Our group has isolated a hepatic progenitor cell populationfrom adult murine liver without a preceding injury to the liver. (Fig. 1A, 1B show two images of colony formation.) Earlyin culture, these cells express oval cell-like markers [32,33]. During prolonged culture, the expression profile shiftsaway from oval-cell markers toward albumin and cytokeratin,suggestive of differentiation along hepatocytic and biliarylineages . Mitaka et al. have described a similar populationof cells, from adult rat liver, termed "small hepatocytes" .These cells are smaller than their mature counterparts, approximatelyone-third to one-half the size. They are mononuclear and havea less differentiated morphologic appearance . These smallhepatocytes proliferated for more than 2 months in primary culture,whereas the mature cells stopped replicating after one to twocycles. The small hepatocytes formed colonies in culture anddifferentiated into functional mature hepatocytes, as demonstratedby an increasing albumin concentration within the culture media[34, 35]. Overturf et al. demonstrated liver recovery with thesecells by transplanting them into livers of fumarylacetoacetatehydrolase-deficient mice, a model of hereditary tyrosinemia. After transplantation, these adult hepatic cells replicatedand formed colonies, displaying a growth potential similar toembryonic-derived stem cells . Fujikawa et al. isolatedcells from adult murine livers that were -fetoprotein-positivewith immature endodermal characteristics . They found thatduring in vitro culture, these cells were capable of differentiatinginto both hepatic and biliary cell lineages, suggesting cellularbipotency .
Other groups have described novel methods for isolating progenitorcell populations, including isolating them under hypoxic conditionswhile simultaneously inducing cell aggregate formation .Cells isolated from adult murine livers using this method expressalbumin, AFP, and CK-19, markers consistently found on ovalcells and hepatic progenitor cell populations. However, theinvestigators did not find markers for mature hepatocytes, suchas tryptophan-2,3-dioxygenase (TO) or glucose-6-phosphatase(G6P). After the cells proliferated in culture, they began todifferentiate, and at day 40 they expressed both TO and G6P,suggesting that the cells had differentiated to a mature hepatocyte.
Although the potential for many of these adult-derived progenitorcells is promising, there is still a tremendous amount of investigationto be done before their therapeutic potential can be realized.Perhaps most significantly, there is an ongoing challenge withrespect to identifying unique markers that will support theisolation and purification of these cells from the mature hepatocyteand nonparenchymal cell populations within the liver.
The issue of dedifferentiation as a process that generates stemcell populations has been debated in recent years. Tateno etal. demonstrated that hepatocytes in culture expressed biliarymarkers [39, 40]. They also found that a small population ofthe mature hepatocytes began expressing the immature hepaticmarker -fetoprotein [39, 40]. Koenig et al. found that maturehepatocytes placed in culture formed colonies and with the rightmitogen could be stimulated into expressing biliary as wellas extrahepatic progenitor markers .