The process of cartilage-to-bone transition (CBT) is a key event for the achievement of rigid bone healing during fracture repair. Since mineralization of cartilaginous matrix is a prerequisite for the initiation of CBT, the genetic localization of mineralization-related bone matrix proteins in CBT was examined in this study. An in situ hybridization method used on decalcified sections with digoxigenin-11-UTP labelled probes identified the cellular localizations of these genes in CBT. Cessation of osteonectin mRNA together with induction of osteopontin mRNA in chondrocyte maturation was observed during the process of CBT in the fracture callus on day 12 after fracture; osteocalcin mRNA was absent in chondrocytes of the CBT area. Induction of osteopontin mRNA in maturated chondrocytes was followed by the expression of mRNAs for osteonectin, osteopontin and osteocalcin in osteogenic cells in the ossification front of CBT. The data suggest that the switch from osteonectin to osteopontin mRNA expression in chondrocyte maturation is one of the key events during CBT. Transcriptional disorders of the expression of these molecules may be linked to the failure of fracture repair, i.e. delayed or prevented hypertrophic osteosynthesis.
