Spatial and temporal gene expression in chondrogenesis during fracture healing and the effects of basic fibroblast growth factor

Chondrogenesis is an essential component of endochondral fracture healing, though the molecular and cellular events by which it is regulated have not been fully elucidated. In this study, we used a rat model of closed fractur e healing to determine the spatial and temporal expression of genes for car tilage-specific collagens. Furthermore, to determine the effects of basic f ibroblast growth factor (bFGF) on chondrogenesis in fracture healing, we in jected 100 mug recombinant human bFGF into the fracture site immediately af ter fracture. In normal calluses, pro-alpha1(II) collagen mRNA (COL2A1) was detected in p roliferative chondrocytes beginning on day 4 after the fracture, and pro-al pha1(X) collagen mRNA (COL10A1) in hypertrophic chondrocytes beginning on d ay 7. In FGF-injected calluses, the cartilage enlarged in size significantl y. On day 14, both COL2A1- and COL10A1-expressing cells were more widely di stributed, and the amounts of COL2A1- and COL10A1 mRNAs were both approxima tely 2-fold increased when compared with uninjected fractures. Temporal pat terns of expression for these genes were, however, identical to those found in normal calluses. The number of proliferating cell nuclear antigen-posit ive cells was increased in the non-cartilaginous area in the bFGF-injected calluses by day 4. The present molecular analyses demonstrate that a single injection of bFGF enhances the proliferation of chondroprogenitor cells in fracture callus, a nd thus contributes to the formation of a larger cartilage. However, matura tion of chondrocytes and replacement of the cartilage by osseous tissue are not enhanced by exogenous bFGF, and this results in the prolonged cartilag inous callus phase. We conclude that, in the healing of closed fractures of long bones, exogenous bFGF has a capacity to enlarge the cartilaginous cal luses, but not to induce more rapid healing. (C) 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.