Scanning electron microscopy studies on the origin and structure of matrix vesicles in epiphyseal cartilage from young rats

Epiphyseal plates of young rats were examined by scanning electron microscopy to study the distribution, morphology and origin of the matrix vesicles (calcifying globules) and the results were compared with the findings on transmission electron microscopy. The matrix vesicles, 0.1 μm in diameter, were found to cover the intercolumnar septa and to increase gradually in number from the zone of proliferation to the zone of provisional calcification. In the intercolumnar septa of hypertrophic and calcifying cartilage, the vesicles aggregated to form calcospherites, i.e., mineralized globules about 1 μm in diameter. Chondrocytes showed numerous cell processes that were longer and more abundant in the more mature cells. In the zone of maturation, many processes had enlarged tips with small globular projections ('bulges'), about 0.1 μm in diameter, which resembled the matrix vesicles covering chondrocytic lacunae. In the zones of hypertrophy and provisional calcification, many of these bulges aggregated to form larger, spherical structures. It is suggested that matrix vesicles in cartilage originate by budding from cell processes, and that in the zone of provisional calcification they aggregate to form the calcospherites, which are the loci of initial calcification of cartilage.