Macrophage-lineage [TRAP.sup.+] cells recruit periosteum-derived cells for periosteal osteogenesis and regeneration

Cortical bones account for more than 80% of human bone mass. The periosteum, a thin tissue that covers almost the entire bone surface, is essential for bone formation and regeneration. However, its osteogenic and bone regenerative abilities are not well studied. In this study, we found that macrophage-lineage cells recruit periosteum-derived cells (PDCs) for cortical bone formation. Knockout of colony-stimulating factor-1 eliminated macrophage-lineage cells and resulted in loss of PDCs with impaired periosteal bone formation. Moreover, macrophage-lineage tartrate- resistant acid phosphatase-positive ([TRAP.sup.+]) cells induced transcriptional expression of periostin and recruitment of PDCs to the periosteal surface through secretion of PDGF-BB, where the recruited PDCs underwent osteoblast differentiation coupled with type H vessel formation. We also found that subsets of [Nestin.sup.+] and [LepR.sup.+][CD45.sup.-][Ter119.sup.- ][CD31.sup.-] cells ([LepR.sup.+] PDCs) possess multipotent and self-renewal abilities and contribute to cortical bone formation. [Nestin.sup.+] PDCs are found primarily during bone development, whereas [LepR.sup.+] PDCs are essential for bone homeostasis in adult mice. Importantly, conditional knockout of Pdgfr-[beta] in [LepR.sup.+] cells impaired periosteal bone formation and regeneration. These findings uncover the essential role of periosteal macrophage-lineage cells in regulating periosteum homeostasis and regeneration.