The illustrations highlighted that cell renewal in the liver under all
these situations occurs predominantly (but not exclusively) with phenotypic fidelity, with only a small percentage of hepatocytes during liver regeneration potentially being contributed by biliary epithelial cells. Is it possible to reconcile the different conclusions arising from these models of careful cell lineage tagging? Are all the assumptions made for each model fully validated? Is it possible that the limitations of wildtype animal manipulations, decried for many years as subject to multiple interpretations, have been replaced by more elegant find protocol methodologies with genetically modified mice, which nonetheless have limitations of their own that are more difficult to expose? If we examine the studies of the last two decades, and employing only wildtype nongenetically modified rats and mice, transdifferentiation of cells from the biliary compartment to form progenitor cells that eventually also transdifferentiate to hepatocytes occurs only when hepatocyte proliferation is suppressed or when hepatocyte death is so overwhelming that there no residual hepatocytes sufficient to provide restoration of the lost liver tissue. The publication by Furuyama et al. reaches different conclusions from the articles
by Lemaigre and colleagues and by Willenbring and colleagues, who argue that in the absence of the above limits to hepatocyte proliferation, contribution of RAD001 cost biliary cells to formation of new hepatocytes is either absent or miniscule. Currently, there is no “clean” model to suppress hepatocyte proliferation Thymidylate synthase after partial hepatectomy
in the mouse as it exists for the rat (i.e., AAF plus partial hepatectomy) and the rat model cannot be evaluated by lineage tagging. Despite the apparently contradictory studies with genetic mouse models, the majority of workers in liver growth biology seem to agree that the biliary compartment (portal ductules, canals of Hering, glands around gallbladder) is the source of progenitor cells and the formation of hepatocytes from biliary-derived progenitor cells under extreme conditions mentioned above is also generally accepted. The demonstration of expression of HNF4α and HEPPAR in proliferating biliary cells in fulminant hepatic failure in humans also strongly argues that this pathway is a clinically important SOS mechanism to salvage the liver from total collapse under extreme circumstances.17 The transdifferentiation in the opposite direction, i.e., hepatocytes giving rise to biliary epithelial cells, is much debated. The article by Willenbring and coworkers, using simple bile duct ligation, did not observe evidence for formation of biliary epithelial cells from hepatocytes.