Histochemical localization of calcium in the fracture callus with potassium pyroantimonate. Possible role of chondrocyte mitochondrial calcium in callus calcification.

Potassium pyroantimonate was employed as a histochemical stain for calcium at the ultrastructural level in the cartilaginous fracture callus in the rat rib. In areas of the callus showing no matrix mineralization, the electron-dense precipitate of the antimony-calcium complex was heavily deposited in chondrocyte mitochondria, lipid, and cell membrane. In areas showing early mineralization the mitochondria, lipid, and cell membrane showed a smaller amount of antimony-calcium complex, and in areas of more advanced matrix mineralization the mitochondria, lipid, and cell membrane were completely void of any stain. In the matrix, the initial site of mineralization was associated with matrix vesicles located in areas of early matrix mineralization. These findings suggest the hypothesis that mitochondria play an important role in matrix calcification in cartilaginous fracture callus.