Liver stem cells: a two compartment system.

Hepatocytes and biliary epithelia are phenotypically very dissimilar, but share a common ancestry. Hepatocytes regenerate very efficiently, and their division potential indicates that many of them are functional stem cells. When hepatocyte-damaging agents also impair the regenerative ability of surviving hepatocytes, a potential stem cell system of biliary origin is activated to generate new hepatocytes - a reversal of ontogeny. Now both bile duct derived cells and hepatocytes can be isolated from the liver, genetically modified in vitro and returned to their in vivo origins where, after considerable population expansion, they can function as hepatocytes - paving the way for ex vivo gene therapy.