Effect of hypoxia on MSC plasticity. Oil-Red-O stain of MSC in adipogenic induction medium at atmospheric and reduced oxygen concentrations after 14 days of culture: (A) 21% O2. control; (B) 21% O2. adipogenic induction; (C) 1% O2. control; (D) 1% O2. adipogenic induction. Alizarin Red-S staining for the deposition of calcium precipitates is an indicator for the osteogenic differentiation. Viability and osteogenic differentiation of MSC from most donors was drastically reduced in this medium under 1% O2 as compared to 21% O2: (E) 21% O2. control; (F) 21% O2. osteogenic induction; (G) 1% O2. control; (H) 1% O2. osteogenic induction. However, a slight increase of oxygen to 3% allowed for efficient differentiation of MSC into osteoblasts: (I) 3% O2. control; (J) 3% O2. osteogenic induction. Three independent experiments of adipogenic and osteogenic differentiation were performed.