Altered cartilage phenotype expressed during intramembranous bone formation.

The sequential phenotypic expression occurring during intramembranous bone formation was investigated using the tooth extraction socket created in rat alveolar bone in vivo. The differential expression of bone extracellular matrix genes, such as collagen I and osteocalcin, was confirmed by RNA transfer blot analysis and in situ hybridization during the active healing period of the bony socket. To clarify the possible involvement of the chondrogenic phenotype during the process of intramembranous bone formation, the expression of cartilage collagen II and IX was further examined in this model. It was found that both alpha 1(II) and alpha 1(IX) mRNAs were present, but the alpha 1(IX) mRNA was a transcript from the downstream start site/promoter, which is a different site in the alpha 1(IX) gene from that used in hyaline cartilage. In situ hybridization indicated that the alpha 1(IX) message was expressed by cells associated with bone matrix in the early formation stage. This finding led to the investigation of type IX collagen expression by osteogenic cells isolated from newborn rat calvariae, in which only the truncated form of alpha 1(IX) mRNA was indicated by RNA transfer analysis. The expression of collagen II and a truncated form of collagen IX may represent an early phenotypic feature of osteoblast differentiation.