Genetic expression of extracellular matrix proteins correlates with histologic changes during fracture repair

We characterized gene expression in the reparative callus that formed after fracture of the rat femur. The callus was divided into regions of bone formation (hard callus) and cartilage formation (soft callus), and gene expression was examined separately in each region. Expression of extracellular matrix protein genes varied with the progression of repair and differed between hard and soft calluses. Messenger ribonucleic acids (mRNAs) for osteonectin, alkaline phosphatase, and type I procollagen were detected in the hard callus at maximal levels during endochondral ossification and bone remodeling (day 15) and at 50% maximal levels during intramembranous bone formation (day 7). Messenger RNAs for these proteins in the soft callus were detected at low levels during chondrogenesis (day 9) but increased to 80% of maximal levels with chondrocyte hypertrophy and mineralization of the cartilage matrix (day 13). Messenger RNAs for type II procollagen and proteoglycan core protein were detected at maximal levels in the soft callus during chondrogenesis (day 9). Osteocalcin gene expression was detected in the hard callus during endochondral ossification and remodeling but not during intramembranous bone formation or at any time in the soft callus. Osteonectin mRNA was detected in both the hard and soft callus throughout the entire course of fracture repair. Expression of cartilage and bone-related genes correlated with the temporal sequence of histologic changes, suggesting transcriptional regulation of gene expression during repair. Differences in gene expression between hard and soft callus and in each of these regions as repair progressed suggest local regulation of gene expression during cell differentiation and matrix synthesis.