Scanning and transmission electron microscopic observations on the origin and structure of matrix vesicles in normal and papain-digested epiphyseal cartilage of young rats

Epiphyseal plates of young rats were examined by light, scanning (SEM) and transmission (TEM) electron microscopy in order to study the distribution, morphology and origin of the matrix vesicles (calcifying globules).SEM studies showed two types of globular structure. Small globules, 0.02 to 0.06 μm in diameter, were probably proteoglycan granules as they were removed by papain extraction. Larger globules, 0.1 to 0.2 μm in diameter, covered the longitudinal and, to a lesser extent, the transverse intercartilaginous septa and increased gradually in number from the zone of proliferation to the zone of provisional calcification. In the longitudinal septa of hypertrophic and calcifying cartilage, these globular structures calcified to form larger, globular calcospherites, about 1 μm in diameter, which contained hydroxyapatite crystals. Chondrocytes had many cell processes, which were longer and more numerous in the more mature cells. In the zone of hypertrophy, many of the cell processes had enlarged tips, with small globular bulges about 0.1 μm in diameter, which could not be extracted by papain and resembled the larger globules that covered the intercartilaginous septa. TEM studies confirmed that the larger globules seen by SEM are membrane-bound matrix vesicles that are attached to the outer surface of chondrocytic membranes and processes and also cover the inner aspects of chondrocytic lacunae. These matrix vesicles were not removed by papain digestion. In the zone of provisional calcification, mineralized vesicles were seen within longitudinal septa, as were larger spherical structures - the calcospherites - that contained mineral crystals but were devoid of any membrane. It is therefore assumed that matrix vesicles in cartilage arise by budding from cell processes and that in the longitudinal septa of the hypertrophic zone they mineralize, lose their membrane and form larger calcospherites. With advancing calcification, the whole longitudinal septum calcifies.