Distribution of CD166+chondrocytes within the osteoarthritis (OA) joint cartilage matrix. CD166+ cells, characterized by an intense brown staining of the cell membrane and pericellular matrix, are localized primarily in the superficial and middle cartilage zones of OA cartilage. In contrast, almost no CD166+ cells are in the deep and calcified cartilage zones. The corresponding isotype control of the immunohistochemical mesenchymal progenitor cell staining shows an absence of a positive signal, supporting the specificity of the above-named CD166 staining. Magnifications: 100 × (large micrograph), 400 × (insets). Results of semiquantitative analysis of cartilage samples from six patients with OA are expressed as mean ± standard error of the mean (SEM).