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.