Figure 7.

Gastrointestinal stromal tumors (GISTs). A: GISTs are subepithelial polyps that can arise from anywhere in the gastrointestinal (GI) tract. They are commonly accompanied by an ulcerated center. B: These lesions arise from the interstitial cells of Cajal, the pacemaker cells of the GI tract that reside between the inner circular and outer longitudinal layers of the muscularis propria (MP). By definition, therefore, these lesions must arise in the MP. GISTs need not be considered in the differential diagnosis if a lesion arises in the mucosa (M) or submucosa (SM; hematoxylin and eosin stain, 20× magnification).