Schwannoma. A 66-year-oldman had a nonenlarging retroperitoneal mass for 3 years that was composed of Schwann cells. The spindle cells and their extensive, narrow cellular processes were encased in basal lamina (arrows). The tumor cells lacked surface vesicles, bundles of microfilaments, dense bodies and dense surface plaques—features characteristic for tumors of smooth muscle cell origin. Immunoperoxidase stains were positive for vimentin and S-100 and negative for muscle-specific actin. Original magnification × 7,500. (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)