Tumors of the peripheral nerve are predominantly those of the nerve sheath but can also include some peripheral primitive neuroectodermal tumors, gastrointestinal autonomic nerve tumors, and peripheral neuroblastoma. This presentation will concern tumors of the peripheral nerve sheath - schwannoma, neurofibroma, perineurioma, and malignant peripheral nerve sheath tumor, and their variants.

THE CELLS OF THE NERVE SHEATH
The epineurium is a layer of connective tissue enclosing the other layers of the nerve. The nerve sheath cells are principally the Schwann cell and the perineurial cell, both characterized by cytoplasmic processes.

Schwann cells, the inside layer of the endoneurium, surround the axon and produce myelin, external lamina and collagen. They have interdigitating cytoplasmic processes and continuous external lamina. In myelinated nerves there is one Schwann cell per axon, and in non-myelinated nerves there are several axon segments within a single Schwann cell, but with very few layers of schwannian plasma membrane. The Schwann cell is neural-crest derived and is positive for S100 protein, Leu7, laminin, and myelin basic protein, and negative for cytokeratin, epithelial membrane antigen (EMA), desmin and muscle actins.

Perineurial cells form a few circumferential layers outside the endoneurium. They are bipolar (rarely tripolar) cells with very long, thin processes. The perineurial cell differs from a fibroblast by the presence of external lamina (continuous or interrupted), pinocytosis, and intercellular junctions. Perineurial cells derive from the arachnoid, and display positivity for EMA, but not for cytokeratin, S100 protein or (usually) Leu7, although perineurial-like cells occasionally show actin positivity. A third cell type has been identified within the endoneurium as a slender dendritic spindle cell which displays CD34 positivity, and which appears to be distinct from Schwann, perineurial or endothelial cells. It may be analogous to the dermal dendritic fibroblast. CD34 positive cells are found in increased numbers in neurofibromas and in Antoni B areas of schwannomas, but only in about 15% of MPNST.

Pseudomesaxon	Schwannoma

BENIGN PERIPHERAL NERVE SHEATH TUMORS
Schwannomas show strong diffuse S100 protein-positivity, and also Leu7. GFAP has been reported in about a third of schwannomas (more frequently in those in the GI tract (Daimaru et al, 1988)) and also in 40% of neurofibromas (Gray et al, 1989). Electron microscopy shows Schwann cells only; attenuated interdigitating cell processes lie in undulating layers adjacent to cell body, covered in continuous external lamina 500 Å thick, some of which is also redundant in extracellular space. The cytoplasm contains microfibrils, lysosomes, mitochondria. In Antoni B areas, the cells are dispersed in a myxoid stroma, the cytoplasm has more lysosomes and myelin figures and a fragmented external lamina, so that these perhaps represent degenerate Antoni A areas. Long-spacing collagen in seen in the stroma of benign peripheral nerve sheath tumors and occasionally in malignant ones, but is not specific.

In cellular schwannoma, the ultrastructure is that of well differentiated schwann cells but with less interdigitation than in regular schwannoma. External lamina can be absent when processes are closely apposed. EM is occasionally useful in the differential with smooth muscle tumor.

Granular cell schwannoma is the suggested term for granular cell tumor (previously myoblastoma), because of its S100 positive immunophenotype, occasional neural connection, and occasional appearance in neurofibromatosis. EM shows numerous autophagosomes with various lipoid and membranous inclusions.

Neurofibromas have focal S100 positivity in schwann cells but less strongly and in fewer cells than in schwannoma . The majority of neurofibromas are negative for EMA but about 25% show some positivity in irregular fascicles of cells, as well as around the edge (residual nerve). Ultrastructurally there is a mixture of Schwann cells, with or without nerves, perineurial-like cells and fibroblasts, with intermediate forms between the cell types.

In the plexiform type, the early stage is increased endoneurial matrix, separating nerve fibres, which are later replaced by Schwann cells with thick wavy collagen bundles, and scattered small axons. EM shows Schwann cells, fibroblasts, and perineurial cells.

