Fine structure, enzyme histochemistry, and immunohistochemistry of liver in zebrafish

The fine structures, enzyme histochemical, and immunohistochemical characterization of liver in zebrafish were investigated using light microscopy and electron microscopy. The results showed that liver was separated into three lobes and each lobe had a central vessel comparable to the mammalian central vein. However typical hepatic lobules, portal areas, and hepatic arteries were not observed. A portal vein entered the liver and its tributaries were connected directly to the sinusoids, which then converged to the central vessel. Three central vessels in lobes finally carried the blood out of liver. The polygonal and bilayered hepatocytes were arranged as twisting, branching, and anastomosing cords. Ultrastructurally, they showed apparent morphological features of protein synthesis and secretion. Bile entered the biliary tree through the intracellular canaliculus, the ramifications of intercellular canaliculi that originated near the hepatic nucleus and then extended to the hepatocyte surface where two adjacent hepatocyte membranes formed intercellular canaliculi, and then ran sequentially through bile preductules, bile ductules, and bile ducts to be secreted out of the liver. Bile preductular epithelial cells (BPs) were cells located between bilayered hepatocytes in one hepatic cord. Occasionally, some tight junctions were detected forming the link between BPs and hepatocytes, which led us to assume that BPs might have a close relationship with hepatocytes during evolution. The present results indicate that zebrafish liver has its own specific fine structure.