The role of muscle in bone repair: the cells, signals, and tissue responses to injury.

Bone repair is a complicated process that includes many types of cells, signaling molecules, and growth factors. Fracture healing involves a temporally and spatially regulated biologic process that involves recruitment of stem cells to the injury site, tissue specific differentiation, angiogenesis, and remodeling. In light of its proximity to bone and abundant vascularity, muscle is an important potential source of cells and signals for bone healing. More complete understanding of the role of muscle in bone formation and repair will provide new therapeutic approaches to enhance fracture healing. Recent studies establish that muscle-derived stem cells are able to differentiate into cartilage and bone and can directly participate in fracture healing. The role of muscle-derived stem cells is particularly important in fractures associated with more severe injury to the periosteum. Sarcopenia is a serious consequence of aging, and studies show a strong association between bone mass and lean muscle mass. Muscle anabolic agents may improve function and reduce the incidence of fracture with aging.