Osteoclasts drive bone formation in ectopic and orthotopic environments

To date, cell-based approaches to stimulate bone formation have primarily focused on mesenchymal stromal cells (MSCs) for their supposed osteogenic potential, but despite some pre-clinical successes, clinical outcomes have remained unsatisfactory. Emerging data suggest that osteoclasts play crucial roles in stimulating bone formation beyond their catabolic function in bone resorption. Interestingly, osteoclastic activity precedes osteoblastic bone formation in the physiological bone remodeling cycle. To explore the role of osteoclasts in bone formation further, we prepared osteoclast-based constructs and implanted them (i) ectopically to evaluate their potential to induce bone formation, and (ii) orthotopically to evaluate effects on bone regeneration. Remarkably, constructs containing primary mouse osteoclasts showed consistent and robust de novo bone formation, which presented comparable osteogenic efficacy to BMP-2 treatment. Additionally, we observed de novo bone marrow formation upon ectopic implantation of osteoclast-based constructs (incidence 73 %) and BMP-2 loaded controls (incidence 91 %). Importantly, constructs containing macrophages (MФs) or scaffold only (negative control) showed neither bone nor bone marrow formation. Further, a mouse cranial defect model confirmed the stimulatory bone regeneration capabilities of Osteoclast-based constructs, evidenced by 2.5-fold increased bone formation compared to scaffold only. These findings demonstrate the osteoinduction and osteogenesis capacity of osteoclasts, reshaping our understanding of their role in bone formation and opening new avenues for the design and development of cell-based constructs for bone repair.