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The authors report a malignant peripheral nerve sheath tumor (MPNST) that developed in the body of the pancreas of a 76-year-old woman. The well-demarcated tumor, 4.5 x 3.5 cm in size, was composed of spindle-shaped cells arranged in a storiform pattern or sweeping fascicles. Ultrastructurally, the tumor cells had elongated cytoplasmic processes with numerous pinocytotic vesicles, and were covered with discontinuous basal lamina-like substances. Immunohistochemically, tumor cells were diffusely positive for epithelial membrane antigen, vimentin, and CD34, but negative for S-100 protein and Leu 7. It was therefore concluded that the tumor was a MPNST with perineurial cell differentiation. Primary MPNST of the pancreas is extremely rare and the immunohistochemical and ultrastructural studies are necessary for its diagnosis.

The participation of the perineurial cell in peripheral nerve tumors is the subject of much debate. The case of a 75-year-old female with a soft tissue tumor on her left shoulder is presented. The tumor had histological, ultrastructural, and immunohistochemical characteristics of a pure perineurial cell neoplasm. Ultrastructurally, distinctive ribosome-lamella complexes were found in the cytoplasm of the perineurial cells. This may be the first time that these structures have been described in perineurioma.

The present ultrastructural evaluation of 12 acquired intradermal melanocytic nevi revealed that in contrast to the nested epithelioid melanocytic nevus cells of the upper dermis, the spindle nevus cells of the deep dermis showed perineurial differentiation, exhibiting a spindly configuration characterized by a melanosome-free cytoplasm that showed extremely slender bipolar contour and contained abundant intermediate filaments, a decreased number of cytoplasmic organelles, and, significantly, a fair number of plasmalemmal pinocytotic vesicles. The nevic corpuscles were found to consist of laminated slender cytoplasm showing subcellular conformation similar to that of the spindle nevus cells. By immunohistochemistry, many spindle nevus cells and nevic corpuscles were immunoreactive for nerve growth factor receptor. All the nevus cells were immunoreactive for vimentin and S-100 protein, and negative for protein gene product 9.5, epithelial membrane antigen, Leu-7, and myelin basic protein. Characteristically, protein gene product 9.5 immunohistochemistry revealed numerous immunoreactive axons intermingled with the spindle nevus cells in the deep portion. All the PGP9.5-immunoreactive axons were observed by immunoelectron microscope to be unmyelinated and always ensheathed by a thin cytoplasmic process of Schwann cells but not nevus cells. These findings indicate that differentiation plasticity exists in the various nevus cells, with the epithelioid nevus cells and the spindle nevus cells displaying more ultrastructural and immunophenotypical characteristics of melanocyte and perineurial cells, respectively, suggesting that a pluripotential cell of neural crest origin accounts for the histogenesis of this lesion.

The perineurial cells that make up the perineurium of peripheral nerve fascicles are characterized by distinct ultrastructural features, including non-branching thin cytoplasmic processes coated by an external lamina and joined at their ends by a tight junction, few organelles, actin and vimentin filaments, and numerous pinocytotic vesicles. Perineurial cells are immunoreactive for vimentin and epithelial membrane antigen (EMA) but not for the Schwann cell markers S-100 protein and Leu-7. The cytogenesis of the perineurium remains disputable, with morphologic, immunohistochemical, and experimental evidence supporting origin from the fibroblast, Schwann cell, and arachnoid cap cell. Ultrastructural studies more recently supported by immunolocalization of EMA have detected hyperplastic and neoplastic perineuriallike cells in a number of pseudoneoplastic lesions and true neoplasms, notably localized hypertrophic neuropathy, neurofibromas of various types, and perineurioma.

This review introduces the traditionally defined anatomic compartments of the peripheral nerves based on light and electron microscopic topography and then explores the cellular and the most recent molecular basis of the different barrier functions operative in peripheral nerves. We also elucidate where, and how, the homeostasis of the normal human peripheral nerve is controlled in situ and how claudin-containing tight junctions contribute to the barriers of peripheral nerve. Also, the human timeline of the development of the barriers of the peripheral nerve is depicted. Finally, potential future therapeutic modalities interfering with the barriers of the peripheral nerve are discussed.

A case of soft tissue tumor in the left brachialis muscle of a 49-year-old Japanese female patient was studied by electron microscopy. The tumor was diagnosed as intramuscular myxoma by light microscopy, but electron microscopic observation revealed that the tumor almost entirely consisted of cells similar to normal perineurial cells. The tumor cells possessed long, slender cytoplasmic processes covered by well-developed but discontinuous basal laminae, clusters of pinocytotic vesicles, and infrequent intercellular junctions. Perineurial cells have also been observed in other peripheral nerve lesions: neurofibromas, nerve sheath myxomas, and localized hypertrophic neuropathies. However, the term "perineurioma" or "perineurial cell tumor" should be reserved for discrete tumorous masses that are almost entirely composed of perineurial cells without evidence of residual axons, Schwann cells, fibroblasts, or tactile corpusclelike structures. Perineurioma may represent a third category of peripheral nerve sheath tumors, ultrastructurally distinct from schwannomas and neurofibromas.

A case of a malignant spinal schwannoma in a 5-year-old boy is reported. Electron microscopic findings permitting the classification of the tumor and its possible origin from the nerve sheath (perineurial cell) are discussed.