Calluses flex their muscles to align bone fragments during fracture repair

Abstract

Neonatal animals spontaneously reduce fractures, yet the mechanical forces influencing this process are poorly understood. In this issue of Developmental Cell, Rot et al. (2014) show that muscle and the fracture callus actively position fractured neonatal bone fragments to restore their alignment, highlighting the multifaceted roles of mechanical cues in skeletal regeneration.

Figure 1. Mechanical forces mediate natural reduction

Initially, an inflamed soft callus forms at the neonatal fracture site. Mechanical asymmetry in the callus directs differential cellular responses. On the convex side where cells sense tension, fibrocartilage forms. On the concave side where cells sense compression, endochondrogenesis ensues (magnified image). The distinct bidirectional chondrogenesis on the concave side produces mechanical forces that push on the bony fragments and contribute to their realignment along with muscle contractions. Once properly aligned and mechanically stabilized, ossification of the callus progresses and remodeling of the bone finalizes the structure to restore function.