Lateral domain:

   1. 4. This goes throughout entire liver cord to form a fluid filled channel into which bile can be excreted.

      5. The walls of the liver cell membrane at this point have microvili which pariticpate in exocytosis.

      10. Short microvilli protturde from the lumenal surface of the canaliculi.

   2. Bile canaliculi form a ring around hepatocytes and connect with those of adjacent hepatocytes to form a complex branched network which converges on the portal region of the lobule.

1. surface of the hepatocyte that is adjacent to the sinusoid (and space of Disse).

2. hepatocytes are not in direct contact with the sinusoid epithelia but are separated from it by the space of Disse.

Gall bladder

Histology of the gall bladder:

1. Gall bladder is a hollow pear shaped organ attached to the posterior – inferior surface of the liver

6. The wall of the gallbladder has 5 layers.

   1. Lumenal surface: a simple columnar epithelium composed of

   4. A layer of perimuscular CT

   5. A thin outer simple squamous epithelium (serosa).

1. 3. Excretion of excess cholesterol synthesized in the liver

   4. Elimination of heme breakdown products also removed with bile

6. Bile empties into the duodenum at an expanded junction with the common bile duct = ampulla of vater. (location of sphincter ampullae)

Gallstones:

1. the pancreas is another accessory GI organ that promotes digestion in the GI tract by supplying enzymes that assist in the digestion of proteins, fats, carbohydrates.

2. is an oblong tubuloacinar gland nestled within the curvature of the duodenum.

1. 1. synthesize / secrete pancreatic enzymes.

   2. They empty into specific duct system which empties into the duodeneum.

4. 2. 3-4 centrally located low cuboidal centroacinar cells, which form the beginnings of the duct system.

1. the centroacinar cells lead to the initial duct of the duct system leaving a secretory unit = intercalated ducts.

Pancreatic acinar cells: pyramidal shaped cells with an ultrastructure typical of a cell polarized for protein synthesis and secretion. These cells are somewhat polarized.

1. Basal region:

2. Apical region:

   1. Numerous secretory zymogen granules whose number fluctuates with the state of fasting (are acidophilic)

1. enzymes are secreted as inactive proenzymes which are activated by trypsin in the duodenum.

2. pancreas is protected from autodigestion from premature activation of proteolytic enzymes in the acinus or ducts by a trypsin inhibitor secreted by the acinar cells.

3. The nuclei of these cells can be observed in the central lumen of the acini and are diagnostic for pancreatic tissue.

4. these cells along with other duct cells secrete bicarbonate ions which maintain the duodenal pH within limits appropriate for pancreatic enzyme function.

2. basal surface of acinar cell contain CCK receptors.

3. Centroacinar cells: the secretion of centroacinar cells is regulated by levels of the hormone secretin (also by GI motility) produced by duodenal epithelium.

Liver disease:

1. hepatitis:

   5. Treatment: anti viral drugs (interferon and ribaviron).

   6. Most common is type C. types include: A,B,C,D,E
2. 3. Associated with alcoholism, chronic hepatitis, bile obstruction

   4. Drainage of blood and bile disrupted resulting in:

   5. Is the most common liver disease.

3. Inherited diseases of the liver:

   2. Hemochromatosis:

      1. Inherited disease of iron overload

1. 1. Aspartate aminotransferase: AST

   2. Alanine aminotransferase: ALT

1. reduced protein synthesis

   1. decreased albumin synthesis leads to decreased osmotic pressure in the blood, resulting in edema

3. decreased bile secretion

   1. the array of hepatic cords have been disturbed.