RESUMO
To evaluate the differentiation status of smooth muscle in gastric stromal tumors which were negative for S-100 protein, immunohistochemistry using desmin, actin, myosin and vimentin was performed in 14 cases of gastric smooth muscle tumors. Ultrastructural Examination was also performed. For comparison a case of leiomyoma of the esophagus, a case of the sigmoid colon, 10 cases of the uterus were also examined. The results obtained were as follows. All gastric smooth muscle tumors showed vimentin-positivity. Six of 14 gastric smooth muscle tumors, (5 of 8 leiomyoma and 1 of 4 leiomyosarcoma) showed positivity for desmin, actin, and myosin(42.9%). All esophageal, colonic, and uterine leiomyomas showed diffuse positive reaction for desmin, actin, and myosin. Vimentin positivity was also noted in leiomyoma of the colon and uterus. Ultrastructurally, a few cells in the gastric stromal tumors had scattered microfilaments with dense bodies, subplasmalemmal dense plaques, and micropinocytic vesicles. However, most of the tumor cells did not have any of the ultrastructural features of smooth muscle differentiation. Leiomyomas of the esophagus and uterus showed many cytoplasmic microfilaments with dense bodies. These results suggest that most of the benign and malignant tumor cells of gastric stromal tumors have features of the undifferentiated cells, immunohistochemically as well as ultrastructurally, although a few cells have. It is speculated that most gastric stromal tumors may have lost their smooth muscle differentiation.