Perineuriomas are rare, and not all those reported as such are acceptable. However, they do appear to be monomorphic proliferations which have the gross morphology (a discrete, circumscribed but unencapsulated mass) of a tumor rather than hyperplasia. Such tumors (not to be confused with intraneurial perineurioma or localised hypertrophic neuropathy, see below) are reported in soft tissues, more commonly in women and affecting the back or shoulder, as similar to neurofibroma but more cellular (although hypocellular examples have been reported). The cases described by Giannini et al had evidence of deletion of part of chromosome 22(M-bcr locus at 22q11). Histologically, they are neurofibroma-like, fibromatosis-like or even meningioma-like variably cellular spindle cell lesions with whorls or lamellar formations. Myxoid change is common. Lacking both Schwann cells and S100 immunoreactivity, this tumor is best recognized ultrastructurally and by EMA positivity, although this can be faint as the cytoplasmic processes are very slender. Sclerosing perineurioma is a lesion affecting predominantly the thumb, fingers and palm, composed of small epithelioid and spindled cells in cords, trabeculae and whorls in a fibrous stroma. In addition to EMA positivity, some of these expressed cytokeratin.

Cellular Schwannoma	Perineuroma

Localized Hypertrophic Neuropathy (intraneural perineurioma, onion bulb perineurial cell hyperplasia) affects young adults, males more than females, and especially the posterior interosseous nerve. This forms a large cylindrical enlargement a few cms long. Histologically, in cross section there are sheets of onion bulb-like transections. EM shows each with a central axon plus schwann cell, surrounded by layers of cells with curved thin processes, pinocytotic vesicles, and interrupted external lamina. These are EMA-positive and S100 negative and are regarded as perineurial cells. The spatial relationship of the three elements has suggested a hyperplastic rather than neoplastic process, but the recent observation in intraneural perineuriomas of a clonal abnormality involving loss of 22q11.2 (Emory et al, 1995) has suggested the converse (the NF2 tumor suppressor gene is localized to 22q11.2-q12). This lesion should not be confused with extraneural perineurial cell tumor in soft tissue.

Ossifying Fibromyxoid Tumor This recently recognized entity is of uncertain nature but there is evidence to suggest that it might be a schwann cell tumor with low malignant potential. It is a tumor of adults, commoner in men, occurring in the subcutis or muscle mostly of the extremities. The tumor has a thick fibrous capsule from which septa extend in, giving a partially lobulated pattern. 80% have foci of mature benign lamellar bone within the capsule and its extensions. The tumor cells are small and round with vesicular nuclei and ill-defined cytoplasm and cell margins, arranged in cords or small nests or focally glomus-like. The stroma is myxoid and focally fibrosed or hyaline, in places suggesting osteoid formation. Mitotic activity is usually minimal and there is no necrosis or vascular invasion. Atypical variants have been described. Some cases recur, and an occasional metastasis has been reported.

The majority of cells are strongly positive for S 100 protein and for NSE, and some have GFAP positivity. No reactivity is detected with HMB45, or with antibodies to cytokeratin, EMA, or neurofilament. However, in some examples of this tumor a few cells have desmin and actin positivity. Electron microscopy shows the tumor cells to have rather short cytoplasmic processes with occasional cell membrane thickening suggestive of attempted junction formation but no definite desmosomes. Many cells have a discontinuous, thick external lamina which is focally reduplicated into the stroma. There are tangles of cytoplasmic intermediate filaments but no characteristic myofilaments. Ribosome-lamella complexes have been described. The tumor differs ultrastructurally from myxoid chondrosarcoma.

MALIGNANT PERIPHERAL NERVE SHEATH TUMORS
MPNST has a variety of morphologic patterns - neurofibroma-like, fibrosarcoma-like with alternating cellular and myxoid zones, neuroid whorls and subintimal invasion, epithelioid, and with divergent differentiation. Only 50-70% of MPNST (including the epithelioid type) are S100 protein positive, often in more differentiated, loose, wavy areas. S100 immunoreactivity is helpful diagnostically in tumors of a spindle cell type, but other spindle cell tumors can be positive, including synovial sarcoma. MPNST with heterologous elements will contain the corresponding antigens. MPNST lacks neurofilament, but occasional tumors are NSE positive, and some express myelin basic protein. Muscle-specific actin or smooth muscle actin may be seen in many non-muscle tumors including MPNST. Cytokeratin may be present in scattered cells in the spindle component, causing a problem with distinction from synovial sarcoma in some cases. CD34 and bcl2 are variably expressed, but usually only focally.

