It is currently estimated that approximately 5.6 million fractures occur annually in the United States and between five and ten percent of these will result in delayed or impaired healing. Although the biology of fracture repair has been extensively studied several aspects of this process remain incompletely understood and thus clinical problems continue to occur. Recent evidence that a variety of intercellular signals and growth factors are involved in the healing of the soft tissue wounds suggests that this area may represent a gap in our knowledge of fracture repair. Cytokines are a group of immunomodulatory proteins formerly thought to be unique products of immune cells. In the broad sense: cytokines are considered to be a class of growth factors. Recent data indicate that certain musculoskeletal tissues both produce and respond to specific cytokines. Since cytokines regulate a variety of cellular events in soft tissue wound healing, and since these polypeptides are now known to be important modifiers of cellular responses in cartilage and bone, this project proposes to investigate the role of specific cytokines in fracture healing. To lay foundation for a long term project to elucidate how specific cytokines may regulate certain aspects of fracture healing, this project proposes to do the following: 1. Develop a fracture callus organ culture system and characterize it extensively with respect to sequential changes in its histology, histomorphometry, biochemistry and the expression of specific genes associated with developing callus tissues. 2. Determine the pattern of production of Specific cytokines during fracture healing and relate these Changes to the histological, histomorophometric biochemical and molecular phenotypic changes which will have been established in this model. Once it is known which cytokines are produced by fractures and at what times during healinq, future proposals may be designed to elucidate which cells produce and respond to these polypeptides and-how they are regulated at the molecular level. Ultimately, it may be possible to develop therapies using cytokines to prevent fracture complications and enhance healing. Since a variety of traumatic conditions and surgical procedures require that bone repair itself in a fashion similar to that which occurs during fracture healing, the knowledge derived from this project could have wide spread application to musculoskeletal disease.