Relation between in vitro and in vivo osteogenic potential of cultured human bone marrow stromal cells

The use of cell therapies in bone reconstruction has been the subject of extensive research. It is known that human bone marrow stromal cell (HBMSC) cultures contain a population of progenitor cells capable of differentiation towards the osteogenic lineage. In the present study, the correlation between the in vitro osteogenic potential of HBMSC cultures and their capacity to form bone in vivo was investigated. HBMSC cultures were established from 14 different donors. Fourth passage cells were examined for the expression of alkaline phosphatase (ALP), procollagen I (PCI) and osteopontin (OP), through flow cytometry and the effect of the osteogenic differentiation factor dexamethasone (Dex) on this expression was evaluated. In addition, the capacity of the cultures to induce in vivo bone formation was analysed by culturing the cells on porous hydroxyapatite (HA) scaffolds followed by subcutaneous implantation of these constructs in nude mice. Results showed expression of PCI, OP and ALP in all cultures, irrespective of the presence of Dex in the culture medium. Dex failed to have a significant effect on the expression of PCI and OP but it induced a consistent increase in the relative amount of cells expressing ALP. Nevertheless, although in vitro testing clearly indicated osteogenic potential in all cultures, HBMSC from six of the 14 tested donors did not form bone in vivo. The results, therefore, demonstrate that neither the expression of PCI, OP and ALP nor the absolute increase in Dex-stimulated ALP expression can as yet be used as predictive markers for in vivo bone formation by HBMSC. However, preliminary data indicate that not the absolute, but the relative increase in the percentage of ALP expressing cells caused by Dex stimulation may be related to the ability of the HBMSC to form bone.