Matrix vesicles in bone tumors. Ultrastructural analysis and their significance in neoplastic bone formation

Bone tumors were categorized into alkaline phosphatase (ALPase)-positive (2 ossifying fibromas, 1 benign osteoblastoma and 16 osteosarcomas) and negative (2 chondromas, 2 chondrosarcomas, 3 non-ossifying fibromas, 2 malignant fibrous histiocytomas and 6 giant cell tumors of bone) groups. Production and distribution of matrix vesicles (MVs) in the tumor tissues were examined to clarify their role in neoplastic bone formation. Four distinct types of MV were isolated primarily in ALPase positive bone tumors: empty, amorphous, crystalline and ruptured MVs. They were formed by budding off from the cytoplasmic projections of the osteoblastic tumor cells. The significance of differences in the production rate of MVs between ALPase-positive and negative bone tumors was investigated in view of the predominantly high production of MVs in ALPase-positive bone tumors. Many more mature MVs (crystalline and ruptured) were observed in the osteoblastic lesions of osteosarcoma than in the fibroblastic and MFH-like lesions, suggesting an intimate relationship with maturation and differentiation of the osteoblastic tumor cells. The above findings indicate that production of MVs is one of the diagnostic parameters for osteoblast-derived bone tumors, as well as ALPase activity, and that vesicle-induced mineralization is a major mineralization mechanism in neoplastic bone formation.