The ultrastructure of osteosarcoma. A review of twenty cases.

The ultrastructural features of 20 cases of osteosarcoma are discussed. Characteristically, the malignant cells (osteoblasts) contained large quantities of dilated, anastomosing rough endoplasmic reticulum, ofter forming large lakes. Mitochondria were sparse. Similar features are seen in osteoblasts in normal developing bone. Marked ultrastructural similarities of cells from chondrosarcomatous, fibrosarcomatous and typical osteosarcomatous areas of these tumors strongly supports the concept that these neoplastic cells all arise from the same progenitor cell. Malignant giant cells were readily differentiated from benign osteoclasts. The most significant ultrastructural findings were the varying degrees of calcification in the intercellular areas (osteoid), ranging from an extensive dense deposition of hydroxyapatite crystals which obscured the underlying collagen fibers, to focal collectors, or puffs, of slender needle-shaped hydroxyapatite crystals deposited on or along collagen fibers. The earliest apparent evidence of calcification was the presence of small electron-dense particles on or in collagen fibers.