Submicroscopic and immunohistochemical profile of surface osteosarcomas

While the analysis of the clinical, radiologic, and histopathologic feature s of surface osteosarcomas has been the subject of several papers, identifi cation of the phenotypic features of these tumors has so far received littl e attention. The aim of the present study was to characterize the neoplasti c cells of surface osteosarcomas using an ultrastructural and immunohistoch emical approach. Glutaraldehyde-fixed, epoxy resin-embedded archival pieces of tissue from 8 surface osteosarcomas (4 parosteal low-grade osteosarcoma s, 3 dedifferentiated parosteal osteosarcomas, 1 periosteal osteosarcoma) w ere investigated using transmission electron microscopy. Sections of formal in-fixed, paraffin-embedded tumor specimens were employed for the immunohis tochemical analysis of osteonectin and osteocalcin, two markers of cells of osteoblastic lineage, and alpha-smooth muscle actin and muscle specific ac tin. By electron microscopy, the tumors were composed of a mixture of neopl astic cells with varied differentiation, i.e., osteoblast-like, fibroblast- like, myofibroblast-like, and chondroblast-like. The latter were particular ly abundant in the periosteal osteosarcoma. Osteocalcin expression was dete cted in the cytoplasm of neoplastic cells in 6 cases (66.6%), while osteone ctin was expressed at least focally in all cases. The expression of the non collagenous bone proteins was higher in low-grade osteosarcomas than in ded ifferentiated osteosarcomas. alpha-Smooth muscle actin and muscle-specific actin expression were detected in 4 (44.4%) and 5 (55.5%) cases respectivel y, and the distribution was similar in both low-grade and dedifferentiated lesions. The results do not confirm previous observations regarding the pre valence 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, ost eonectin, and osteocalcin.