Eosinophilic cytoplasmic inclusions in sporadic equine motor neuron disease: an electron microscopic study.

Equine motor neuron disease (EMND) is a sporadic, progressive neurodegenerative disorder that has been identified recently in horses of different breeds in North America. The cause is unknown. Pathologic changes which occur in spinal and certain brain stem motor neurons include chromatolysis, swelling, neurofilamentous accumulation, and development of eosinophilic cytoplasmic inclusions. Punctate eosinophilic inclusions, the type usually encountered in degenerating neurons, resembled Bunina bodies at the light microscopic level, but differed in their ultrastructural composition. These and less common but larger juxtanuclear inclusions appeared to be aggregated vesicular residues of membranous organelle degradation. The third kind of eosinophilic inclusion was a marginated derivative of endoplasmic reticulum and consisted of large membrane-bound accumulations of finely granular material. It was concluded that, although the distribution and nature of the lesions in EMND appeared similar to those of human motor neuron disease, none of the equine eosinophilic inclusions duplicated the ultrastructure of Bunina bodies.