It has been suggested that when the regenerative ability ofmature hepatocytes is insufficient, then the capacity of theliver for innate cellular recovery occurs secondary to a separateand unique population of liver cells. The origin, nomenclature,and function of these cells have been a longstanding area ofstudy and debate. Hepatic progenitor cells have been broadlycharacterized as a wide range of cell populations by multiplescientific teams. These cells have been investigated from thevery early embryonic stages through adulthood, using a diverserange of experimental models.
A stem cell is typically characterized by its capacity for self-renewaland ability to give rise to multiple differentiated cellularpopulations (often termed cellular plasticity) . Despiteprogress in characterizing select stem cell populations fromdistinct organs (e.g., bone marrow, liver, the nervous system,etc.), a pattern of differences has become evident. These distinctionsare associated with the host and/or the injury model involvedwith isolating a specific cellular population. An additionaland oftentimes greater challenge when working with stem cellsis the lack of unique markers to identify these cells. Althoughsome markers are observed in stem cell subsets (e.g., the effluxof certain dyes appears to be a common property of hematopoieticstem cells),  stem cells are routinely characterized by theabsence rather than the presence of specific lineage-relatedmarkers.