SMOOTH TUBULAR AGGREGATES ASSOCIATED WITH PLASMALEMMAL INVAGINATION IN ALVEOLAR SOFT PART SARCOMA

The ultrastructure of four alveolar soft part sarcomas was examined to search for ultrastructural signs of myogenic or neural origin. A new ultrastructural structure, an unusual tubular structure, was found in two of four cases. The structure appeared as a large, smooth, tubular aggregate in the cytoplasm of some tumor cells but did not show a hone ycomb arrangement of tubules. The aggregate was composed of long, serp entine, branching, smooth, irregularly arranged tubules without riboso mes that ran in various directions. The aggregates intermingled with s mall amounts of cytoplasmic organelles. Because the aggregated tubules were at times continuous with cell membranes, it was shown that they were the complex extensions or invaginations of cell membranes. Neithe r myelin-axon complexes nor myofilaments, including Z band material, w ere seen in any case. There was a possibility that the smooth tubular aggregate was a T-tubule-like structure, suggesting that the tumors we re derived from skeletal muscle cells.