Congenital neural hamartoma ("fascicular schwannoma"). A light microscopic, immunohistochemical, and ultrastructural...

A previously undescribed form of a congenital neural hamartoma composed entirely of Schwann cells in a fascicular pattern was found on the leg of a male infant. The lesion was thought to be an unusual variant of plexiform Schwannoma or a newly recognized unencapsulated form of Schwannoma. On light microscopic examination, the lesion, which measured 5 x 4 cm when it was surgically removed when the infant was 7 months old, showed an unencapsulated dermal mass composed of fascicles of spindle cells with frequent Verocay body-like structures. The intervening stroma was collagenous and contained an increased number of mast cells. Special stains did not demonstrate any axons in the tumor. There was a strongly positive immunohistochemical reaction for S-100 protein and collagen type IV in the spindle cells. These cells were weakly or focally positive for Leu-7 and vimentin, and completely negative for neural filaments, neuron-specific enolase, glial fibrillary acidic protein, epithelial membrane antigen, desmin, and muscle-specific actin. On electron microscopic examination, the spindle cells were found to be surrounded by basal lamina and showed frequent cytoplasmic invagination filled with collagen bundles. No unmyelinated nerve fibers were identified.