Background Osteopontin (OPN), also known as bone sialoprotein I or secreted phosphoprotein 1, is a major non-collagenous bone matrix protein. A broad distribution has been detected in embryonic bone, osteoid, and fracture callus [Nomura et al. 2000] pointing out its central role in bone development and healing. It remains unclear weather mechanical conditions influence OPN synthesis and thereby osteoprogenitor cell differentiation. We investigated OPN mRNA-levels of bone marrow derived mesenchymal stem cells (bm-MSC) cultured in a previously described compression bioreactor (CBR) [Matziolis et al. under review] under dynamic compression (DC).

Materials Bm-MSCs of 5 different individuals (mean age 61y) were seeded in a fibrin-alginate mix-matrix placed between two slices of lyophyliced cancellous bone. One group of constructs (n=10) underwent DC with 7kPa at 0.05 Hz, resulting in a matrix compression of 1mm at an heigh of 5mm, for 24 hours in the CBR. Constructs cultured under similar conditions but without DC served as control group (n=10). mRNA was extracted out of each construct after ending the DC, following the Trizol®-protocol. After cDNA-synthesis, GEArray Q series (Human Osteogenesis Gene Arrays) were performed and normalized versus GAPDH.

Results We found an increase of OPN-expression in all dynamically compressed matrices. In the DC-group we found a mean of 5-fold increase of OPN mRNA compared to the control group (median: 0.43 vs. 0.09, p< 0.001).

Discussion and Conclusion The results of this study demonstrate that an in vitro DC of bm-MSCs for 24 hours leads to an increased expression of OPN. We conclude that DC is an important element of early fracture healing by increasing the expression of OPN and thereby modulating progenitor cell differentiation immediately after mechanical instability caused by a fracture.