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It can be difficult to differentiate between epithelial and mixed type mesothelioma and metastatic adenocarcinoma. To distinguish between the two, it has recently been suggested that the presence of abluminal microvilli making contact with collagen fibrils through discontinuities in the basal lamina was characteristic of mesothelioma but not of metastatic adenocarcinoma. This study presents three patients out of 39 who had adenocarcinoma metastatic to the pleural and abdominal cavities, in which direct contact was observed between abluminal microvilli and collagen fibrils. All 39 examples of mesothelioma which were also examined demonstrated this feature. This phenomenon should therefore not be used as a discriminator between mesothelioma and metastatic adenocarcinoma. An additional patient with metastatic adenocarcinoma in an axillary lymph node demonstrates that this phenomenon is not limited to the pleural or abdominal cavities.

(1992). Anemone Cell Tumor Revisited. Ultrastructural Pathology: Vol. 16, No. 4, pp. iii-v.

Chordoid tumor--synonymous with chordoid sarcoma, parachordoma, and peripheral chordoma--is a very rare neoplasm with histologic similarity to chordoma that is found outside the axial skeleton. A soft tissue chordoid tumor in the gluteus maximus muscle of a 42-year-old man is presented. This tumor had morphologic features identical to a chordoma: nodular growth with vacuolated cytoplasm and myxomatous stroma by light microscopy, positive immunoreaction for cytokeratin and epithelial membrane antigen by immunohistochemistry, desmosomes, intercellular lumina lined with microvilli, and the presence of basal lamina material by electron microscopy. Two similar cases have been reported in the English literature.

Granular cell tumors (GCTs) were originally characterized ultrastructurally by the presence of many large lysosomes and angulate bodies (ABs). ABs appear to have been forgotten, and the diagnosis of GCT currently depends solely upon the presence of large lysosomes. Most investigators favor a Schwann cell origin for GCT. Recently, reports of granular tumors with large lysosomes but without ABs have suggested other cells of origin. If the ultrastructural criteria for the diagnosis of GCT were maintained as originally described, many of these granular tumors would not qualify as true GCTs. Two granular tumors that were at first considered GCTs are described. Based on the presence of large lysosomes and the presence or absence of ABs, one tumor was considered a true malignant GCT and the other a benign fibroblastic tumor.

A metastatic spindle cell malignant melanoma with positive immunoreactivity for S-100 protein and HMB-45 antigen and the presence of premelanosomes is described in a lymph node of the neck 11 years after removal of a superficial spreading malignant melanoma from the arm. Ultrastructurally, long, slender microvilli created a pattern with great similarity to an anemone cell tumor. A more solid pattern without microvilli was also present. Despite the abundance of microvilli in the anemone part of the tumor, intracytoplasmic lumina were not observed. In addition, crystalline structures within enlarged mitochondria and intracisternal tubules were noted.

A 72-year-old man presented with a 15 × 12 × 5 cm cystic and necrotic tumor involving the superior region of the stomach and the retroperitoneum down to the transverse colon and extending laterally from the left lobe of the liver to the spleen without involving either organ. Light microscopy showed the tumor cells to be arranged in an organoid pattern. Immunocytochemistry showed a strong reaction for desmin and vimentin. Electron microscopy demonstrated features suggestive of an alveolar soft part sarcoma.

Membrane-bound cytoplasmic crystals were found in the intrafusal fibers of a muscle spindle from a patient with neurogenic atrophy. The crystals have a periodicity of 10 nm and an intersecting axis angle of approximately 80 degrees. This makes the crystals similar to those described in alveolar soft part sarcoma (ASPS). Because the crystals in ASPS may not be quite as specific as previously believed, and because similar crystals have been described in various other neoplasms, the present findings should not be taken as evidence for a muscle spindle derivation for ASPS.

We report a rare case of malignant oncocytic carci-noid of the pancreas including the diagnostic immu-nocytochemical and electron microscopic findings. In addition the tumor shows some unusual ultra-structural features: lamelliform cristae and cyto-plasmic and nuclear crystalline structures.

Biliary tract obstruction in a 30-year-old man was found to be caused by a malignant melanoma in the common bile duct. Melanin pigment was demonstrated by immunohistochemistry and electron microscopy. Extensive search for a primary malignant melanoma elsewhere was unsuccessful. No pigmented lesions had been removed previously. There were junctional changes in the mucosa of the common bile duct close to the tumor. The malignant melanoma in the common bile duct therefore is considered to be primary. Only one other case of primary malignant melanoma in the common bile duct has been described in the literature, whereas metastases to the major bile ducts in one autopsy study of malignant melanoma in the more common locations were found with a frequency of 6 per cent.

Malignant melanoma originating outside the skin, juxtacutaneous mucous membranes, eyes, and leptomeninges is a very rare neoplasm. Two such primary visceral malignant melanomas from the lung and adrenal are described in detail.

We have recently had the opportunity to examine two cases, where malignant melanoma was suspected on light microscopy and Fontana-Masson stain was focally and faintly positive. Electron microscopy of four to five different blocks failed, however, to demonstrate the presence of premelanosomes and thereby to confirm the diagnosis of malignant melanoma. The areas in the paraffin blocks that on Fontana-Masson stain had showed a positive reaction were deparaffinized, embedded in plastic, and examined by electron microscopy. Now the characteristic premelanosomes were found and the diagnosis of malignant melanoma was confirmed.

The Weibel-Palade body (WPB), a cytoplasmic marker for endothelial cells, was utilized in this ultrastructural study to confirm the light microscopic diagnosis in four different neoplasms of suspected endothelial origin. The WPB was prominent in benign endothelial neoplasms but was less so or absent in the malignant counterpart.

Four cases of small cell carcinoma of the lung were studied ultrastructurally. The tumor cells contained scattered neurosecretory granules. Areas of the tumor that showed the characteristic crush phenomenon were found to be surrounded by myofibroblasts. It is proposed that the crush phenomenon is caused by contraction of the encircling myofibroblasts. The myofibroblasts and collagen may also cause degeneration and necrosis of the tumor cells. Myofibroblasts have not been observed in other histologic types of carcinoma of the lung.

A unique ultrastructural feature in a duodenal carcinoid is described, viz., large juxtanuclear accumulations of microfilaments arranged in whorls. Similar microfilaments are found in the D1 cell of the gastro-entero-pancreatic endocrine system. Apparently identical whorl-like accumulations of filaments have been previously described in bronchial cells of animals treated with nitrosamines and in human giant cell carcinoma of the lung. We suggest that the D1 cell is the cell of origin for the described carcinoid as well as for the above-mentioned tumours of the lung.