The bile canaliculus is a small canal formed by apposed grooves in the surface of adjacent hepatocytes.
The smallest branches of the biliary tree are the bile canaliculi into which the hepatocytes secrete bile. They form a complete loop around four sides of the idealized six-sided hepatocytes (Fig. 18.14 and Plate 66, page 658). They are approximately 0.5 m in luminal diameter and are isolated from the rest of the intercellular compartment by tight junctions, which are part of junctional complexes that also include zonulae adherentes and desmosomes. Microvilli of the two adjacent hepatocytes extend into the canalicular lumen. Adenosine triphosphatase (ATPase) and other alkaline phosphatases can be localized on the plasma membranes of the canaliculi, suggesting that bile secretion into this space is an active process. Bile flow is centrifugal, that is from the region of the terminal hepatic venule (central vein) toward the portal canal (a direction opposite to the blood flow). Near the portal canal but still within the lobule, bile canaliculi transform into the short canals of Hering.
A characteristic feature of the canal of Hering is its lining made of two types of cells, hepatocytes and cholangiocytes.
chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas LIVE R 641
FIGURE 18.14 • Photomicrograph of bile canaliculi. This high-magnification photomicrograph shows several one-cell-thick plates of hepatocytes separated by hepatic sinusoids. The plane of section in certain areas is parallel to the bile canaliculi. In this plane, the canaliculi reveal their arrangement on four sides of the hepatocytes (arrows). Arrowheads indicate those bile canaliculi that appear only in cross-sectional profile. 1,240.
The canal of Hering is a channel partially lined by hepatocytes and partially by cuboidal shaped cholangiocytes. Similar to other cholangiocytes, hepatocytes possess microvilli at their apical surface and tight junctions, and their basal domain is rested on basal lamina, as is the rest of the distal biliary epithelium. Functionally, as demonstrated by video-microscopy, the canal of Hering exhibits contractile activity that assists with unidirectional bile flow toward the portal canal. Because the canal of Hering represents the smallest and most proximal tributary of the biliary tree containing cholangiocytes, it often is involved in the same diseases that affect small bile ducts. Functional disturbance in contractile activity as well as injury or destruction of the canals of Hering may contribute to intrahepatic cholestasis (obstruction of the bile ﬂow).
Canal of Hering serves as a reservoir of liver progenitor cells.
Due to their location at the crucial interface between hepatocytes and cholangiocytes, it has been proposed that the hepatic stem cells’ niche is present either in the canals of Hering or in their vicinity. This hypothesis was supported by the appearance of liver cell precursors near the canals of Hering in most of pathologic conditions characterized by extensive damage to hepatocytes. These cells could migrate and differentiate into either hepatocytes or bile duct cells. Recently, the three-dimensional reconstruction of ductular reactions in liver necrosis suggests that small cholangiocytes lining the canals of Hering proliferate extensively and migrate into the parenchyma of the liver. In immunocytochemical staining, these cells express dual markers of both biliary and hepatocyte antigens and appear to be involved in the repair of liver tissue damaged by chronic pathologic processes. Therefore, it has been concluded that the canal of Hering consists of or harbors specific hepatic stem cells. Laboratory studies suggest that in the future, hepatic stem cells may ultimately have therapeutic use in the treatment of liver diseases.