T H E HISTOGENESIS OF NERVE SHEATH MYXOMA: R E P O R T O F A CA SE W I T H E L E C T R O N M IC R OS C OP Y

J. N. WEBB Department of Pathology, Western General Hospital, Edinburgh, EH4 2 X U , and the University of Edinburgh, Medical School.

PLATES XXIII AND XXIV BENIGNtumours of nerve sheath origin, other than the so-called traumatic neuromas may be subdivided into two categories : (i) neurofibromas, which embrace a variety of clinical and histological variants and (ii) Schwannomas. A much rarer tumour, of myxomatous character, is also considered to be of nerve sheath origin and has been designated simply as nerve-sheath myxoma by Harkin and Reed (1969). On account of its rarity and uncertain histogenesis it does not appear to be a clearly defined or widely recognised entity. Tumours of apparently identical character have also been described as Pacinian Neurofibromas (MacDonald and Wilson-Jones, 1977) but they do not closely resemble neurofibromas and an origin from the sensory end-organs of Vater-Pacini has not been proved. In this paper I describe the histology and electron-microscopy of a tumour of this type. The findings provide convincing evidence that these tumours are derived from the perineural cell. CASEREPORT A woman of 71 years of age presented in 1976 with a swelling in the pulp of her left index finger, the swelling having increased in size over the previous 5 yr. Clinically it was thought to be an inclusion dermoid. The excised tumour measured 2 . 0 1.5 ~ x 1.5 cm. The cut surface was grey and gelatinous with distinct lobulations. METHODS Parafin sections of formalin-hed tissue were stained by the following methods: HE, PAS, alcian green, luxol fast blue and Glees and Marslands method for axis cylinders. For electron-microscopy 1 mm3 of the formalin-hed tissue was washed in cacodylate buffer and post-fixed in Osmium tetroxide. Thin araldite sections were stained with uranyl acetate and lead citrate. Histology (Figs. 1, 2 and 3.) The tumour is situated in the dermis and subcutis and consists of well-defined lobules of myxomatous tissue separated by fibrous septae. The cellularity of the lobules varies considerably but the general pattern is that of hyperchromatic bipolar or stellate cells scattered within a myxomatous matrix. Some tumour cells are multinucleate and appear to form a syncytium. In one or two areas the tumour cells form fascicles with

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Received 30 April 1978; accepted 15 May 1978. J. PATH.-VOL.

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distinct nuclear palisading reminiscent of a Schwannoma. No mitotic figures are seen. The matrix is rich in acid mucopolysaccharide staining intensely with alcian green stain. No axis cylinders are identified within the tumour. A normal Pacinian corpuscle is identified adjacent to the tumour (fig. 4). Electron microscopy (Figs. 5 and 6.) The tumour cells are all of similar type and no cells with the features of fibroblasts or smooth muscle cells are identified. They show complex interdigitations of their cytoplasm with occasional junctional complexes (zona adherens). The cytoplasm contains abundant filaments of about 10 nm diameter; no dense areas are seen in relation to these filaments. Mitochondria are few and small. There is some rough endoplasmic reticulum as well as small numbers of free ribosomes. Some cells contain a few lysosomes. Many cells have a well-defined basal lamina applied to the plasmalemma which appears to be a condensation of the stroma. This membrane is generally incomplete and is absent in some cells.

DISCUSSION The myxomatous tumour described in this paper closely resembles the myxomas of nerve sheath described by Harkin and Reed (1969) and the Pacinian neurofibromas described by MacDonald and Wilson-Jones (1977). These latter authors have reviewed the literature of these apparently rare tumours: they point out that six out of eight cases (which included four of their own) arose in the hands or feet. The reason for supposing that these tumours arise in the sense-organs of Vater-Pacini would appear to rest on (i) the characteristic site-where these structures are particularly plentiful and (ii) on the histology of the tumour lobules which bear a resemblance to Pacinian corpuscles (figs. 3 and 4). The electron-microscopic findings in the present case establish that the tumour cells are of nerve sheath origin, the ultra-structural features resembling those of the perineural cell; a cell derived from neuro-ectoderm. The neuroectodermal sheath forms the covering of the central nervous system, the piaarachnoid (Harvey and Burr, 1926), and of the peripheral nervous system, the perineurium (Shanthaveerappa and Bourne, 1926). In fact the perineurium may be considered as an extension of the pia-arachnoid. The similarity of the tumour cells to those of meningiomas (Napolitano et al., 1963) and normal arachnoid villi (Shabo and Maxwell, 1968) is striking and provides convincing evidence for an origin from the perineural cell; the complex cytoplasmic interdigitations, cytoplasmic filaments, cell junctions and basal lamina being the characteristic features. The tumour cells are clearly more akin to the aforementioned neuro-ectodermal cells than they are to the cells of neurofibromas and Schwannomas as revealed by electron microscopy (Waggener 1966), these tumours being composed either of Schwann cells and fibroblasts or solely of Schwann cells. Whether or not such tumours are derived from the Vater-Pacini corpuscles is more debatable. However, anatomical studies suggest that the lamellar cells of the corpuscles are derived from perineural cells, being an extension of the cellular elements of the perineural sheath of the peripheral nerve (Shanthaveerappa and Bourne 1963). An electron-microscopic study of a tumour described as a Pacinian neurofibroma revealed that the cells were of

