Normal Liver

The liver is a unique organ that has numerous structural and physiological functions. It is most important when discussing liver pathology that one understands first the normal liver histology before one can best understand the basic pathophysiologic concepts of the numerous liver diseases. The pathologist plays a fundamental role in assessing the various morphologic features seen in liver tissue, whether by fine needle aspirates, needle or wedge biopsies, partial hepatectomies, liver explants, or autopsy material. The pathologist also has not only routine but also numerous special histochemical and immunohistologic stains as well. Yet correlating the histologic findings with the most pertinent clinical and laboratory data enables the pathologist to better arrive at a diagnosis and the most pertinent differential possibilities. This introductory chapter addresses all aspects of the normal liver, reviewing the embryologic development, gross and microscopic features, the pertinent intracytoplasmic components and how their function varies with their location within the hepatic lobule, and the importance of stem cell function within the liver. Additionally the various useful stains and laboratory values will also be presented, as well as a brief outline of how best to organize pathologic readings and signouts of liver biopsy specimens.

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Embryology

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The hepatic primordium anlage initially appears at the end of the third week of gestation and is first seen as a hollow midline outgrowth stalk (hepatic diverticulum) of the endodermal epithelium at the distal aspect of the foregut. By the fourth week, the diverticulum enlarges from proliferation of the endodermal cell strands (hepatoblasts) and projects cranially into the mesoderm of the septum transversum, eventually giving rise to the liver hepatic parenchyma and intrahepatic ducts. The cephalic end ultimately develops into the right and left hepatic lobes, while the stalk between the diverticulum and foregut narrows and forms the extrahepatic biliary system and gallbladder. Solid cords are initially formed by proliferating endodermal cells. These eventually anastomose to form vesicles and cribriform tubules with centrally located lumenal structures (biliary canaliculi). The cords eventually merge and develop small channels and capillaries that subdivide the cords to eventually fo
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Gross Anatomy

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The adult liver weighs from 1200 to 1800 g, dependent on the overall body size, takes up the majority of the right upper abdominal cavity beneath the rib cage, and extends from the right lateral aspect of the abdomen 15–20 cm transversely toward the xiphoid process. Although the weight of the adult liver constitutes about 1.8–3.1% of the total body weight, at birth the liver is larger compared with adjacent thoracic and abdominal viscera and constitutes about 5–6% of the body weight. Anatomically, the liver has four lobes: right, left, caudate, and quadrate. The right lobe accounts for one-half to two-thirds of the total liver volume and is divided from the left lobe by the falciform ligament on gross inspection; however, functionally the right and left lobes are of about equal size and are divided by a line extending from the inferior vena cava superiorly to the middle of the gallbladder fossa inferiorly. A total of eight functional segments are present, each having its own vascular
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