ABSTRACT. Malignant histiocytosis was diagnosed in 4 cows. In all cases the tumor tissues were composed of cytologically atypical histiocytes with evidence of erythrophagocytosis. The tumor in case 1 appeared highly anaplastic with marked nuclear pleomorphism, and had areas of spindle cell differentiation, but had no relation to malignant fibrous histiocytoma. The neoplastic tissue in case 2, characterized by cohesive growth of tumor cells, was distinguishable from anaplastic carcinoma cells by cytokeratin immunostaining. There were many hemosiderin-laden neoplastic cells suggestive of high phagocytic activity in a lymph node of case 3. The neoplastic cells in case 4, frequently multinucleated, were less atypical than in the other cases. All cases expressed histiocyte-associated markers (lysozyme and HAM56), and were negative for cytokeratin, S100, and T-and B-cell lineage-specific markers (CD3 and CD79a). The most frequent HAM56 immunoreactivity was detected in case 4, and the giant, multinucleated forms, reminiscent of epithelioid cell differentiation, seemed not to indicate cytological pleomorphism as a result of neoplastic transformation. KEY WORDS: bovine, erythrophagocytosis, HAM56, histiocytic marker, malignant histiocytosis.

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ABSTRACT. A neuroendocrine carcinoma originating in the thymus was found in a 7-month-old, castrated male, Japanese Black calf. The neoplasm consisted largely of very primitive cells, characterized by the paucity of cytoplasmic organelles, but a few cells were immunoreactive for somatostatin or neurofilaments. The expression of both cytokeratin and neurofilament protein was a feature of neuroendocrine differentiation. This neoplasm considered to be a tumor of a thymic stem cell, with little but indubitable evidence of differentiation into somatostatin-producing cells.-KEY WORDS: bovine, somatostatin, thymic carcinoma.J. Vet. Med. Sci. 61 (7): [853][854][855] 1999 electron microscopical examination, small blocks taken from formalin-fixed tissues were post-fixed in 1% osmium tetroxide, embedded in epoxy resin, stained with uranyl acetate and lead citrate, and examined by electron microscopy. Histologically, the neoplastic tissue in the mediastinum consisted of clusters or sheets of closely packed neoplastic cells, enclosed by variously developed collagenous fibrous tissue (Fig. 1), and was accompanied with necrotic or hemorrhagic areas. Neoplastic cells invaded the pleura and were present mainly within lymphatics, but only few neoplastic cells were detected within pulmonary alveoli. The neoplastic cells were large and polyhedral, and on occasion very large cells were detected. The nuclei were large and oval, with prominent or moderately prominent nucleoli and finely clumped chromatin. The cytoplasm was eosinophilic and relatively narrow. Mitotic figures were occasionally seen. Similar morphological features were observed in the other invasive or metastatic lesions.Immunohistochemically, somatostatin-, chromogranin-, or NF-positive cells were found in the neoplastic tissue albeit extremely rare (Fig. 2). Many neoplastic cells were positive for cytokeratin (Fig. 3) or NSE.Ultrastructurally, the neoplastic cells had ovoid nuclei containing plentiful euchromatin, but at times showed indented nuclear contours. There were poorly developed organelles in the cytoplasm. It was rare to find cells containing small numbers of dense-core secretory granules (Fig. 4), sparsely distributed glycogen granules, one or more low density lipid droplets, or globular bodies composed of intermediate filaments. Tight junctions and immature desmosomes were detected between adjoining cells. Cell borders were smooth, but on rare occasions intercellular lumina bearing microvilli were formed (Fig. 5).

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