31. 0 Gi: liver, gallbladder, pancreas



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31.0 GI: LIVER, GALLBLADDER, PANCREAS


  1. Define and use properly the following words: Liver, Parenchyma, Hepatocytes, Sinusoids, Sinusoidal endothelial cell, Stroma, Hepatic lobules, Hepatic artery, Portal Vein, Portal triad, Central veins, Hepatic vein, Bile canaliculi, Bile duct, Portal tracts, Polyhedral cells, Space of Disse, Kupffer cells, Gallbladder, Mucosa, Exocrine Pancreas, Septa, Lobule, Tubuloacinar gland, Serous acini, Acinar Cells, RER, Basophilia, Centroacinar cells, Intercalated ducts, Intralobular ducts, Interlobular ducts, Proteolytic enzymes.

  2. Describe and associate basic structure/function for the following: 1) all structures listed above; 2) major functions of the liver; 3) trace an erythrocyte through the liver from the portal vein or hepatic artery to the central vein; 4) trace bile from its formation to the small intestine; 5) structure of the hepatic lobule, portal lobule, and hepatic acinus; 6) lining epithelium of the gall bladder; 7) duct system of the exocrine pancreas versus the duct system of salivary glands; 8) three major secretory products of the exocrine pancreas and the function of each.

  3. Identify by microscopy: Liver, Hepatocytes, Sinusoids, Sinusoidal endothelial cell, Stroma, Hepatic lobules, Hepatic artery, Portal Vein, Portal triad, Central veins, Hepatic vein, Bile canaliculi, Bile duct, Portal tracts, Polyhedral cells, Space of Disse, Kupffer cells, Gallbladder, Mucosa, Exocrine Pancreas, Septa, Lobule, Tubuloacinar gland, Serous acini, Acinar Cells, RER, Basophilia, Centroacinar cells, Intercalated ducts, Intralobular ducts.



GI: LIVER and GALLBLADDER

I. Introduction and Anatomy of Liver and Gallbladder


  1. Both are extramural organs outside of the tubular GI tract.

  2. Liver is arranged into lobules.

  3. This is most prominent in the pig, which has CT septae.


II. Liver Organization


  1. Parenchyma

    1. Hepatocytes arranged into anastomosing plates or cords.

      1. radiate out from the central vein, which results in meshwork arrangement

      2. can appear hexagonal.

      3. the main parenchymal cells of the liver

    2. Each hepatocyte contacts at least one, usually two sinusoids (tremendous vascularity in this region.)




  1. Stroma

    1. Not much CT matrix in most species

    2. In the pig the CT helps to differentiate the different hepatic lobules.

    3. There are a lot of reticular fibers that make up the supporting network.




  1. Vasculature

    1. Dual blood supply

      1. Hepatic artery

        1. Brings in freshly oxygenated blood.

        2. Supplies only about 1/4 of the liver's blood supply.

      2. Portal Vein

        1. Originates from the GI organs (pancreas, intestines, stomach, and spleen).

        2. Thus, this blood won't have much oxygen in it.

        3. helps to bring the products of digestion to the liver to get filtered out here.

        4. 3/4 of the liver's blood supply comes from the portal vein




    1. These vessels occur in a branching connective tissue tree.

      1. The area where they come together is called the portal tract (triad)




    1. Both vessels open into liver sinusoids found next to the hepatocytes.

      1. These will drain into the central vein located in the middle of an hepatic lobule.




    1. The central veins eventually collect into the large hepatic vein

      1. The hepatic vein will then go into the caudal vena cava and back to the heart.


  1. Exocrine Ducts

    1. Bile initially secreted into channels (canaliculi) between adjacent liver cells.

      1. Canaliculi are membrane adaptations of the hepatocytes.

      2. Usually don't see bile canaliculi.

      3. Between adjacent plates of hepatocytes.

    2. Canaliculi connect with the ducts of Herring

    3. will then lead into the cellular ducts called bile ducts

    4. Function of bile

      1. to aide in fat absorption in the small intestine




  1. Portal tracts

    1. Located in the CT stroma

    2. Structures that are found here include:

      1. Hepatic artery

        1. thick walled with layers of smooth muscle

      2. Portal vein

        1. thin walled and possibly collapsed

      3. Bile duct

        1. lined by cuboidal epithelium

      4. Lymphatics

        1. Normally hard to see

        2. Thin walled and tend to collapse

        3. Won't see RBC in them.




    1. Another name for it is the Portal triad, which refers to 3 structures in it:

      1. Hepatic artery

      2. Portal vein

      3. Bile duct




  1. The liver lobule

    1. Pig liver shows the lobules the best.

    2. May not see the lobular structure as well in animals that don't have much CT septae

      1. eg. the cat


III. Cytology of Hepatocytes


  1. Polyhedral with 3 types of surfaces.

  2. Organelles that they have an abundance of include:

    1. Mitochondria because need a lot of energy

    2. Lysosomes to help break down toxicants, pesticides, etc.

    3. SER which is involved in lipid synthesis and detoxification

    4. RER for protein synthesis




  1. General characteristics

    1. Multipurpose cell

      1. produce secretions (including bile)

