Ultrastructure of human osteosarcoma. Malignant transformation of a multipotential connective tissue cell.

Ten cases of osteosarcoma were studied by electron microscopy. The tumors consisted of six cell types: fibroblastic, myofibroblastic, chondroblastic, osteoblastic, unclassified and histiocytic cells. Disturbed structure of dilated endoplasmic reticulum was a common feature. The neoplastic character of myofibroblastic and histiocytic cells is controversial. Myofibroblastic differentiation was most abundant in parosteal osteosarcoma and in fibrosarcomatous intraosseal osteosarcoma. The malignant cells sometimes formed giant cells and many aggregates of these cells were seen. Osteoclasts and other reactive cells were encountered and this may indicate host reaction against the tumor cells. Formation of collagenous and cartilaginous ground substance was poor, and the capacity of collagen to mineralize was decreased. It is concluded that osteosarcoma is the malignancy of a multipotential connective tissue cell which forms callus in normal osteogenesis.