LIVER (slides 137,139,140 and 141)

In some ways the liver is structurally an endocrine gland, with the movement of stored substances, such as glucose from glycogen stores, taking place directly between the parenchymal cells and the blood sinusoids. On the other hand, the secretion of bile is via ducts, making the liver an exocrine gland. Read about the blood supply and drainage of the liver. The histological classification of microscopic lobules is based upon the arrangement of parenchymal cells in relation to the finer branches of the hepatic artery, hepatic vein, portal vein, and bile duct.

Look first at slide 137, a section of pig liver, which contains much more connective tissue than the human liver and thus defines the boundaries of the classical liver lobule most clearly. If your slide is stained in H&E, compare later with a neighbor's slide stained with Mallory's connective tissue stain (and vice versa). Consult a textbook diagram and then locate, under low power, several central veins and portal areas (portal triads, portal canals). Under high power, identify the branches of the hepatic artery, portal vein and bile duct that typically run together in the septal portal canals. In what direction does the blood flow through a liver lobule as defined here?

Note that the epithelial parenchymal cells are arranged in anastomosing and branching plates or cords. Between these cords lie the sinusoids containing rbc's. In the Mallory preparation you will notice very fine blue connective tissue lying between the epithelial cords and the sinusoids, thus forming the stroma of the liver. In the H&E preparation the nuclei of endothelial cells lining the sinusoids are clearly visible. Nuclei may also belong to special stellate cells of  Kupffer which span the sinusoidal lumen. As part of the macrophage system of the body, Kupffer cells function as phagocytes if necessary. In some slides they are loaded with black particles.

While looking at sinusoids, find an area where you can see them emptying into a central vein, and notice the endothelial lining as it continues over all luminal surfaces. With electron microscopy we know that the lining of the sinusoids is discontinuous, including the basal lamina. Where else in the body have you seen sinusoids with loosely aligned endothelial cells? What is the space of Disse that is seen in  electron micrographs of liver? Consider the location of this space as it would relate to what you can see in light microscopy.

Before leaving this slide, look in low power for any portion of the dense connective tissue (Glisson's) capsule which may be present on one or two surfaces of your section. Consider also the boundaries used  in  demarcating  two  other  classifications of hepatic lobule than the classical one already defined;

1.            the portal lobule, using the portal canal as its center, and

2.            the liver acinus, or functional unit, which is used by pathologists in discussing liver damage.

NOTE: Don't forget to compare slide 137 with your neighbor's so as to see the lobular arrangement of the pig liver in both Mallory's and H&E.