An immunohistochemical study of spontaneous histiocytic tumours in the rat.

Immunocytochemistry was used to examine 18 cases of rat fibrous histiocytic tumours (11 malignant; seven benign). The diagnosis was made by light microscopic criteria and all cases were categorized as pleomorphic-storiform. A selection of polyclonal antibodies to histiocytic, muscle, neural and mesenchymal antigens was used. Fifteen tumours were positive with alpha 1-antitrypsin, four with alpha 1-chymotrypsin, ten with muramidase, five with desmin, 15 with neuron-specific enolase, 14 with S100, one with glial fibrillary acid protein and 12 with vimentin. Many tumours expressed several antigens, highlighting the confusion which has arisen with regard to the histiogenesis of fibrous histiocytic tumours in man, and supporting the concept of differentiation from a primitive mesenchymal common precursor able to differentiate in several directions.