SPONTANEOUS EQUINE PULMONARY GRANULAR-CELL TUMORS - MORPHOLOGIC, HISTOCHEMICAL, AND IMMUNOHISTOCHEMICAL CHARACTERIZATION

Spontaneous equine pulmonary granular cell tumors were diagnosed in si x mature horses at slaughter. These tumors were grossly recognized as multiple (5/6) or single (1/6) creamy white, firm nodules. The tumors, located adjacent to bronchi and bronchioles, often invaded airways, r esulting in partial to complete occlusion of the lumina. Neoplastic ce lls were rounded to polyhedral with numerous eosinophilic cytoplasmic granules that reacted uniformly positive with S-100 and neuron-specifi c enolase antibodies and multifocally with glial fibrillary acidic pro tein antibodies. These cells were negative for muscle-specific actin, lysozyme, cytokeratin, chromogranin A, and myelin basic protein antige ns and did not stain with silver by the Grimelius technique. Uniformly blue-green and scattered pink intracytoplasmic granules were evident with luxol fast blue and periodic acid-Schiff counterstain for myelin and myelin breakdown products. Histochemical and immunohistochemical s taining results of these tumors suggest that they are composed primari ly of myelinating Schwann cells with lesser numbers of scattered nonmy elinating Schwann cells. The morphologic features of the equine pulmon ary granular cell tumors are strikingly similar to those of endobronch ial granular cell tumors of human beings.