[Biliary epithelium, hepatocytes and oval cells]

Two types of epithelial cells derived from a unique embryonic stem cell coexist in the liver: hepatocytes, which form the liver parenchyma, and cholangiocytes which line intrahepatic bile ducts. Both cell types represent more than 70% and around 3% of hepatic cells, respectively. Initially secreted in the canalicular space formed by the apical domain of hepatocytes, bile is subsequently collected and modified in bile ducts. Both types of epithelia can regenerate as a result of differentiated cell proliferation. However, massive or chronic injury may induce the emergence and proliferation of oval cells, a heterogeneous population of small cells expressing embryonic markers that may subsequently differentiate into hepatocytes or cholangiocytes. Oval cells may also give rise to hepatocellular or cholangiocellular carcinomas. They may derive not only from hepatic cells (including hepatocytes and cholangiocytes themselves), but also from extrahepatic haematopoietic stem cells.