Ultrastructural and cytochemical studies on developing tendon-bone joints (fibrocartilage) of rats indicated that the initial calcification loci were, as in some other calcifying tissues, matrix vesicles. These membrane-bounded vesicular structures were concentrated in the spaces between the longitudinal cell columns of the fibrocartilage.
Where calcification was going to start, the vesicles became denser and more osmiophilic in appearance. The first identifiable needles of apatite crystals were deposited in and close to the matrix vesicles. Where calcification was more extensive and crystals radiating from each calcification center formed a calcified spherule, the matrix vesicles were no longer visible in the spherule. No ultrastructural relation between collagen fibrils and the initial deposits of minerals was noticed at the site of the initial calcification.
Alkaline phosphatase activity was demonstrated in the matrix vesicles as well as in the plasma membrane of fibrocartilage cells, most intensely at the initiating site of calcification.
Following potassium pyroantimonate fixation to produce electron opaque deposits of calcium, the deposits were located mainly in the mitochondria and plasma membrane of fibrocartilage cells as well as in matrix vesicles. The closer to the initial site of calcification, the stronger the cells and vesicles reacted. In the area where calcification was in progress, however, calcium was gradually lost from the cell and reversely accumulated in the matrix vesicles.
The ultrastructural and cytochemical findings from this study suggested that they were of cellular origin and liberated into the matrix from cells, partly by way of budding- off and partly by cell disintegration. The possible roles of matrix vesicles at the initiating sites of calcification in fibrocartilage were discussed.