Multilineage potential of the side population (SP) derived from normal human articular cartilage. (a) Safranin O staining of 14-day SP and non-SP (NSP) pellet cultures (magnification 40×). (b) Gene expression analysis of 14-day pellet cultures relative to NSP cells. Higher Sox9, Aggrecan, and both Col2a1 IIA and Col2a1 IIB expression in SP cells. (c) SP cultured in pro-osteogenic medium for three weeks show higher levels of Col1a1 and Runx2 gene expression relative to NSP cells.