Osteogenic differentiation influences stem cell migration out of scaffold-free microspheres

Complete bone regeneration of critical-size defects frequently fail because of the use of acellular bone substitutes and because of partially negative influences of artificial scaffolds. However, the supply of cells to critical-size defects is essential for the regeneration. Therefore, engineered scaffold-free tissues, with outgrowing cells that fill up spaces in the bony defect, are promising candidates for bone regeneration approaches. Here, we demonstrate such a scaffold-free tissue construct (microspheres) that, if osteogenic differentiated, mineralizes while maintaining the capability to let cells grow out of the united cell structure. A superior outgrowth capability of microspheres composed of human cord blood-derived unrestricted somatic stem cells compared with murine embryonic stem cells was found and a time-dependent reduction in outgrowth was evident in vitro. Even after 5 days of osteoinduction and strong mineralization, the cells migrate out of the microsphere. As migration of cells out of unrestricted somatic stem cell microspheres was also found in extracellular matrix gel, we suggest that cells would migrate also in vivo. Thus, microspheres could serve as the scaffold and the source of osteogenic cells in future bone regeneration approaches. Further, microspheres permit the precise administration of large amount of cells into an area of interest.