Molecular aspects of healing in stabilized and non-stabilized fractures

Bone formation is a continuous process that is initiated during fetal devel opment and persists in adults in the form of bone regeneration and remodeli ng. These latter two aspects of bone formation are clearly influenced by th e mechanical environment. In this study we tested the hypothesis that alter ations in the mechanical environment regulate the program of mesenchymal ce ll differentiation, and thus the formation of a cartilage or bony callus, a t the site of injury. As a first step in testing this hypothesis we produce d stabilized and non-stabilized tibial fractures in a mouse model, then use d molecular and cellular methods to examine the stage of healing. Using the "molecular map" of the fracture callus, we divided our analyzes into three phases of fracture healing: the inflammatory or initial phase of healing, the soft callus or intermediate stage, and the hard callus stage. Our resul ts show that indian hedgehog(ihh), which regulates aspects of chondrocyte m aturation during fetal and early postnatal skeletogenesis, was expressed ea rlier in an non-stabilized fracture callus as compared to a stabilized call us. ihh persisted in the non-stabilized fracture: whereas its expression wa s down-regulated in the stabilized bane. IHH exerts its effects on chondroc yte maturation through a feedback loop that may involve bone morphogenetic protein 6 [bmp6 (S. Pathi, J.B. Rutenberg. R.L. Johnson, A. Vortkamp, Devel opmental Biology 209 (1999) 239-253)3] and the transcription factor gli3, b mp6 and gli3 were re-induced in domain adjacent to the ihh-positive cells d uring the soft and hard callus stages of healing. Thus, stabilizing the fra cture, which circumvents or decreases the cartilaginous phase of bone repai r, correlates with a decrease in ihh signaling in the fracture callus. Coll ectively, our results illustrate that the ihh signaling pathway participate s in fracture repair, and that the mechanical environment affects the tempo ral induction of ihh, bmp6 and gli3. These data support the hypothesis that mechanical influences affect mesenchymal cell differentiation to bone. (C) 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.