Notch Signaling is Enhanced During Endochondral Bone Regeneration Relative to Intramembranous Regeneration

INTRODUCTION: Bone regeneration occurs through a series of spatiotemporal events influenced by the mechanical environment, various local and systemic factors, and the embryological origin of the specific bone. Long bone fractures treated with flexible fixation heal primarily through endochondral ossification. Following an initial inflammatory phase, mesenchymal cells proliferate and undergo chondrogenesis to produce a cartilaginous callus that serves as a matrix for new bone formation. As the cartilage is resorbed, the callus becomes highly vascularized. Secondary bone remodeling ultimately reestablishes the original bony architecture. Alternatively, fractures treated with rigid fixation heal solely through intramembranous ossification, which involves direct bone formation without a cartilage precursor. The Notch signaling pathway plays an important role in maintenance and proliferation of mesenchymal progenitor cells. Briefly, notch activation promotes proliferation while inhibiting differentiation, whereas loss of Notch signaling enhances both chondrogenic and osteogenic differentiation while depleting the progenitor pool [1-3]. Preliminary studies in a murine tibial fracture model have shown transient Notch upregulation during endochondral bone regeneration [4]. However, Notch activation during intramembranous bone formation has not been investigated. Therefore, the objective of this study was to characterize the expression of Notch ligands, receptors, and target genes during intramembranous ossification in a murine calvarial defect (CD) injury, and to compare these results to Notch activation during endochondral ossification in a murine tibial fracture (TF) injury.