Ultrastructure of benign cartilaginous tumors of intraosseous origin.

The ultrastructure of chondroblastoma, chondromyxoid fibroma, and enchondroma is described. In chondroblastoma the tumor cells often show nuclear indentations and a thick nuclear fibrous lamina, and have features indicative of poorly differentiated cartilaginous cells. Chondroblastoma is a tumor thought to arise from immature chondrocytes related to the epiphyseal cartilage. In chondromyxoid fibroma the cells are pleomorphic with irregular cell processes and scalloping of the cell membrane. The nuclei are also often indented and demonstrate the presence of a thick nuclear fibrous lamina. The matrix consists of proteoglycan granules and fine fibrils. The large bizarre atypical cells often seen in this tumor, except for their size, do not differ from the typical cells. Chondromyxoid fibroma arises from cells related to the epiphyseal cartilage and demonstrates moderate degrees of cartilaginous differentiation. The close relationship between chondromyxoid fibroma and chondroblastoma is emphasized. The nuclear irregularities and the presence of a thick fibrous lamina are features shared by these two lesions. The tumor cells of enchondroma have features of well differentiated cartilage, and the pericellular matrix tends to be sparse around most cells. In addition to proteoglycan granules and fine fibrils, the matrix contains many dense membrane bound bodies. Numerous tumor cells undergoing necrosis are noted. Studies comparing enchondroma and well differentiated chondrosarcoma show no morphologic differences and both tumors have similar features. The application of electron microscopy does not provide pathognomonic features to distinguish between these lesions. However, it helps to correlate the ultrastructural findings with light microscopy.