Crystalloid formation in gastrointestinal schwannoma.

We found intracytoplasmic crystalloids in two of six cases (33.3%) of gastrointestinal (GI) schwannomas. The crystalloid inclusions were periodic acid-Schiff (PAS) positive with diastase-resistance and stained blue with Masson's trichrome. They were needle-shaped and about 1 to 15 microm in length at microscopic levels. They had varying electron density revealed by electron microscopy and some of them showed distinct lattice structure with periodicity of about 9 nm. Survey of soft tissue schwannomas (n = 20) and S-100-negative GI stromal tumors (n = 41) did not detect such crystalloids. Although the origin and differentiation of GI stromal tumors (GISTs) have been a source of controversy, these intracytoplasmic crystalloids may be a marker for Schwann cell differentiation in some GIST.