Histological staining and gene expression analysis of ovine chondrocytes for osteogenic differentiation in three-dimensional scaffold-assisted cartilage grafts. Von Kossa staining of mineralized osteogenic extracellular matrix was negative (A) with mild staining of scaffolds fibers (B, black double-arrowhead) and no staining of chondroctyes (B, black arrowhead). The gene expression level of the osteogenic marker gene osteocalcin was low (C). The expression level was calculated as the percentage of the expression level of the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The mean (n = 3 donors) is plotted and the error bars represent SD.