Many cases of MPNST, so categorized by a neural connexion and appropriate histology, show no differentiation at the ultrastructural level. Electron microscopy of well differentiated MPNST of usual spindle cell type can show numerous cytoplasmic processes which interdigitate with those of other cells and occasionally form junctional complexes. These are of variable shape, size and development. MPNST can also contain basal lamina; however, this can be detached, incomplete or undeveloped in poorly differentiated tumors. Long spacing collagen is also found on occasion.

Correlation between ultrastructure and immunohistochemistry. Cases with good schwann cell differentiation (interdigitating cytoplasmic processes, external lamina) have strong S100 positivity and also Leu7. A spectrum of decreasing morphological resemblance to schwann cells is demonstrable (Fisher et al), accompanied by loss of immunohistochemical markers. Hirose et al (1989) have reported EMA positivity in MPNST with schwannian ultrastructure, and also cytokeratin (although not necessarily in their EMA + cases).

Malignant perineurial cell tumor Some poorly differentiated (S100 negative) MPNST are found ultrastructurally to have features of perineurial cell differentiation. Sporadic cases have been described (Hirose 1989; Fisher, 1990, ), and a series of 7 cases in adults was reported in 1998 by Hirose et al, as occasionally necrotic spindle cell sarcomas with mitoses, which were EMA positive; two displayed CD34. EM showed perineurial-like bipolar cells with pinocytosis and fragments of lamina in 3 cases, and cells intermediate between perineurial and schwann cells in one. Four examples recurred and two metastasized but there were no deaths in the period of follow up.

Malignant Epithelioid Schwannoma (MES) is usually S100 protein positive, if there is good schwannian differentiation. Superficial examples of these tumors are sometimes difficult to distinguish from melanomas but they lack HMB45 (as do some melanomas). Also, some clear cell sarcomas of tendons and aponeuroses have ultrastructural features of nerve sheath differentiation.

Divergent Differentiation Tumors with nerve sheath and skeletal muscle differentiation include: neuromuscular hamartoma, medulloblastoma with rhabdomyosarcoma, rhabdomyosarcoma with ganglion cells (ectomesenchymoma), and malignant schwannoma with rhabdomyosarcoma (malignant Triton tumor). In the latter, rhabdomyoblasts are scattered through the schwannomatous background. They are usually spindled or rounded and relatively mature, with eosinophilic cytoplasm, and perhaps cross-striations Muscle markers are positive and S100 usually so and EM shows features of both cell types. Glandular malignant schwannoma has a spindled background and may have other heterotopic elements such as cartilage or osteoid. The glands are well differentiated nonciliated cuboidal or columnar cells with clear cytoplasm, occasional goblets, and neuroendocrine cell differentiation (demonstrable by chromogranin, etc) (Christensen et al, 1988; Woodruff & Christensen 1993). Mucin is demonstrable and there is ultrastructural evidence of intestinal differentiation. Occasionally there are malignant looking glands. EM of the spindle cells can assist diagnosis from S100 positive biphasic synovial sarcoma.

Differential Diagnosis: The principal differential diagnosis of MPNST is provided in Tables 1 and 2.

TABLE 1
	Monoph synovial	Leiomyosarcoma	Fibrosarcoma	MPNST
Nuclei	Plump, pale, ovoid, overlapping	Blunt-ended	Tapered	Buckled, wavy
Patterns	Parallel, alternating, nodules	Criss cross, 90 degrees	Parallel, herringbone	Alternating, neuroid whorls, subintimal spread
Immunohistochemistry
SMA / MSA	A few	Yes	Focal	Rarely
Desmin	No	Yes	No	Rarely (& Triton)
S100 protein	30%	Some	No	60%
Cytokeratins	Yes	Some	No	Very rarely
EMA	Yes	Some	No	Very rarely
CD34	No	Some	No	Some
bcl2	All	No	No	Some

TABLE 2
Tumor type	EM features
MPNST	Interdigitating processes, lamina, mesaxons, long-spaced collagen
Synovial sarcoma (mono)	Short processes in intercellular spaces, junctions
Leiomyosarcoma	External lamina, pinocytosis, myofilaments, attachment plaques
Fibrosarcoma	Abundant ER, Golgi, no lamina or junctionns
Myofibrosarcoma	RER, Myofilaments, fragmented lamina, fibonexus