Cutaneous plexiform schwannoma in a pig.

A plexiform schwannoma (PFS) observed as a solitary mass in the dermis of a 6-month-old pig consisted of schwannoma cells of Antoni A type and B type. Neoplastic cells in Antoni A type areas sometimes showed cord-like outgrowths or a neurofibromatous pattern. Neoplastic cells in Antoni B type areas showed erythrophagocytosis, some encircling the microvasculature. Immunohistochemically, neoplastic cells were strongly positive for S-100 protein and vimentin. Peripheral parts of the nodules were cytokeratin (clone AE1/AE3)-positive, as in normal swine perineurial cells. Double immunostaining clearly demonstrated neoplastic cells doubly positive for both S-100 protein and cytokeratin, suggesting that S-100-positive Schwann cells and cytokeratin-positive perineurial cells are functional variants of the same cell type. Ultrastructurally, neoplastic cells in Antoni A type areas possessed characteristics of Schwann cells, such as cytoplasmic interdigitation, external laminae and intercellular junctions. At the periphery of the nodules, features of perineurial cells were detected. Neoplastic cells in Antoni B type areas seemed to be undergoing degenerative processes similar to those in Antoni A type regions and they contained many lysosomes. The neoplasm was generally similar in both location and histology to that seen in man, but there were some histological, immunohistochemical and ultrastructural differences. This is the first reported case of PFS in domestic animals.