SWITCH OF OSTEONECTIN AND OSTEOPONTIN MESSENGER-RNA EXPRESSION IN THEPROCESS OF CARTILAGE-TO-BONE TRANSITION DURING FRACTURE REPAIR

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