There are three ways of describing the liver parenchyma in terms of a functional unit, "classic" lobules, portal lobules, or acini (see Fig. 17.5, page 538). The classic lobule is a roughly hexagonal block of tissue that has at its center the terminal hepatic venule (central vein) and at its six corners the portaI canals (portal triads) containing in each a branch of the portal vein, hepatic artery, and bile duct. The portal lobule is a triangular construct that emphasizes the exocrine secretory function. It has as its axis the bile duct of the portal triad of the classic lobule, and its outer margins are imaginary lines drawn between the central veins closest to that portal triad. The liver acinus provides the best correlation among blood perfusion, metabolic activity, and liver pathology. The acinus is a small diamond- or lozenge-shaped mass of tissue that has as its short axis the fine branches of the portal triad that lie along the border of two classic lobules and as its long axis a line drawn between the two central veins closest to the short axis. The hepatocytes in each acinus are described as arranged in three concentric elliptical zones around the short axis; zone 1 is closest to, and zone 3 is farthest from, the axis.

Figure 1, liver, human, H&E X500; inset x800.

The central vein and surrounding hepatocytes from Figure 2 of Plate 61 are shown here at higher magnification. The cytoplasm of the hepatocytes in this specimen has a foamy appearance because of extraction of glycogen and lipid during tissue preparation. The boundaries between individual hepatocytes are discernable in some locations but not between those cells where the knife has cut across the boundary in an oblique plane. Frequently, when cell boundaries are observed at still higher magnification (inset), a very small circular or oval profile is observed midway along the boundary. These profiles represent the bile canali-culi (BC).

The cells that line the sinusoids (S) show little, if any, cytoplasmic detail in routine preparations. Perisinusoidal macrophages (Kupffer cells f KC)) are generally recognized by their ovoid nuclei and the projection of the cell into the lumen. The endothelial cell, in contrast, is a squamous cell that has a smaller, attenuated or elongated nucleus. Some nuclei of this description are evident in the micrograph.

The termination of two of the sinusoids and their union with the central vein (CV) is indicated by the curved arrows. Note that the wall of the vein is strengthened by connective tissue, mostly collagen, which appears as homogeneous eosin-stained material (asterisks). Fibroblasts (F) within this connective tissue can be identified and distinguished from the endothelial cell (EN) lining of the vein.

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