Prolonged propagation of human embryonic stem (ES) cells is currently achieved by co-culture with primary or immortalized mouse embryonic fibroblast (MEF) cells. In order to replace the heterologous with homologous co-culture systems, an attempt was made using mononuclear cells derived from human fetal liver. Human fetal liver-derived mesenchymal-like stem cells (FL-MLSC) can be maintained for the prolonged period of time. They showed the characteristics of mesenchymal stem cells in various aspects. They retained a normal diploid karyotype and growth characteristics over the successive culture. Human ES cells cultured on human FL-MLSC cells up to 8 passages displayed the unique morphology and molecular markers characteristic for undifferentiated human ES cells as cultured on MEF cells. Alkaline phosphatase activity was detected in human ES cells co-cultured on human FL-MLSC. Immunocytochemical analyses showed that expressions of stage-specific embryonic antigen-3, -4 and Oct-4 were not altered on human ES cells cultured on human FLDSC. Reverse-transcriptase PCR analyses showed that similar expressions of Oct-4 and Nanog genes, markers for undifferentiated ES cells, were also observed in human ES cells cultured on both human FL-MLSC and MEF cells. Furthermore, human ES cells cultured on human FL-MLSC retained unique differentiation potentials in culture when allowed to form embryoid body. Results of this study suggest that human FL-MLSC can support the maintenance of human ES cell in vitro.

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