IDENTIFICATION OF BIPOTENTIAL PROGENITOR CELLS IN HUMAN LIVER DEVELOPMENT

Intermediate filament proteins have been reported to be expressed in a cell lineage-specific manner during morphogenesis. We studied the exp ression of cytokeratin (CK) 14, CK19, and vimentin and of the hepatocy te-specific HepPar1 antigen during the development of human liver. Nin eteen fetal livers (gestational ages 4 to 40 weeks), 3 normal infant L ivers, and 3 normal adult livers were studied by immunoperoxidase stai ning of paraffin sections with monoclonal anti-CK19, anti-vimentin, an d HepPar1 antibodies and polyclonal anti-CK14 antibodies. Double-immun ostaining for CK14 and CK19 as well as bile duct cytokeratin and HepPa r1 antigen was also done. CK19 and HepPar1 antigen were the first mark ers detected in immature progenitor cells of the liver primordium at 4 weeks' gestation. During subsequent Liver development, the progenitor cells expressed HepPar1 antigen, CK14, and CK19, from 8 to 14 weeks' gestation. As hepatocyte differentiation progressed, expression of Hep Par1 antigen increased, and CR14 and CK19 were abrogated hem hepatobla sts at 14 to 16 weeks' gestation. in contrast, as progenitor cells tra nsformed into ductal plate cells, CK19 expression increased and persis ted in differentiated bile ducts, whereas CK14 and HepPar1 antigen wer e lost. Vimentin was detected in ductal plate and biliary epithelial c ells hom 9 to 36 weeks' gestation, but not in hepatoblasts or hepatocy tes. Double-immunostaining confirmed coexpression of CK14 and CK19 in the progenitor cells for a short time (8 to 14 weeks' gestation) durin g early development. Double immunostaining for bile duct CK and HepPar 1 antigen clearly demonstrated the divergence of the hepatocyte and bi le duct epithelial cell lineages. Our findings suggest that hepatic pr ogenitor cells differentiate in steps marked by the acquisition or los s of specific phenotypic characteristics. Commitment of the HepPar1(+) CK19(+) progenitor cells to either hepatocyte or bile duct epithelial cell lineages results in increased expression of one marker and loss of the other marker. These characteristics clearly identify bipotentia l hepatic progenitor cells in the developing human Liver.