Skeinoid Fibers: An Ultrastructural Marker of Neurogenic Spindle Cell Tumors

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Abstract
The article describes novel stromal fibrillar aggregates found in three cases of neurogenic spindle cell tumor and eight cases of small intestinal stromal tumor. The aggregates were composed of tangles of curvilinear fluffy fibrils with a periodicity of from 41 to 48 nm with a staining pattern similar to that of collagen fibrils. The overall ultrastructural appearance simulated skeins of yarn, hence they are designated skeinoid fibers. No similar stromal fibers were found in more than 5000 other tumors studied. Their exclusive presence in neurogenic spindle cell tumors suggests the possibility that they are an ultrastructural marker for neurogenic spindle cell tumors and that the eight cases of small intestinal stromal tumors with skeinoid fibers may be of neurogenic origin.