Immunoreactive calbindin-D9K localization in matrix vesicle-initiated calcification in rat epiphyseal cartilage: an immunoelectron microscope study.

Calbindin-D9K immunoreactivity was localized by electron microscopy in rat calcifying epiphyseal plate cartilage. Antigen-antibody reaction sites were visualized by the presence of protein A-gold complex particles on undecalcified material embedded in Lowicryl K4M. Immunoreactive calbindin-D9K was found in the hyaloplasm of hypertrophic chondrocytes and inside and at the ends of their cell processes. It was localized outside the cells, inside matrix vesicles (MVs), often against the inner face of the delimiting membrane, and inside the trilaminar membrane. Immunoreactive calbindin-D9K appeared to be extruded from the chondrocytes into the matrix vesicles when the latter were formed during the budding of cell processes. In calcifying MVs, gold particles were detected over the needle-shaped crystallites and often over the crystallites lying against the inner leaflet of the vesicular membrane. At a later stage of matrix vesicle calcification after MV membrane disruption, the number of gold particles remained unchanged over the clusters of crystallites at the loci from which the crystallites appeared to have grown and radiated. At a yet more advanced stage of calcification, they remained in the same areas, which were limited to the lateral edges of calcified cartilage longitudinal septa. These results suggest that immunoreactive calbindin-D9K plays a role in calcium input to matrix vesicles and may be involved in matrix vesicle calcification, perhaps in the initial event of matrix vesicle crystal nucleation.