Apoptosis in hNPCs. Rate of apoptotic hNPCs during differentiation at 20% O2 (black bars) and 3% O2 (white bars) detected by TUNEL staining. Up to day 4 of differentiation, the percentage of apoptotic cells is continuously increasing up to 32.43 ± 4.26%, with no difference between normoxia, hypoxia, EPO-treated and control cells. At day 4 of differentiation, 10 IU/ml EPO did significantly reduce the number of apoptotic cells (18.80 ± 2.27%)%. The same decrease was observed when culturing the cells under hypoxia (15.82 ± 4.65%, A) (n = 3, mean ± SEM, *p < 0.05). Western blot analysis of Bcl-2 of hNPCs differentiated and treated with either normoxia, hypoxia or 10 IU/ml EPO over a time course of 4 days revealed an increase of signal with the onset of differentiation. GAPDH was used as loading control (B).