Effect of chordin knockdown on the expression of markers of osteogenic differentiation in human MSCs. (a) Alkaline phosphatase (ALP) activity at day 10. (b) Calcium deposit at day 21. (c) Alizarin Red staining after 21 days in culture. Human mesenchymal stem cells (MSCs) were exposed to basal media, osteogenic media only (Blank), transfection reagent only in osteogenic media (Lipo), control small interfering RNA (siRNA) in osteogenic media, or chordin siRNA in osteogenic media. ALP activity is expressed as the mean ± standard deviation (nmol nitrophenol/minute/absorbance) of independent experiments of cells from five donors. Calcium deposit is expressed as micromoles/well. *P < 0.005 versus the other groups.