Discovery of neuro- and vascular-associated periosteum-resident macrophages essential for cortical bone regeneration [neg vs pos]

The periosteum contains a highly osteogenic and neuro-vascular microenvironment that is essential for cortical bone formation and regeneration. Resident tissue macrophages (RTMs) are critical for maintaining tissue-specific niches and participating in tissue regeneration. However, the characteristics, origin and functions of periosteum-resident macrophages (PRMs) remain largely unknown. In this study, we demonstrated that PRMs are generated during embryonic hematopoiesis and are self-maintained locally during regeneration. Furthermore, by single-cell RNA sequencing analysis of periosteum myeloid cells, CX3CR1+CD45+ACE+ and CX3CR1+CD45+CD74+ cells were identified as vascular-associated and neuro-associated PRMs, respectively. Both cell types were found to arise from CX3CR1+CD168+CD45+ PRMs and shown to play critical roles in maintaining the periosteum neuro-vascular niche and promoting early-stage cortical bone regeneration. Importantly, the neuro-vascular characteristic of PRMs are directly modulated via Periostin, the essential ECM distributed in periosteum. These findings elucidate the characteristics and origins of PRMs and reveal their essential roles in maintaining the local niche and cortical bone regeneration as well as highlighting the potential value of PRMs as a therapeutic target in bone regeneration.