EXPRESSION OF BONE-MATRIX PROTEINS MESSENGER-RNA DURING DISTRACTION OSTEOGENESIS

Distraction osteogenesis is a recently advanced principle of bone leng thening in which a bone separated by osteotomy is subjected to slow pr ogressive distraction using an external fixation device. Appropriate m echanical tension-stress is believed not to break the callus but rathe r to stimulate osteogenesis. To study the molecular features of this p rocess, the expression and localization of the mRNAs encoding osteopon tin (OPN), osteocalcin (OC), matrix Gla protein (MGP), osteonectin (ON ), and collagen type I and ZI during distraction osteogenesis were exa mined by in situ hybridization and Northern blot analysis. The process can be divided into three distinct phases: the lag phase for 7 days b etween osteotomy and the beginning of distraction, the distraction pha se for 21 days, and the consolidation phase for several weeks. The his tologic and molecular events taking place during the lag phase were si milar to those observed in fracture healing. The osteotomy site was su rrounded by external callus consisting of hyaline cartilage. As distra ction started at the rate of 0.25 mm/12 h, the cartilaginous callus wa s elongated, deformed, and eventually separated into proximal and dist al segments. The chondrocytes were stretched along the tension vector and became fibroblast-like in shape. Although morphologically these ce lls were distinguishable from osteogenic cells, they expressed OPN, OC , and alkaline phosphatase mRNAs. As distraction advanced, the cartila ginous callus was progressively replaced by bony callus by endochondra l ossification and thereafter new bone was formed directly by intramem branous ossification. OPN mRNA was detected in preosteoblasts and oste oblasts at the boundary between fibrous tissue and new bone. ON, MGP, and OC mRNAs appeared early in the differentiation stage. The variety of cell types expressing mRNA encoding bone matrix proteins in distrac tion osteogenesis was much greater than that detected in the embryonic bone formation and fracture healing process. Moreover, the levels of OPN, ON, MGP, and OC mRNA expression markedly increased during the dis traction phase. These results suggested that mechanical tension-stress modulates cell shape and phenotype, and stimulates the expression of the mRNA for bone matrix proteins.