Metabolic activity of hNPCs. Effect of oxygen tension on the metabolic activity of differentiating hNPCs. Cells differentiated for 24 h or 72 h either at 20% (A, C, E, black bars) or 3% O2 (B, D, E, white bars). EPO was applied with the onset of differentiation at various concentrations (10 - 300 IU/ml). Treatment with EPO resulted in a significant increase of the absorbance from 25 IU/ml onwards at 20% O2 (n = 3-7) and 100 IU/ml at 3% O2 (n = 3-6) indicating a higher metabolic activity of cells after one day of differentiation (A, B). Samples treated with EPO and differentiated for 3 days showed similar absorbance levels at 20% (n = 3-4) and 3% oxygen (n = 3-4), whereas the increase of absorbance levels after EPO treatment were not any longer significant. Cultivation of untreated cells at low oxygen concentration resulted in a 2.4 fold increase of the absorbance after one day of differentiation and a 1.7 fold increase after 3 days of differentiation (E), indicating a higher metabolic activity (n = 4-6), compared to controls. (F) includes the metabolic activity of proliferating cells for comparison with differentiated cells (E). * p < 0.05, **p < 0.01.