PLATEXXIII

WEBB NERVE SHEATH

MYXOMA

FIG.1 .-Low-power of tumour showing lobular structure. Haematoxylin and eosin (HE). x 40.

FIG.2.-Lobules of myxomatous tissue intersected by fibrous septae. Areas in lobules show nuclear palisading. HE. x 85.

FIG.3.-High-power of tumour showing hyperchromatic bipolar spindle cells within abundant myxomatous matrix. HE. x 225.

FIG.4.-Pacinian corpuscle at tumour marginfor comparison with fig. 3. HE. x 225.

PLATEXXIV

WEBB NERVESHFATH

MYOMA

FIG.5.-Electron micrograph of tumour cell. Note complex cytoplasmic interdigitations. Mitochondria are few and small. Basal lamina (b). Matrix (M). UA/LC ~ 2 0 , 0 0 0 . Insetshowing basal lamina (b). x 60,000.

FIG.6.-Detail

of tumour cell. Note abundant micro-filaments (F) within cytoplasm. Note small mitochondria. UA/LC x 60,000.

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perineural type (Weiser 1975). This accords with the frndings and interpretation of the present case. MacDonald and Wilson-Jones have suggested that Weiser’s case may have shown Meisserian or pseudo-Meisserian differentiation. It may very well be however that these tumours of perineural origin are derived from more than one type of sensory nerve ending. Since the tumour cells are all of one type it must be assumed that the perineural cells are capable of producing acid mucopolysaccharides, as fibroblasts do not contribute to these tumours. The term myxomatous perineuroma would appear to describe these tumours most exactly while leaving open the question of an origin from Vater-Pacini corpuscles or other sensory nerve endings. Such a derivation does however seem probable. SUMMARY

The case is described of a 71-yr-old female with a tumour in the pulp of the left index finger. The histology was that of a myxoma of nerve sheath or Pacinian neurofibroma. Electron microscopy of the tumour provides convincing evidence for an origin from the perineural cells. The location of these tumours, suggestive histology and perineural origin provides support for the belief that they arise from sensory nerve endings. REFERENCES HARKIN, J. C., AND REED,R. J. 1969. Tumours of the peripheral nervous system. Fascicle 3. Second Series. Armed Forces Institute of Pathology, Washington, D.C., p. 60. HARVEY,S. C., AND BURR,H. S. 1926. The development of the meninges. Arch. Neurol., 15, 545-567. MACDONALD, D. M., AND WILSON-JONES, E. 1977. Pacinian Neurofibroma. Histopath., 1, 247-255. NAPOLITANO, L., KYLE,R., AND FISHER, E. R. 1963. The ultra-structure of meningiomas and the derivation and nature of their cellular components. Cancer, 17, 233-241. SHABO, A. L., AND MAXWELL, D. S. 1968. The morphology of the arachnoid villi: a light and electron-microscopicstudy in the Monkey. J. Neurosurg., 29,451-463. SHANTHAVEERAPPA, T R ,AND BOURNE, G. H. 1962. The perineural epithelium. A metabolically active, continuous protoplasmic cell barrier surrounding peripheral nerve fasiculi. J. Anat., 96, 527-537. SHANTHAVEERAPPA, T. R., AND BOURNE, G. H. 1963. Observations on the structure of the Pacinian corpuscle and its relation to the perineural epithelium of peripheral nerves. Amer. J. Anat., 112, 97-109. WAGGENER, J. D. 1966. Ultra-structure of benign nerve sheath tumours. Cancer, 19, 699-709. WEISER,G . 1975. An electron-microscope study of Pacinian Neurofibroma. Virchows Archiv fur pathologische Anatomie und Histologie, 366, 331-340.

The histogenesis of nerve sheath myxoma: report of a case with electron microscopy.

T H E HISTOGENESIS OF NERVE SHEATH MYXOMA: R E P O R T O F A CA SE W I T H E L E C T R O N M IC R OS C OP Y J. N. WEBB Department of Pathology, Weste...
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