Cell biology of hepatic repopulation in diseased liver

My overall objective was to determine whether transplanted hepatocytes or cells derived from a putative stem cell lineage can be used to replace damaged liver parenchyma in mice. First, I tested the hypothesis that hepatocyte transplantation could provide long-term therapy in mice with severe toxin-induced hepatic parenchymal damage using a model I developed to reproduce this type of hepatic injury. Hepatocyte transplantation only slightly improved post-transplant survival despite the fact that many non-survivors displayed moderate liver repopulation by donor cells. Perhaps accounting for this outcome, donor parenchyma in non-survivors did not display typical lobular organization. These results suggested that re-creation of functional parenchyma by transplanted hepatocytes will require time during which cells first must proliferate and then reestablish normal lobular architecture. My second objective was to assess the proposal that a non-hepatocytic stem cell lineage is responsible for replacing damaged parenchyma following severe hepatic injury. Several treatments in rodents combine destruction of liver parenchyma with hepatocyte mitoinhibition. These treatments induce proliferation of bile epithelial-like cells termed oval cells, development of foci composed of small hepatocytes, and eventual replacement of damaged parenchyma by healthy hepatocytes. Oval cells are proposed to represent transitional cells in a non-hepatocytic facultative liver stem cell lineage that can produce the small hepatocyte foci. I used in vivo cell lineage marking in genetically chimeric livers to test the hypothesis that hepatocytes can serve as the precursor cell type to the small hepatocyte foci that develop in severely damaged mouse liver, I demonstrated that hepatocyte-derived foci are present following this type of liver damage, indicating that a stem cell lineage is not necessary for regeneration to occur under these conditions. Finally, I used transgenic mice with chimeric livers consisting of diseased parenchyma and regenerating foci of healthy hepatocytes to assess whether the local hepatic microenvironment influences the cellular response in liver disease. I found that oval cells co-localized with Ito cells and increased laminin protein, and that both oval cells and stromal changes were excluded from foci of healthy hepatocytes. This provides strong evidence that regeneration in chronically diseased liver is highly dependent upon the local microenvironment.