The periosteum, endosteum, and Haversian canals tend to be the sources of pluripotent mesenchymal stem cells that initiate the development from the healing tissues. The bridging callus viewed on radiographs mostly occurs in the periosteum. It is because the fractured bone would not protrude out from your pores and skin https://www.theorthopaedicandpainpractice.com/what-is-a-long-bone-fracture/