Electron microscopy of adipose tissue tumors: comparative features of hibernomas, myxoid and pleomorphic liposarcomas.

Two hibernomas, 16 myxoid, and eight pleomorphic liposarcomas were studied by transmission electron microscopy. Neoplastic cells in hibernomas and the type I cells--adipocytes--in myxoid liposarcomas consistently display abundant lipid droplet populations and prominent basal lamina deposition. Despite some variability in their distribution, these features clearly pertain to the mature and/or maturing characteristics of brown and white adipose cells. On the other hand, the more primitive type II cells of myxoid liposarcomas display highly variable, and occasionally absent, lipid droplet populations and basal lamina deposition. Moreover, the type II cells may also display other features such as prominent cytoplasmic filaments and rough endoplasmic reticulum that, while not characteristic of typical adipose cells, are still consistent with the more primitive mesenchymal cells from which they are assumed to derive. The cells of pleomorphic liposarcomas exhibit a variable lipid droplet population and apparently total absence of basal lamina deposition. Moreover, they often display atypical cell junctions that are neither characteristic for adipose cells nor for the "primitive reticular cells" from which adipocytes presumably derive. These observations would suggest that some "high grade" sarcomas may no be truly "undifferentiated"; rather, they may often display structural characteristics indicative of multiple as well as variable lines of differentiation. Therefore, classifications of such tumors based on quantitatively limited observations may not truly reflect their evident complexities.