Stem cell markers in human osteoarthritis (OA) articular cartilage. (a) Safranin O and Notch-1 staining in clusters (10× and 40×). (b) Safranin O and Stro-1 staining of OA cartilage sections (10× and 40×). (c) OA cartilage sections immunostained for VCAM-1 (10× and 40×). Positive staining indicated by black arrows and negative with white arrows.