Ultrastructure of low-grade intraosseous osteosarcoma of bone: a comparative study with fibrous dysplasia and parosteal osteosarcoma.

The ultrastructure of low-grade intraosseous osteosarcoma (LGOS) is not well documented in the literature. Four cases of LGOS are described with an emphasis on its distinguishing characteristics as well those it shares with other lesions. The predominant cells of LGOS are fibroblasts with well-developed rough endoplasmic reticulum and mild focal immunoreactivity to SMA and MSA. Few osteoblasts and myofibroblasts are present. Transition cells between fibroblasts and osteoblasts are also noted. The fibroblasts are closely related to osteoblasts from a histological and functional point of view, and phenotypically are probably modified osteoblasts. Comparative ultrastructural studies between LGOS and fibrous dysplasia (FD) reveal basic similarities, although the cells in LGOS are larger with more of an abundance of organelles. Therefore, accurate differentiation between these two lesions rests at the histological and radiological levels. LGOS and parosteal osteosarcomas (PO) also share similar ultrastructural features. In the case samples in this study, an unusual type of multilayered amorphous material was found in the osteoid matrix of a case of LGOS and one of PO. This probably emphasizes the morphologic similarities between these 2 tumors.