Age-related changes in the role of matrix vesicles in the mandibular condylar cartilage

A combined approach of light microscopy, immunofluorescence, transmission electron microscopy and electron energy loss spectroscopy (EELS) was used to study age-related changes in the condylar cartilage in mice. Chondrocalcin, a cartilage matrix calcium-binding protein, was demonstrated by indirect immunofluorescence microscopy using monospecific antibodies. In one week old animals the most intense staining was observed in the matrix around the hypertrophic cells in the mineralising zone, to a lesser degree around the cells in the zone of chondroblasts, while no staining was noted in the zone of chondroprogenitor cells and in the matrix around the early hypertrophic cells. In the mineralisation zone the distribution of chondrocalcin correlated with that of mineral deposits as revealed by the von Kossa stain. The matrix between the early hypertrophic cells as shown by transmission electron microscopy revealed the presence of matrix vesicles and demonstrated a gradual accumulation of hydroxyapatite in the mineralising zone. In one month old animals chondrocalcin localisation was mainly confined to the lower hypertrophic zone which also demonstrated positive von Kossa staining was seen along the articular surface. In older animals multiple electron-dense structure that resembled matrix vesicles were observed in the non-mineralising portions of the condylar cartilage. Use of the EELS method confirmed the almost complete lack of calcium ions in these structures. In contrast, with the use of the same method, detectable amounts of calcium were recorded in vesicles in the mineralising zones of all age groups. Hence what appear ultrastructurally as structures similar to matrix vesicles represent atypical vesicles that might characterise an ageing and degenerative articular cartilage and are not necessarily associated with the mineralisation process.