D espite decades of research, the processes that govern liver development and regeneration are only partially understood. A good understanding of the mechanisms that play a role in these processes is important because it may lead to new treatments of human liver diseases. Several in vivo experimental conditions have been used to study liver regeneration and repair and the interaction of different cell types in these processes. These include partial hepatectomy, administration of toxic compounds, or a combination of both, and transgenic expression of certain proteins. 1,2 In contrast to in vitro experiments, these approaches raise fewer questions concerning the influence of artificial matrices on the function and behavior of hepatocytes and other liver cell types.Although these procedures have generated useful information on rodent liver regeneration, stem cell activation, and other processes, extrapolating these findings to the Abbreviations: uPA, urokinase plasminogen activator; HBV, hepatitis B virus; HCV, hepatitis C virus; HBsAg, hepatitis B surface antigen; HBeAg, hepatitis B e antigen; PAS, periodic-acid-Schiff. From the