      2. Filter things out of the blood

    2. These cells can be binucleated

    3. Polyhedral shape

    4. They have one surface that faces the sinusoids

  2. Bile canaliculi

    1. Between hepatocytes

    2. Hepatocytes have one surface that faces it

    3. Usually don't see bile canaliculi that well in LM.

  3. Gap junctions common between hepatocytes.


IV. The Cells of the Liver Sinusoids


  1. Endothelium

    1. NO basement membrane

      1. allows easier movement in and out of the liver

    2. Space of Disse

      1. an actual space (not an artifact)

      2. between the sinusoids and hepatocytes

        1. Kupffer cells can be found

    3. Endothelium is discontinuous (fenestrated).

      1. Thus, the sinusoids are very leaky

      2. Materials can easily pass to and from the hepatocytes into the sinusoids




  1. Kupffer cells

    1. Phagocytic and mobile cells

    2. Found in the Sinusoids by the endothelium or in the Space of Disse

    3. Not easy to visualize by normal light microscopy

    4. To identify

      1. inject dye (eg. India ink or trypan blue) and the Kupffer cells will ingest these carbon particles

      2. will result in them staining intensely (usually dark blue)




  1. Fat storing cells

    1. located in the Space of Disse

    2. Very little lipid content normally

    3. Can have increases in lipid with pathological diseases (eg. fatty liver)


V. Liver Functions


  1. The liver plays a very crucial role in metabolism, digestion, and maintaining homeostasis

  2. Add albumin to blood

  3. Removes steroids and other compounds

  4. Detoxification



VI. Gallbladder Histology


  1. General salient features

    1. It is intimately and functionally associated with the liver

    2. Many students confuse it with the pyloric stomach or small intestine

    3. At the low mag view

      1. note that it is a very thin walled organ




  1. Mucosa organized into high folds in some species

    1. eg. dog and cat

    2. This helps histologists to identify it

    3. One will see different sections of these folds

      1. The folds will also have open areas

      2. actual infolding of the mucosa

  2. Lamina propria

    1. Thin

    2. NO muscularis mucosa

  3. Smooth muscle layer

    1. mostly circular fibers

  4. Outer serosa


THE EXOCRINE PANCREAS
I. Morphology pancreas


  1. It varies grossly.

    1. Can appear banana or star shaped.

  2. Histologically very similar.


II. CT Capsule pancreas


  1. Septa pass from the capsule into the gland.

    1. Divide into lobes.

  2. Lobule groups

    1. consist of numerous primary lobules

    2. surrounded by more extensive CT


III. Compound Tubuloacinar Gland


  1. Organized into serous acini

  2. Acinar Cells

    1. Broad basally and narrow apically.

    2. Pyramidal shape.

    3. Have an abundance of RER at base of cell.

      1. They need a lot of RER because they are producing enzymes (proteins).

      2. RNA gives basophilic appearance near the basement membrane.



    1. Closest to the apical surface they have an abundance of zymogen granules.

      1. storage form of the pancreatic enzymes.

      2. an acidophilic appearance near the apex (closest to the lumen).




    1. cells are basophilic closest to the basement membrane and acidophilic near the lumen.

    2. The acinar cells will define the lumen of the gland.




  1. Centroacinar cells.

    1. The initial portion of the pancreatic duct system

      1. actually extends into the acinus.

    2. Smaller than the acinar cells.

    3. Squamous to cuboidal epithelial

    4. Stain very lightly (pale staining.)

    5. Will be located in the center of the acinus.

      1. Thus, usually surrounded by the zymogen granules of the acinar cells.

    6. Form the beginning of the intercalated ducts

    7. Secrete bicarbonate ion


IV. Duct System of the Pancreas


  1. The centroacinar cells form the termination of the intercalated ducts.

    1. Intercalated duct epithelium secretes bicarbonate ion to help neutralize the acid coming from the stomach.

  2. Intralobular ducts

  3. Interlobular and main ducts


V. Function of Exocrine Pancreas


  1. Duct cells secrete sodium bicarbonate and water.

    1. Help to neutralize HCl from the parietal cells before the acid reaches the small intestine.

    2. Secretions of the acinar cells.

      1. Proteolytic enzymes

        1. involved in digestion of fats, carbohydrates, and proteins.







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