Major fibrillar collagens and fibronectin in an experimental nonunion: an immunohistochemical study.

We studied in a rat pseudoarthrosis model the time sequence of expression and distribution of fibronectin and collagens I, II, III and V. Collagens and fibronectin were immunolocalized at the light microscopic level. The major difference from the normal healing pattern was the extension of collagen II and cartilage into the interfragmentary area and at the circumference of the periosteal callus, without any bridging chondral or bony elements in the fracture gap. The formation of a fibrous bond, consisting mostly of collagen III and fibronectin, was observed. This speaks in favor of the failure of the multipotentional mesenchymal stem cells to change the fracture-healing process towards fibroblast proliferation and the production of tissue unable to mineralize. The decisive zone for mineralization of the callus appeared to be the area of the hypertrophied chondrocytes near the periosteal ossification front.