Expression of bone morphogenetic protein-6 messenger RNA in bovine growth plate chondrocytes of different size.

Bone morphogenetic protein-6 (BMP-6) is an osteoinductive factor that may have a regulatory role in the terminal differentiation of chondrocytes. We investigated the expression of BMP-6 messenger RNA in freshly isolated newborn calf rib chondrocytes separated by density gradient centrifugation into five highly enriched subpopulations at different stages of maturation as assessed by cell size and alkaline phosphatase activity. Expression of BMP-6 mRNA was compared with expression of type II collagen mRNA and type X collagen mRNA using Northern analysis. Type X collagen mRNA expression increased with increasing cell size whereas type II collagen mRNA varied little with cell size. BMP-6 mRNA expression was highest in small cells and lowest in the largest cells, which were maximally expressing type X collagen mRNA. This suggests that up-regulation of the BMP-6 gene may precede chondrocyte hypertrophy.