In vitro expansion of adipose-derived adult stromal cells in hypoxia enhances early chondrogenesis

Cartilage is an avascular tissue, and chondrocytes in vivo experience a severely hypoxic environment. Using a defined in vitro model of early chondrogenesis, we attempted to enrich for cells with an enhanced ability for chondrogenic differentiation by pre-exposure of mouse adipose-derived adult stromal cells (ADASs) to a hypoxic (2% oxygen) environment. ADASs were subsequently expanded in 2% or 21% oxygen environments, resulting in 2 groups of cells, and then early chondrogenic differentiation was induced at 21% oxygen tension using a 3-dimensional micromass culture system. ADAS chondrogenesis was assessed using Alcian Blue staining for proteoglycans and quantification of sulfated glycosaminoglycans. Osteogenesis of the 2 cell groups was also studied. Two percent oxygen tension profoundly increased the proliferation of ADASs. ADASs expanded in 2% oxygen tension exhibited enhanced early chondrogenic differentiation and diminished osteogenesis, suggesting that the reduced oxygen environment may favor chondroprogenitors. Gene expression analysis suggested that matrix metalloproteinase synthesis was inhibited in cells expanded in 2% oxygen. Furthermore, re-oxygenation of the 2% oxygen-expanded ADASs before differentiation did not significantly affect early chondrogenesis. Thus, priming ADASs with 2% oxygen may have selected for chondrogenic progenitors with an enhanced ability to survive and differentiate. This study is relevant for the future application of cell-based therapies involving cartilage tissue regeneration.