Gene expression pattern of chondrocytes after primary culture (P0). Chondrocytes isolated from cartilage of patients undergoing autologous chondrocyte transplantation (ACT; n = 40, white bars) or osteoarthritis (OA) patients (n = 26; black bars) were expanded in primary culture for 10 to 12 days and the gene expression of type I and II collagen (CI and CII, respectively), aggrecan (AGG), IL-1β and activin-like kinase (ALK)-1 was determined using qRT-PCR. The mRNA levels were normalized to GAPDH and amplified by a factor of 106. In comparison to cells ex vivo, the collagen type I to collagen type II mRNA ratio is increased, especially in ACT chondrocytes. Statistically significant differences (p < 0.05) are marked by asterisks (*).