Bone tissue is constantly replaced throughout life in a process known as remodelling. Bones also have a remarkable capacity to heal following fracture. However, in some circumstances, including in aging, or traumatic injuries, healing is poor, and surgery is required. Osteoblasts, the cells that form bone, are thought to originate from mesenchymal stem cells (MSC), but methods to identify MSCs are controversial. There is also evidence that there may be multiple osteoprogenitor populations rather than a single MSC population, which can contribute to bone formation in different settings, such as during growth, or following injury. I propose to study two types of osteoprogenitor populations that have not been well characterized: periosteal progenitors and bone lining cells.