Ultrastructure and acid phosphatase activity of matrix vesicles and cytoplasmic dense bodies in the epiphyseal plate

Abstract

The ultrastructure and acid phosphatase activity of matrix vesicles and cytoplasmic dense bodies in the epiphyseal plate of young guinea pigs were investigated. With respect to matrix structure, shape, and location within the intercellular space the vesicles could be divided into two distinct types. Both types were bounded by a triple-layered membrane and occurred in all zones of the cartilage. Matrix vesicles type I were rounded in shape and had a finely granular, electron-dense matrix. They were located in the longitudinal septa and were most numerous in the hypertrophying and calcifying zones where they often occurred in close vicinity of early mineral deposits. Vesicles type I were structurally indistinguishable from cytoplasmic dense bodies, and probably represent extruded dense bodies. The presence of acid phosphatase activity, as demonstrated by electron microscopic histochemistry, in cytoplasmic dense bodies and matrix vesicles type I strongly suggests that these structures are lysosomes. Matrix vesicles type II usually were markedly irregular in shape, contained a matrix which structurally was similar to the ground cytoplasm of the chondrocytes, and were most often found pericellularly.