Histochemical localization of calcium in growth plate mitochondria and matrix vesicles.

K-pyroantimonate is an anion that forms an electron-dense precipitate with cellular cations that is readily visualized at the ultrastructural level. The staining process is made relatively specific for calcium by comparing pyroantimonate treated sections to sections pretreated with ethylene glycol-bis-N,N'-tetraacetic acid, a chelating agent that removes calcium but not other cellular cations. By these means, it is shown that the antimonate-calcium complex is located predominantly in mitochondria and cell membranes throughout most of the growth plate. In the degenerating zone, however, there is a gradual loss of stain complex from the mitochondria and cell membranes and a concomitant accumulation of the stain complex by matrix vesicles. The latter are the initial site of mineralization in the growth plate as detected by these means. Thus, this study suggests that intracellular calcium plays a significant role in matrix calcification.