The tissue was further sectioned, embedded in paraffin, and stained with hematoxylin-eosin. Immunohistochemical stains for α–smooth muscle actin (SMA) (prediluted mouse monoclonal IgG; Ventana Medical Systems, Tucson, Arizona) and pancytokeratin (mouse monoclonal IgG, 1:80 and 1:160; Becton Dickinson, San Jose, California, and Signet Laboratories, Dedham, Massachusetts) were used following standard staining protocols9,10 on Ventana Benchmark automated immunostainers (Ventana Medical Systems) at the Massachusetts General Hospital. In 2 cases, some of the tissue that had been peeled off of the back plate was submitted for transmission electron microscopy after fixation in Karnovsky fixative (glutaraldehyde, 2.5%, and formaldehyde, 2%, in 0.1M cacodylate buffer with 2.5mM calcium chloride). Tissue was processed through aqueous osmium tetroxide, 2%, and graded ethanol, transitioned with propylene oxide, and embedded in an epon substitute (t-Epon; Tousimis Research Corporation, Rockville, Maryland). One-micron sections were stained with toluidine blue and 70-nm-thin sections were stained with saturated uranyl acetate and Sato lead stain. Sections were examined with transmission electron microscopy (Philips CM 10; Philips Scientifics, Eindhoven, the Netherlands).