The occurrence of hydroxyapatite crystals in extracellular matrix vesicles after surgical manipulation of the rabbit knee joint.

Free autologous grafts of synovial tissue were transplanted into experimental defects produced in the articular cartilage of rabbit knee joints. The grafted tissue underwent transformation into fibrocartilage. Extracellular matrix vesicles associated with calcified areas were present at the grafted sites. Hydroxyapatite crystals were found within these vesicles and in their vicinity. No calcification occurred in articular cartilage from sham operated joints in which defects were produced but no grafts made and in normal controls. These tissues showed abundant matrix vesicles devoid of crystalline mineral. A careful study of normal synovial tissue did not reveal matrix vesicles and calcifications. The present observations suggest that matrix vesicles in normal articular cartilage exist in a latent form. Vesicle mineralization following surgical manipulations of the joint is probably a manifestation of the metabolic stage of the tissue.