Ultrastructural and Cytochemical Studies on the Calcification of the Tendon-Bone Joint

Abstract

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-bound vesicular structures were concentrated in the spaces between the longtudinal cell columns of the fibrocartilage. In areas where calcification was beginning, the vesicles became denser and more osmiophilic in appearance. The 1st 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 and 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 Ca, 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, Ca was gradually lost from the cell and reversely accumulated in the matrix vesicles.