Hepatic Biopsy – Open

Normal Anatomy

Several structural and functional units have been proposed in the microanatomy of the liver. Of these, the most relevant for the diagnostic histopathologist are the liver lobule and the liver acinus. For several years the liver acinus was the preferred concept in textbooks of pathology and hepatology, as it appeared to better explain the “bridging” type of lesions connecting afferent and efferent vascular landmarks: portal–central bridging necrosis and fibrosis. The simple liver acinus corresponds to the clump of parenchyma vascularized by a single terminal branch of the portal vein (portal venule) and hepatic artery, and drained by the neighboring terminal twigs of the hepatic vein (terminal hepatic venules). In spite of its popularity in recent years, the liver acinus did not stand the scrutiny of further detailed angioarchitectural, morphologic and histochemical investigations, and virtually all later studies support the lobular concept of liver architecture, albeit with several types of subunits.

The classic hexagonal liver lobule of Kiernan is equivalent to the more recently described secondary lobule of Matsumoto et al., which itself is composed of six to eight primary lobules. Whereas the acinus is centered on the “afferent” portal tract, the classic liver lobule is surrounded by (on average) six portal tracts and centered on a terminal twig of the hepatic vein: the centrolobular (or central) vein. The portal vein ramifications in the portal tracts give off a series of branches in the plane between adjacent portal tracts; these in turn give rise to sinusoids that drain the blood toward the center of the lobule. The lobular equivalents of the acinar zones 1, 2 and 3 are the periportal zone, midzonal area (or midlobular zone), and centrolobular area.

The liver parenchymal cell or hepatocyte is a polygonal cell with a mostly central, single nucleus. Hepatocytes are arranged in plates one cell thick (muralia) with a sinusoid on either side, thus exposing each hepatocyte to portal blood on two surfaces. Within the muralium each hepatocyte adjoins adjacent parenchymal cells with its intercellular surface. This intercellular domain of the cell membrane carries a groove (hemicanaliculus) as a girdle around the cell. The hemicanaliculi of two adjacent hepatocytes form the intercellular bile canaliculus, which is separated from the rest of the intercellular space and the blood compartment by tight junctions.

The virtual cleft between the sinusoidal lining cells and the sinusoidal domain of the hepatocyte surface is the space of Disse. The hepatic sinusoids are lined by different cell types. The sinusoidal endothelial cells are of the fenestrated type. Kupffer cells bulge out on the luminal side of the sinusoids; they are resident members of the mononuclear phagocyte system that undergo enlargement and proliferation in response to a wide variety of stimuli. Hepatic stellate cells (previously also termed Ito cells, fat-storing cells or lipocytes) are contractile perisinusoidal “pericytes” located in the space of Disse. They store vitamin A in their cytoplasmic lipid droplets and are able to switch their phenotype to myofibroblastlike cells under appropriate stimulation. They are matrix producers in normal liver and in pathologic intralobular fibrosis. Hepatic stellate cells are only identified with difficulty in routinely stained preparations; immunostaining for synaptophysin marks both resting and activated cells, and immunostaining for alpha-smooth muscle actin reveals activated stellate cells.

Located on the inside of endothelial cells are some scattered “pit cells”, which correspond to resident intrahepatic lymphocytes, to a large extent of the natural killer phenotype.

The portal tracts at the lobular periphery are composed of connective tissue, ensheathing branches of the hepatic artery, portal vein, bile duct, and lymphatics. The caliber of portal tracts and their composing structures decreases from the hilum of the liver toward its periphery. This explains why surgical liver biopsies and autopsy specimens usually contain larger portal areas than needle biopsy specimens.

Percutaneous needle biopsies may preferentially sample peripheral tissue, the microarchitectural variability of which is greater than at deeper sites within the liver. Portal dyads (with only two of the three profiles: artery, vein, and bile duct) are almost as common as portal triads in normal peripheral liver tissue. However, due to the presence of more than one branch of artery, vein, or duct, the average number of profiles per portal tract is 6 ± 5. On average, one may expect two interlobular bile ducts, two hepatic arteries, and one portal vein per portal tract.

Interlobular bile ducts, usually located in the center of the portal tract, are connected with the lobular canaliculi by ductules (or cholangioles) and canals of Hering, the latter being short stretches lined by both hepatocytes and cholangiocytes. These finer structures are hardly visible on hematoxylin–eosin (H&E) staining but clearly revealed by cytokeratin immunostains.

Liver cell plates and sinusoids have a more regular radial arrangement in the center of the lobule, whereas the periportal zone is characterized by a less regular, anastomosing network pattern of these structures.

Hepatocytes are polygonal in shape, with clearly outlined margins in H&E preparations. The cytoplasm is granular and eosinophilic, with basophilic aggregates of rough endoplasmic reticulum in a perinuclear distribution. The nucleus is centrally placed and carries one or more easily identifiable nucleoli. Mitotic figures are very rare. Hepatocytes are rich in glycogen, with diurnal and diet-related variations in amount and distribution. Glycogen accumulation in nuclei occurs as clear vacuoles, predominantly in periportal hepatocytes. This is common in childhood and may be conspicuous in some adult diseases (for example diabetes and Wilson disease) but is not by itself of diagnostic significance. Lipofuscin occurs as fine, light brown, periodic acid–Schiff (PAS)-positive and acid-fast granules in the pericanalicular region of centrolobular hepatocytes. Stainable iron is absent or demonstrable in only scant amounts. An occasional hepatocyte in normal liver may contain fat. The perisinusoidal space of Disse is not seen in biopsy material, but appears dilated in autopsy specimens.

There is some variation in hepatic microscopic appearance with age. In neonates, extramedullary hematopoiesis may persist during the first weeks of life. In children below the age of 5 years, liver cell plates are two cells thick, whereas in adulthood twin cell plates indicate regeneration. With increasing age, there is more variation in nuclear size (anisokaryosis), somewhat more pronounced centrolobular lipofuscinosis (“wear and tear pigment”), some atrophy of centrolobular hepatocytes and correspondingly broader appearance of perivenular sinusoids, possible hyalinization of the wall of hepatic artery branches, slight infiltration of macrophages and a few lymphocytes in some (not all) portal tracts, insignificant or minimal amounts of stainable iron in hepatocytes, and thickening of the liver capsule with some fibrous extensions reaching for 1–2 mm into the underlying parenchyma.

Physiologic postnatal liver development comprises three structural components: enlargement of the hepatocytes and expansion and multiplication of the liver lobules. Enlargement of the lobules is exclusively responsible for the regenerative liver growth following partial hepatectomy; the number of hepatic lobules does not change during this latter reaction, but they gain a more complex, irregular structure.