Background & Aims: The liver has high regenerative potential. We attempted to establish a novel culture system for extensive expansion of fetal mouse hepatic stem/progenitor cells and to characterize cultured cells. Methods: Hepatic spheroids collected from 6-day floating cultures were cultured on collagen-coated dishes in serum-free conditions in medium containing growth factors. Cultured cells were mainly characterized by immunocytochemistry and flow cytometry or transplanted into adult mice. Results: Approximately 400 expanding hepatic spheroids were generated from every 1 × 106 fetal liver cells. Subsequently, highly replicative colonies were subcultured with maintaining colony formation on collagen-coated dishes. These colonies consisted of small immature α-fetoprotein-positive cells and hepatocytic and cholangiocytic lineage-committed cells. The immature α-fetoprotein- positive cells could be expanded in a reproducible manner at least 5 × 105-fold (which involved at least 30 passages over >6 months) without losing differentiation potential. Flow cytometric analysis showed that all cultured cells expressed CD49f, but not CD34, Thy-1, c-kit, or CD45. Nearly 15% of the cells expressed Sca-1, and approximately 5%-20% of the cells were side population cells. Both sorted side population cells and Sca-1-positive cells (especially side population cells) produced a large number of α-fetoprotein-positive cells and lineage-committed cells. Expanded cells had bidirectional differentiation potential and improved serum albumin levels in mice with severe liver damage. Conclusions: Long-term extensive expansion of transplantable hepatic stem/progenitor cells was reproducibly achieved in a novel serum-free culture system. Moreover, this culture system yielded side population and Sca-1-positive cell populations that included hepatic stem/progenitor cells with differentiation and proliferation properties.
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