Expression of four growth factors during fracture repair

Fracture repair offers an opportunity to study the physiology of bone formation at the fracture site. Isolation of growth factors from bone matrix has implicated growth factors as participants in bone physiology. We therefore examined the expression patterns of aFGF, IGF-I, PDGF, and TGF-beta during fracture repair. An animal model has been developed to study repair of tibial fractures. The model provides both reproducible and quantifiable results, allowing the fracture repair process to be divided into four stages (Bourque et al., Lab. Anim. Sci 42: 369-374, 1992). Fractured tibiae were examined immunohistochemically with polyclonal antibodies to four growth factors. PDGF was visualized in macrophages in close proximity to the periosteum during stage 1. aFGF was visualized in cells of the expanded cambial layer and was associated with a rapid increase in the population of fibroblast-like mesenchymal cells during stage 2. IGF-I was visualized in young chondroblasts at the edge of the cartilage mass replacing the fibrous callus during stage 3. TGF-beta was visualized in calcified matrix producing chondrocytes at the edge of ossification fronts penetrating the cartilage callus during stage 4. The immunohistochemical results suggest that these growth factors act as local simulators of the repair process.