Pigmented
neoplasms of the peripheral nervous system are uncommon. Such lesions, while well documented in the para-axial region, are infrequently encountered in more peripheral extra-axial sites. The authors present a case of a melanocytic
schwannoma arising in the gastric antrum of a 51-year-old woman. Ultrastructural features attesting to the nerve sheath origin of the
neoplasm included the presence of a well-developed basal lamina, complex infoldings of the plasma membrane, and intercellular bundles of long spaced
collagen ("Luse bodies"). Premelanosomes and melanosomes in all stages of development were identified within the cytoplasm of many of the neoplastic cells, providing morphologic support for a close relationship between Schwann cells and melanocytes. Immunoperoxidase studies with
S-100 protein disclosed intense cytoplasmic staining throughout the
tumor. The
S-100 protein antigen has been documented in both nerve sheath elements and melanocytes; this shared antigenicity lends further support to the close relationship between melanocytic and schwannian elements suggested by ultrastructural observations. Additional immunoperoxidase studies employing antibody to
glial fibrillary acidic protein revealed focal cytoplasmic staining, an uncommon phenomenon in peripheral nerve sheath
neoplasms. A review of the literature failed to provide any previous well documented cases of melanocytic
schwannoma of the stomach. The patient was treated with en bloc resection of the
tumor and gastric antrum, and was free of evidence of recurrent or metastatic disease 22 months postoperatively. Theories regarding the histogenesis of pigmented
nerve sheath neoplasms and prognostic implications are discussed, based on experience with this and previously reported pigmented
nerve sheath tumors.