Oxygen-dependent expression of cartilage matrix genes in healthy and osteoarthritic chondrocytes. The relative gene expression levels of COL2A1 (A), ACAN (B), COL1A1 (C) and COL10A1 (D) in pellet cultures following 2 wk of redifferentiation in either 20% oxygen (gray bars) or 2% oxygen (white bars) were analyzed by real-time quantitative polymerase chain reaction. All values are the mean mRNA level normalized to 18S ribosomal RNA for chondrocyte pellets from n = 5 donors. Error bars represent 1 SD. Statistical significance was determined by performing independent t-tests, Mann-Whitney U test (between disease conditions) and paired t-test or Wilcoxon tests as appropriate (between oxygen levels). *P < 0.05.