Ultrastructural and Immunohistochemical Study of a Gastric Bizarre Leiomyoma: Bizarre Nuclei and Prominent Cytoplasmic Processes Extending Into and Displacing the Stroma

This study presents the ultrastructural and immunohistochemical findings of a gastric bizarre leiomyoma arising in the vestibule of a 79-year-old male. Histologically, loosely proliferating tumor cells consist of large, multinucleated, bizarre nuclei with intranuclear inclusions and abundant cytoplasm-containing vacuoles. A murky line was apparent between the tumor cells and eosinophilic and heterogenous stroma-like areas. Immunohistochemically, tumor cells exhibited positively stained dot patterns of α-smooth muscle actin and caldesmon, which were distributed in the cytoplasm of tumor cells and stroma-like regions. Ultrastructurally, tumor cells exhibited extended and complex cytoplasmic processes comprising the fascicles of filamentous fibers. These structures were also detected in the apparent stroma-like regions observed histologically and were consistent with the α-smooth muscle actin- and caldesmon-immunopositive dot structures. The original stromal areas remained as considerably narrow gaps between tumor cells with extended cytoplasmic processes. To the best of our knowledge, this is the first report detailing the unique ultrastructural and immunohistochemical characteristics of tumor cells and the limited stromal composition of an extremely rare primary gastric bizarre leiomyoma.