Signet ring epithelioid stromal tumor of the small intestine

Cells having a signet ring appearance can occur in mesenchymal, lymphoid, and other nonepithelial neoplasms. We report the case of an intestinal stromal tumor with smooth muscle differentiation and a prominent signet ring cell component. The presence of signet ring forms of smooth muscle cells in sections of paraffin-embedded tissue often contrasts with a lack of cytoplasmic spaces by electron microscopy, and the ultrastructural finding of signet ring-like areas in the present case can be attributed to the fact that the tissue for electron microscopy was retrieved from paraffin blocks where this peculiar artifact already existed. Ultrastructural examination of the signet ring-like areas suggests that they originated as retraction spaces which may have resulted from variations in intracellular tension forces related to the distribution of actin filaments.