Gene expression pattern of chondrocytes after first passage (P1). Chondrocytes from cartilage of osteoarthritis (OA) patients (n = 18, black bars) were subcultured in a first passage and further expanded until they reached confluence after an additional 12 to 14 days. The gene expression patterns were enumerated by qRT-PCR for type I and II collagen (CI and CII, respectively), aggrecan (AGG), IL-1β and activin-like kinase (ALK)-1 as indicated. The mRNA levels were normalized to GAPDH and amplified by a factor of 106. The ratio of type I to type II collagen mRNA levels continue to increase in P1 OA chondrocytes.