Serial transplantation reveals stem cell like regenerative potential in parenchymal mouse hepatocytes

Recent work has demonstrated that a fraction of adult mouse hepatocytes has high regenerative potential and that single cells can give rise to repopulation nodules 1-2 mm in diameter (Science 263: 1149, 1994; Nature Genet 12:266, 1996). This appears to also be true for human hepatocytes (J Clin Invest 94:1657, 1994). These nodules contain approximately 30,000 cells representing approximately 15 cell divisions. To test whether this is an upper limit due to a restricted capacity for cell division, we have performed serial transplantation of limiting numbers of adult parenchymal hepatocytes in the fumarylacetoacetate hydrolase deficiency murine model of liver repopulation. 10,000 hepatocytes from an adult donor were serially transplanted four times and resulted in complete repopulation during each cycle. This corresponds to a minimal number of 34 cell divisions or a 1.7 × 10¹⁰ fold expansion. No evidence for abnormal liver function or altered hepatic architecture were found inmost repopulated animals. We conclude that up to 10% of fully differentiated adult mouse hepatocytes have a stem-cell like growth potential. Nodule size therefore is not limited by the replicative potential of the transplanted cells. To gain an understanding of the hepatocyte pool size involved during repopulation we have begun serially transplanting retrovirally tagged hepatocytes. The "hepatic repopulation nodule assay" described by us here is analogous to colony forming assays of hematopoietic cells and will be useful to identify an further characterize highly regenerative cells within the liver. These cell populations may be useful as targets for gene therapy or cellular transplantation strategies in the treatment of genetic disease.