Giant cells in anaplastic mammary carcinoma of the dog and cat.

Four uncommon anaplastic mammary carcinomas containing numerous giant cells are described in three dogs and one cat. The giant cells of all cases were studied by means of monoclonal and polyclonal antibodies to detect epithelial (carcinoembryonic antigen and keratin) and mesenchymal (vimentin, lysozyme and S-100 protein) differentiation. Most of them proved to have an epithelial immunophenotype. Ultrastructurally, scattered bundles of tonofilaments but no lysosome-like bodies could be detected. One tumour had an additional, different type of giant cell, which had a benign multinucleated osteoclast-like appearance, gave positive staining for acid phosphatase, had a histiocytic-stromal immunohistochemical pattern, and was, ultrastructurally, multinucleate with irregular folds and no evidence of tonofilaments. In one case some giant cells had an epithelial immunophenotype and others a stromal immunophenotype, even though their histological and ultrastructural features were the same. In the least histologically differentiated tumour the giant cells presented a coexpression of intermediate filaments. This supported the theory that there might be a stem cell origin for most canine mammary tumours.