Cartilage calcification: normal and aberrant.

Abstract

Hydroxyapatite crystal deposition occurs physiologically in cartilage as a prelude to bone formation via endochondral ossification. Both extracellular molecules and organelles, and the chondrocytes themselves control the initial formation of hydroxyapatite, as well as the growth and orientation of the hydroxyapatite crystals. In this study pathologic calcium containing deposits from 48 patients (27 hydroxyapatite and 21 calcium pyrophosphate dihydrate) were subjected to crystallographic, histologic (light microscopy, scanning electron microscopy, transmission electron microscopy) and chemical analyses and compared to normal controls. It is suggested that both hydroxyapatite and calcium pyrophosphate dihydrate deposition involve elevations in ionic concentrations, exposure of mineral nucleators, and removal of mineral inhibitors. Peculiar to the matrices of pathologic deposits of hydroxyapatite are elevated concentrations of calcium acidic phospholipid phosphate complexes and lower concentrations of hexosamine while collagen (hydroxyprolin) and total lipid contents are not altered. Matrices of deposits of calcium pyrophosphate dihydrate were similar biochemically except that calcium acidic phospholipid phosphate complexes concentrations were not elevated.