Submicroscopic and Immunohistochemical Profile of Surface Osteosarcomas

While the analysis of the clinical, radiologic, and histopathologic features of surface osteosarcomas has been the subject of several papers, identification of the phenotypic features of these tumors has so far received little attention. The aim of the present study was to characterize the neoplastic cells of surface osteosarcomas using an ultrastructural and immunohistochemical approach. Glutaraldehyde-fixed, epoxy resin-embedded archival pieces of tissue from 8 surface osteosarcomas (4 parosteal low-grade osteosarcomas, 3 dedifferentiated parosteal osteosarcomas, 1 periosteal osteosarcoma) were investigated using transmission electron microscopy. Sections of formalin-fixed, paraffin-embedded tumor specimens were employed for the immunohistochemical analysis of osteonectin and osteocalcin, two markers of cells of osteoblastic lineage, and alpha -smooth muscle actin and muscle specific actin. By electron microscopy, the tumors were composed of a mixture of neoplastic cells with varied differentiation, i.e., osteoblast-like, fibroblast-like, myofibroblast-like, and chondroblast-like. The latter were particularly abundant in the periosteal osteosarcoma. Osteocalcin expression was detected in the cytoplasm of neoplastic cells in 6 cases (66.6%), while osteonectin was expressed at least focally in all cases. The expression of the noncollagenous bone proteins was higher in low-grade osteosarcomas than in dedifferentiated osteosarcomas. alpha -Smooth muscle actin and muscle-specific actin expression were detected in 4 (44.4%) and 5 (55.5%) cases respectively, and the distribution was similar in both low-grade and dedifferentiated lesions. The results do not confirm previous observations regarding the prevalence of a specific cellular phenotype in surface osteosarcomas. Further, the myofibroblast-like cells that are present in variable numbers in these tumors are probably modified osteoblasts, since they co-express actin, osteonectin, and osteocalcin.