Expression of bone morphogenetic proteins during membranous bone healing

For the reconstructive plastic surgeon, knowledge of the molecular biology underlying membranous fracture healing is becoming increasingly vital. Understanding the complex patterns of gene expression manifested during the course of membranous fracture repair will be crucial to designing therapies that augment poor fracture healing or that expedite normal osseous repair by strategic manipulation of the normal course of gene expression. In the current study, we present a rat model of membranous bone repair. This model has great utility because of its technical simplicity, reproducibility, and relatively low cost. Furthermore, it is a powerful tool for analysis of the molecular regulation of membranous bone repair by immunolocalization and/or in situ hybridization techniques. In this study, an osteotomy was made within the caudal half of the hemimandible, thus producing a stable bone defect without the need for external or internal fixation. The healing process was then catalogued histologically in 28 Sprague-Dawley rats that were serially killed at 1, 2, 3, 4, 5, 6, and 8 weeks after operation. Furthermore, using this novel model, we analyzed, within the context of membranous bone healing, the temporal and spatial expression patterns of several members of the bone morphogenetic protein (BMP) family, known to be critical regulators of cells of osteoblast lineage. Our data suggest that BMP-2/-4 and BMP-7, also known as osteogenic protein-1 (OP-1), are expressed by osteoblasts, osteoclasts, and other more primitive mesenchymal cells within the fracture callus during the early stages of membranous fracture healing. These proteins continue to be expressed during the process of bone remodeling, albeit less prominently. The return of BMP-2/-4 and OP-1 immunostaining to baseline intensity coincides with the histological appearance of mature lamellar bone. Taken together, these data underscore the potentially important regulatory role played by the bone morphogenetic proteins in the process of membranous bone repair.