Unexpected periosteal bone apposition including newly embedded osteocytes occurs around a mouse calvaria critical defect, independently of the presence of biomaterials.

Osteocytes embedded in mineralized bone have their network disrupted when a bone injury occurs. However, their role in regeneration is still unclear. Bone substitutes, including bioceramics and bone-derived extracts, are commonly implanted in critical-sized defects due to their bioactive properties that promote healing, yet few studies have examined their effects on osteocytes in vivo. We studied early repair phases of a critical defect model in adult male mice calvaria, with or without biomaterial implantation (β-TCP, bovine bone). Using microCT we determined that, after 14 days, bone formation had mainly occurred along the surface of existing bone, increasing its thickness by a factor of 1.6, independently of the biomaterial presence. Using HE staining and fluorescence imaging, we described the newly formed bone and showed the presence of recently embedded osteocytes. We specifically collected osteocytes close, and distant from the defect, using laser-assisted microdissection and analyzed their gene expression. We show that IL6 was mainly dependent on the delay after surgery whereas Dmp1 was spatially regulated. Thus, even with limited bone formation in the defect, bone apposition occurs on the inner and outer periosteal surfaces of the calvaria, a phenomenon that may have been overlooked in the development of bone repair strategies.