The ultrastructural cytology and chemical composition of fracture callus cartilage.

The differentiation and calcification of fracture callus cartilage studied by light microscopy resembles that of the epiphyseal plate cartilage. In this study, a correlation between the ultrastructural cytology and the chemical composition of the various cartilage types found in fracture callus was attempted. Fibrocartilage cells were very active with profuse secretion of collagen into the matrix. Collagen fibres were observed in close proximity to the cell membrane. Cells from hypertrophic and calcifying cartilage showed progressive degeneration while their mitochondria contain dense osmiophilic granules. Mitochondria granules were occasionally seen in fibrocartilage cells. The intercellular matrix of hypertrophic and calcifying cartilage showed needle-shaped crystals quite often arranged as a cluster in a radiated pattern. Chemical analyses of the various types of cartilage found in fracture callus demonstrated the presence of a high molecular weight bound phosphate and an increase of sialic acid in the differentiated types of cartilage.