Studies on the mechanism of callus cartilage differentiation and calcification during fracture healing.

: The morphologic and biochemical events during fracture callus cartilage differentiation and calcification are presented. 1. Histologic studies have demonstrated that unimmobilized fractures heal through endochondral ossification. 2. Biochemical studies have demonstrated an increase in the activities of alkaline phosphatase, enzymes involved with aerobic glucose metabolism, and lysosomal enzymes and a decrease in activities of enzymes involved with glycogen synthesis and anaerobic glycolysis. Hexosamines and hydroxyproline show a net decrease with cartilage differentiation. 3. Electron microscopic studies have demonstrated the intracellular origin and aggregation of collagen molecules, the cellular origin of matrix vesicles, and the early sites of calcification in the fracture callus.