Mouse hepatoblasts at distinct developmental stages are characterized by expression of EpCAM and DLK1: Drastic change of EpCAM expression during liver development

Hepatoblasts are hepatic progenitor cells that expand and give rise to either hepatocyte or cholangiocytes during liver development. We previously reported that delta-like 1 homolog (DLK1) is expressed in the mouse liver primordium at embryonic day (E) 10.5 and that DLK1(+) cells in E14.5 liver contain high proliferative and bipotential hepatoblasts. While the expression of epithelial cell adhesion molecule (EpCAM) in hepatic stem/progenitor cells has been reported, its expression profile at an early stage of liver development remains unknown. In this study, we show that EpCAM is expressed in mouse liver bud at E9.5 and that EpCAM(+)DLK1(+) hepatoblasts form hepatic cords at the early stage of hepatogenesis. DLK1(+) cells of E11.5 liver were fractionated into EpCAM(+) and EpCAM(-) cells; one forth of EpCAM(+)DLK1(+) cells formed a colony in vitro whereas EpCAM(-)DLK1(+) cells rarely did it. Moreover, EpCAM(+)DLK1(+) cells contained cells capable of forming a large colony, indicating that EpCAM(+)DLK1(+) cells in E11.5 liver contain early hepatoblasts with high proliferation potential. Interestingly, EpCAM expression in hepatoblasts was dramatically reduced along with liver development and the colony-forming capacities of both EpCAM(+)DLK1(+) and EpCAM(-)DLK1(+) cells were comparable in E14.5 liver. It strongly suggested that most of mouse hepatoblasts are losing EpCAM expression at this stage. Moreover, we provide evidence that EpCAM(+)DLK1(+) cells in E11.5 liver contain extrahepatic bile duct cells as well as hepatoblasts, while EpCAM(-)DLK1(+) cells contain mesothelial cell precursors. Thus, the expression of EpCAM and DLK1 suggests the developmental pathways of mouse liver progenitors.