PDGFRαreporter activity identifies periosteal progenitor cells critical for bone formation and fracture repair

The outer coverings of the skeleton,which is also known as the periosteum,are arranged in concentric layers and act as a reservoir for tissue-specific bone progenitors.The cellular heterogeneity within this tissue depot is being increasingly recognized.Here,inducible PDGFRαreporter animals were found to mark a population of cells within the periosteum that act as a stem cell reservoir for periosteal appositional bone formation and fracture repair.During these processes,PDGFRαreporter^(+)progenitors give rise to Nestin+periosteal cells before becoming osteoblasts and osteocytes.The diphtheria toxin-mediated ablation of PDGFRαreporter^(+)cells led to deficits in cortical bone formation during homeostasis and a diminutive hard callus during fracture repair.After ossicle transplantation,both mouse PDGFRαreporter^(+)periosteal cells and human Pdgfrα+periosteal progenitors expand,ossify,and recruit marrow to a greater extent than their counterpart periosteal cells,whereas PDGFRαreporter^(−)periosteal cells exhibit a predisposition to chondrogenesis in vitro.Total RNA sequencing identified enrichment of the secreted factors Fermt3 and Ptpn6 within PDGFRαreporter^(+)periosteal cells,which partly underlie the osteoblastogenic features of this cell population.

Comparison of skeletal and soft tissue pericytes identifies CXCR4+ bone forming mural cells in human tissues

Human osteogenic progenitors are not precisely defined,being primarily studied as heterogeneous multipotent cell populations and termed mesenchymal stem cells(MSCs).Notably,select human pericytes can develop into bone-forming osteoblasts.Here,we sought to define the differentiation potential of CD146 f human pericytes from skeletal and soft tissue sources,with the underlying goal of defining cell surface markers that typify an osteoblastogenic pericyte.CD146+CD31~CD45_pericytes were derived by fluorescence-activated cell sorting from human periosteum,adipose,or dermal tissue.Periosteal CD146+CD31—CD45 cells retained canonical features of pericytes/MSC.Periosteal pericytes demonstrated a striking tendency to undergo osteoblastogenesis in vitro and skeletogenesis in vivo,while soft tissue pericytes did not readily.Transcriptome analysis revealed higher CXCR4 signaling among periosteal pericytes in comparison to their soft tissue counterparts,and CXCR4 chemical inhibition abrogated ectopic ossification by periosteal pericytes.Conversely,enrichment of CXCR4+pericytes or stromal cells identified an osteoblastic/non-adipocytic precursor cell.In sum,human skeletal and soft tissue pericytes differ in their basal abilities to form bone.Diversity exists in soft tissue pericytes,however,and CXCR4+pericytes represent an osteoblastogenic,non-adipocytic cell precursor.Indeed,enrichment for CXCR4-expressing stromal cells is a potential new tactic for skeletal tissue engineering.