September 2007

Case A

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A university research project using mdx mice had the occasional mouse that developed a firm unilateral swelling of the muscles in one hind leg.

Example histopathological description

This is a section of haired skin and subjacent skeletal muscle (hind leg). Within the subcutis and skeletal muscle, and causing protrusion of the overlying, intact dermis and epidermis, is a 10 x 10 mm, densely-cellular, well-demarcated, non-encapsulated neoplastic mass (forming 85% of the tissue section) containing isolated skeletal muscle fibres. An adjacent band of skeletal muscle contains a central, small focus of neoplastic tissue similar to the main mass. The tumour cells form closely-associated interlacing bundles and haphazard streams, and are supported by a sparse fibrovascular stroma. The cells are pleomorphic (spindle-shaped, strap-shaped or stellate), with a moderate amount of fibrillar, brightly-eosinophilic cytoplasm with distinct cell boundaries and one or more plump central nuclei which are oval or elongate, with marginated, sparse chromatin and one or two prominent eosinophilic nucleoli. There is moderate anisocytosis and anisokaryosis. In some areas, there are many multinucleated tumour cells containing up to 15 nuclei, in central clumps or rows, and in cells with wide, strap-like cytoplasm, there is a suggestion of cross-striations. Mitotic figures range from 2-5 per high power field (x 400). Although the mass is well-circumscribed, spindle-shaped tumour cells are invading the surrounding skeletal muscle and extending to the surgical margin. The band of skeletal muscle between the mass and the epidermis has marked dystrophic changes, with variation in fibre diameter, fibre splitting and hyaline degeneration.




A phosphotungstic acid haematoxylin (PTAH) stain can be used to optimize demonstration of cytoplasmic striations within neoplastic cells, to support their skeletal muscle origin. Immuno-histochemical stains to help to confirm the diagnosis would include some or all of the following immunocytological markers:

Positive: desmin, muscle specific actin, α-sarcomeric actin*, myoglobin*, myosin /fast myosin* titin, myogenin, myo-D1

Negative: vimentin, smooth muscle actin, cytokeratin.

*Specific for striated muscle

Chamberlain JS, Metzger J, Reyes M, Townsend D, Faulkner JA Dystrophin-deficient mdx mice display a reduced life span and are susceptible to spontaneous rhabdomyosarcoma. Federation of American Societies for Experimental Biology Journal 2007;21:2195-204.

Case B

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An aged cow that was necropsied as part of the national TSE surveillance program had multifocal pale liver lesions. The veterinary practitioner suspected hepatic neoplasia.

Example histopathological description

This is a 15 x 25 mm rectangular section of liver. There are three prominent circumscribed foci (7 x 7 mm, 7 x 4 mm, 4 x 3 mm) of amorphous eosinophilic material, proteinaceous fluid and cellular debris (caseous necrosis) bordered by a narrow zone of necrotic leucocytes and massive numbers of Gram-positive filamentous bacteria. Surrounding the two larger necro-suppurative foci are zones of oedema and mild fibroplasia, infiltrated by lymphocytes, with fewer macrophages and neutrophils. The smaller focus is located within one of several portal areas that are markedly expanded by mature fibrosis, with biliary hyperplasia and infiltrated by small numbers of neutrophils, lymphocytes, and plasma cells. Throughout the hepatic parenchyma there is moderate sinusoidal neutrophilia, and some hepatic venules are oedematous and infiltrated by neutrophils. There is mild to moderate cytomegaly and karyomegaly of hepatocyes. The hepatic capsule is moderately thickened by fibrosis, with a diffuse mild infiltrate of neutrophils and segmental mesothelial hypertrophy and fibrinous exudation.

Morphological diagnosis

1. Hepatitis, necrosuppurative, chronic-active, multifocal, severe, with intralesional Gram-negative filamentous organisms consistent with Fusobacterium necrophorum.

2. Hepatopathy, fibrotic, chronic, portal, severe.

Aetiological diagnosis (common name of this condition)

Hepatic necrobacillosis.