Orchestrating the bone microenvironment to enhance bone-vascular coupling in critical-sized bone defect...

Bone remodeling is driven by the tightly coordinated actions of osteoclast-mediated resorption and osteoblast-driven formation. During the early phase of bone defect healing, resorption predominates. Pro-inflammatory cytokines promote M1 macrophage polarization, and hypoxia-inducible factor-1α (HIF-1α) directs metabolic reprogramming to meet osteoclast energy demands. Moreover, reactive oxygen species (ROS) amplify receptor activator of nuclear factor κB (RANK) - RANK ligand (RANKL) signaling to accelerate osteoclastogenesis, and the resulting acidic microenvironment facilitates hydroxyapatite (HA) dissolution and clearance of necrotic debris. As healing progresses into the reparative phase, macrophages transition toward an M2 phenotype. During this stage, HIF-1α/vascular endothelial growth factor (VEGF) signaling and antioxidant-autophagy pathways become upregulated, enhancing cellular resistance to stress. The microenvironment also shifts from acidity toward mild alkalinity, which supports osteoblast differentiation and HA deposition. This review integrates the pleiotropic actions of osteogenic lineage cells and the spatiotemporal orchestration of bone-vascular coupling. It further examines how convergent microenvironmental stressors, including hypoxia, inflammation, oxidative stress, and acid-base imbalance, inform strategies aimed at reprogramming the bone microenvironment. These approaches strengthen bone-vascular coupling and enable a robust self-repairing program of critical-sized bone defects. By targeting these convergent stress pathways, it is possible to interrupt pathological feedback loops and establish a mechanistic framework for promoting endogenous self-repair of critical-sized bone defects.