Figure 173

Diagram of a "classic" liver lobule. A "classic" liver lobule can be schematically diagramed as a six-sided polyhedral prism with portal triads (hepatic artery, portal vein, and bile duct) at each of the corners. The blood vessels of the portal triads send distributing branches along the sides of the lobule, and these branches open into the hepatic sinusoids. The long axis of the lobule is traversed by the terminal hepatic venule (central vein), which receives blood from the hepatic sinusoids. Note that a wedge of the tissue has been removed from the lobule for better visualization of the terminal hepatic venule. Interconnecting sheets or plates of hepatocytes are disposed in a radial pattern from the terminal hepatic venule to the periphery of the lobule.

cells radiate from the central vein to the periphery of the lobule, as do the sinusoids. At the angles of the hexagon are the portal areas (portal canals), loose stromal connective tissue characterized by the presence of the portal triads. This connective tissue is ultimately continuous with the fibrous capsule of the liver. The portal canal is bordered by the outermost hepatocytes of the lobule. At the edges of the portal canal, between the connective tissue stroma and the hepatocytes, is a small space called the space of Mall. This space is thought to be one of the sites where lymph originates in the liver.

In some species, e.g., the pig (Fig. 17.4a), the classic lobule is easily recognized because the portal areas are connected by relatively thick layers of connective tissue. In humans, however, there is normally very little interlobular connective tissue, and it is necessary, when examining histologic sections of liver, to draw imaginary lines between portal areas surrounding a central vein to get some sense of the size of the classic lobule (Fig. 17.4b).

The portal lobule emphasizes the exocrine functions of the liver

The major exocrine function of the liver is bile secretion. Thus, the morphologic axis of the portal lobule is the interlobular bile duct of the portal triad of the "classic" lobule. Its outer margins are imaginary lines drawn between the three central veins that are closest to that portal triad (Fig. 17.5). These lines define a roughly triangular block of tissue that includes those portions of three classic lobules that secrete the bile that drains into its axial bile duct. This concept allows a description of hepatic parenchymal structure comparable to that of other exocrine glands.

The liver acinus is the structural unit that provides the best correlation between blood perfusion, metabolic activity, and liver pathology

The liver acinus is lozenge shaped and represents the smallest functional unit of the hepatic parenchyma. The short axis of the acinus is defined by the terminal branches of the portal triad that lie along the border between two classic lobules. The long axis is a line drawn between the two central veins closest to the short axis. Therefore, in a two-dimensional view (Fig. 17.6) the liver acinus occupies parts of adjacent classic lobules. This concept allows a description of the exocrine secretory function of the liver comparable to that of the portal lobule.

The hepatocytes in each liver acinus are described as being arranged in three concentric elliptical zones surrounding the short axis (see Fig. 17.6).

• Zone 1 is closest to the short axis and the blood supply from penetrating branches of the portal vein and hepatic

portal canals

ML&f

mm mBk

CLASSIC LOBULE PORTAL L

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