Electron microscopic studies on the initial phases of calcification in guinea pig epiphyseal cartilage

Abstract

The initial calcification in guinea pig epiphyseal cartilage was studied ultrastructurally. The first signs of mineralization of the intercellular substance were found in the central parts of the longitudinal septa. Here, mineral deposits occurred in the form of long, slender crystals which most often were located in the close vicinity of matrix vesicles type I. At more advanced stages of calcification, mineral crystals were sometimes seen also within these vesicles. Almost regularly a diffuse material of low electron density was associated with the clusters of mineral. The amount of collagen fibrils as well as of proteoglycan granules was lower in the central, partly calcified portions, than in the peripheral, noncalcified portions of the longitudinal septa. No clear morphological relation between the collagen fibrils and the initial laying down of mineral was observed. The observations are discussed with reference to a possible role of matrix vesicles in the processes which make the cartilaginous matrix calcifiable.