HIF-1α upregulation in response to low oxygen tension. Exposure of MSC to hypoxia resulted in a more than 3-fold increase of the HIF-1α expression as detected by semiquantitative RT-PCR. The relative index for HIF-1α was normalized against GAPDH relative intensity. Effect of reduced oxygen tension on MSC proliferation kinetics (A). Cell proliferation was measured by an increase in OD using the MTS assay after an incubation period of 7 days under 21% O2, 5% O2 (B), 3% O2 (C), and 1% O2 (D), respectively. Values indicate the mean and standard deviation for 3 cultures of each condition. Significance for all analysis was set at *** p < 0,001, ** p < 0,01, * p < 0,05 using student's t-test.