Objective To evaluate the osteoblastic differentiation and compare the difference in the gene expression of rat bone marrow mesenchymal stem cells（MSCs） affected by a single period of mechanical strain． Methods Bone marrow MSCs were harvested from the femurs and tibiae of SD rats and cultured in vitro. A four-point bending apparatus were used to perform a single 40-minute period of 2 000 με mechanical strain on these MSCs The proliferation of the MSCs was tested by MTT on scheduled date, and the osteoblastic differentiation of the MSCs was measured by testing the expression of osteocalcin and alkaline phosphate（ALP） activity of these cells. In addition, we have investigated the possible mechanisms underlying the action of the single 40 -minute period of 2 000 με mechanical strain on these MSCs, after profile blotted and handled by bioinformation, the gene expressions of these two periods of MSCs were examined. Results The MSCs have grown well in vitro. Our experiment showed that mechanical environment did not weaken the proliferation of the MSCs. However, the ALP activity and the expression of osteocalcin were significantly up-regulated by the 2 000 με mechanical strain. Using the 27 K Rat Genome Array, 416 different expressions were found. The rate of different genes was 2.8%, of which the expressions of 247 genes increased（61 genes remarkably increased） and 169 genes decreased（74 genes remarkably decreased） in these two periods of MSCs. Conclusion Mechanical strain induced the osteoblastic differentiation of the MSCs, which may be attributed to the different gene levels.
