The expression of the fibrillar collagen genes during fracture healing: heterogeneity of the matrices and differentiation of the osteoprogenitor cells

The cells that express the genes for the fibrillar collagens, types I, II, III and V, during callus development in rabbit tibial fractures healing und er stable and unstable mechanical conditions were localized. The fibroblast -like cells in the initial fibrous matrix express types I, III and V collag en mRNAs. Osteoblasts, and osteocytes in the newly formed membranous bone u nder the periosteum, express the mRNAs for types I, III and V collagens, bu t osteocytes in the mature trabeculae express none of these mRNAs. Cartilag e formation starts at 7 days in calluses forming under unstable mechanical conditions. The differentiating chondrocytes express both types I and II co llagen mRNAs, but later they cease expression of type I collagen mRNA. Both types I and II collagens were located in the cartilaginous areas. The hype rtrophic chondrocytes express neither type I, nor type II, collagen mRNA. O steocalcin protein was located in the bone and in some cartilaginous region s. At 21 days, irrespective of the mechanical conditions, the callus consis ts of a layer of bone; only a few osteoblasts lining the cavities now expre ss type I collagen mRNA. We suggest that osteoprogenitor cells in the periosteal tissue can differen tiate into either osteoblasts or chondrocytes and that some cells may exhib it an intermediate phenotype between osteoblasts and chondrocytes for a sho rt period. The finding that hypertrophic chondrocytes do not express type I collagen mRNA suggests that they do not transdifferentiate into osteoblast s during endochondral ossification in fracture callus.