Umbilical cord blood stem cells: induction of differentiation into mesenchymal lineages by cell-cell contacts with various mesenchymal cells.

CD133(+) cells isolated from bone marrow, peripheral blood, or umbilical cord blood (UCB) represent an established source of transplantable hematopoietic progenitors. Further, there is increasing evidence that such CD133(+) cell isolates comprise subpopulations capable of differentiating into several mesenchymal lineages. In this study, we investigated conditions under which mesenchymal differentiation can be induced, particularly the role of cell-cell contacts with mesenchymal cells. A purified, nearly homogeneous CD133(+) population of human UCB cells was expanded by stimulation with platelet-derived growth factor and epidermal growth factor, labeled with the fluorescent marker DiI and cocultivated with rat osteoblasts, C2C12 myoblasts, or rat cardiomyocytes, respectively. In control experiments, the two cell types were separated by microporous membranes to avoid cell-cell contacts. Direct coculture of DiI-labeled UCB cells with the different mesenchymal cell populations resulted in both significant morphological changes and upregulation of lineage-specific markers. Expression of osteocalcin, myosin heavy chain, or alpha-actinin confirmed differentiation of the UCB cells into an osteoblastic, myoblastic, or cardiomyocytic phenotype, respectively. In contrast, coculture of UCB cells with the respective inducer cells under conditions preventing cell-cell contacts yielded minor, if any, evidence for such differentiation. Our data, thus, indicate that UCB cell expansion in vitro and subsequent direct cell-cell contacts with mesenchymal cells can induce their differentiation into mesenchymal lineages specific to the cell type they are in contact with. This finding has important implications for understanding the homing of adult stem cells and the promise of UCB as a cell source for tissue engineering and regenerative medicine.