A nonimmobilized rat tibial fracture model of endochondral osseous repair was examined for the unique localizations of specific collagen genetic types. At various stages of the healing process, the demineralized callus was reacted with immunofluorescent antibodies directed against the type specific forms of matrix collagen. Type III collagen rapidly appeared (day 8-10) and remained in the primitive mesenchymal callus until remodeled. It was particularly prominent in the highly vasoformative regions and the pericallus encapsulation but not present in preexisting cortical and neoformed lamellar bone. The type II collagen, a marker of cartilage, was uniquely located only in areas of chondroid differentiation and calcification. Type II collagen was absent from all bone and was not identified beneath the repairing intact periosteum. The differentiating chondrocytes synthesized type II collagen on an underlayer of type III collagen already within the mesenchymal matrix. From these studies of genetically unique collagen markers, it appears that only in areas of motion or anoxia does an intermediate of chondroid tissue appear. The utilization of specific type II and type III collagen immunofluorescent antibodies has facilitated the understanding of the fracture repair process and has acted as an indicator for unique matrix components.

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