Immunolocalization of BMPs, BMP antagonists, receptors, and effectors during fracture repair

Bone morphogenetic proteins (BMPs) are potent bone inducers used clinically to enhance fracture repair. BMPs have been shown to be produced in the fracture callus; however, the comparative expression of BMPs and BMP signaling components has only been partially examined at the cellular level. The aim of the present study was to establish a detailed spatiotemporal localization of BMPs and BMP signaling components in mouse models of stabilized and nonstabilized fractures. During healing of nonstabilized fractures, which occurs via endochondral ossification, BMP2, 3, 4, 5, and 8, noggin, BMPRIA, BMPRII, and pSmad 1/5/8 were immunolocalized in the activated periosteum as early as 3 days after fracture. BMP2, 4, 5, 6, 7, and 8 and noggin were also found in isolated inflammatory cells within granulation tissue during the early stages of repair, but not BMP receptors and effectors. During the soft callus phase of repair, all BMPs and BMP signaling components were detected in chondrocytes with various intensities of staining depending on the stage of chondrocyte differentiation and their location in the callus. The strongest staining was observed in hypertrophic chondrocytes with decreased intensity during the hard callus phase of repair. All BMPs and components of the BMP pathway were detected in osteoblasts and osteocytes within new bone, with strongest intensity of immunoreaction reported during the early soft callus phase followed by decreasing intensity during the hard callus phase of repair. Most components of the BMP pathway were also detected in endothelial cells associated with new bone. In stabilized fractures that heal strictly via intramembranous ossification, BMPs and BMP antagonists were detected in isolated inflammatory cells and BMP signaling components were not detectable in osteoblasts or osteocytes within new bone. In conclusion, the BMP signaling pathway is primarily activated during fracture healing via endochondral ossification, suggesting that this pathway may influence the mode of healing during the recruitment of skeletal progenitors.