The periosteum, endosteum, and Haversian canals are the resources of pluripotent mesenchymal stem cells that initiate the formation of your healing tissues. The bridging callus observed on radiographs mostly occurs in the periosteum. It's because the fractured bone would not protrude out with the skin and expose your body to https://www.theorthopaedicandpainpractice.com/what-is-a-long-bone-fracture/