Angiosarcoma. The scalp tumor of a 47-year-old woman was excised and found to be a malignant spindle cell tumor by light microscopy. By electron microscopy, the cells were diagnostic for endothelial cells. These polarized cells grew in small clusters and displayed numerous caveolae (inset) that were attached to borders with adjacent cells in areas that were devoid of basal lamina. The caveolae were also located along surfaces interfacing with the extracellular matrix and bound by basal lamina (arrows). Additional cytoplasmic contents included numerous filaments, mitochondria and small strands of nondilated rough endoplasmic reticulum. Note that single caveolae are attached to plasma membranes and are present in clusters attached to the plasma membranes (inset). Collections of caveolae such as these have been termed vesiculo-vacuolar organelles. Immunoperoxidase stains confirmed the endothelial origin for this tumor. (Original magnification ×31,000 [inset, ×72,000].) (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)