Pulmonary granular cell tumour in a horse: anatomo-histopathological, immunohistochemical and ultrastructural investigations.

We report a histological, immunohistochemical and ultrastructural characterisation of a new case of granular cell tumour in the lung of a three^year old trotter mare. The horse presented chronic coughing and progressive dyspnoea. The animal was subjected to euthanasia four months later. At necropsy, examination of the respiratory apparatus revealed three masses arising from the dorsal edge of the medial caudal area of the right lung. The roundish, firm masses measured 4-7 cm in diameter and were yellowish-white y.w.m. colour and also on section. Despite their sharp outlines there was no evidence of a limiting connective tissue capsule. The neoplasm burst" into the right main bronchus and grew caudally for about 4-5 cm causing subtotal occlusion of the bronchus wall. The bronchus distal to the occluding growth was filled with muco-purulent exudate. No metastasis nor abnormal regional lymph nodes were detected. Sampled tissues were processed for histological stains (H-E. PAS-reaction, Gaussen) and immunohistochemical staining (ABC-Peroxidase method). Both monoclonal antibodies (vimentin, NSE, cytokeratin, desmin, SY38) and polyclonal antibodies (GFAP, S100, Muramidase) were tested. Other tissue samples were fixed in glutaraldehyde and embedded in Epon-Araldite for ultrastructural examinations. Nests or strands of round to polyhedral cells with nuclei plus central nucleoli and large amounts of cytoplasmic, eosinophilic, PAS+ granules characterised the histological features. Vimentin, NSE, S-100 and GFAP were demonstrated by immuno- histochemistry. The granular cells contained single membrane bound granules with, variable sizes, shapes and electron density and lacked intracellular junctions on ultrastructural examination. The results confirm the neural crest derivation of the neoplastic cells, namely as Schwann cells precursors. Finally, the positive reaction to GFAP suggested partial- differentiation in some neoplastic cells.