The Fracture Callus Is Formed by Progenitors of Different Skeletal Origins in a Site-Specific Manner

We evaluated repair following a mid-diaphyseal fracture of the tibia in 3-month-old mice. We observed differences in the repair process at three different sites of the callus. Site 1: bone developing from the outer layer of the periosteum of the cortex; site 2: bone developing within the bridge/central region of the fracture; and site 3: bone developing within the marrow of the ends of broken bones. We characterized these sites by correlating datasets from X-ray CT and histology. Correlated data demonstrated the involvement of different cells and different rates of mineralization. The origin of the progenitors and mechanism of progenitor differentiation involved at these sites was then evaluated using lineage tracing of cells expressing Prx1 and Col.2. The Prx1 progeny contributed to intramembranous bone formation (IBF) at site 1 and endochondral bone formation (EndoBF) at site 2 but not to intramedullary bone formation (IMBF) at site 3. IBF at site 1 was confirmed without a chondrocyte intermediate unlike EndoBF at site 2. Additionally, the presence of Col.2 progeny contributed to EndoBF in site 2 and IMBF in site 3 but not to IBF in site 1. However, the Col.2 progeny in IMBF in site 3 appeared to come from Col.2-expressing osteocytes originating in the cortices of the ends of the fractured bone. In conclusion we have identified three sites of bone fracture repair that differ in their origin of cells and their mechanisms of bone formation. © 2019 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.