Are bunina bodies of endoplasmic reticulum origin? An ultrastructural study of subthalamic eosinophilic inclusions in a case of atypical motor neuron disease.

We carried out an electron microscopic study of eosinophilic intracytoplasmic inclusions in the subthalamic neurons in a case of atypical motor neuron disease. These inclusions were identical in light microscopic morphology and staining characteristics to Bunina bodies. Ultrastructurally, most of the intracytoplasmic inclusions observed were divisible into two different types (I and II). Type I inclusions had features essentially identical to those of Bunina bodies demonstrated previously in the anterior horn cells in cases of motor neuron disease; they consisted of electron-dense, granular material without a particular limiting membrane and often contained a number of translucent areas with entrapped cell organelle-like structures. Type II inclusions were very similar to structures known as multilaminated bodies; at their margin, they sometimes showed continuity with the cisternae of endoplasmic reticulum. The remainder of the inclusions were considered to be transitional forms between these two types, and some of them showed the respective features of both types I and II. These findings suggest that Bunina bodies are of endoplasmic reticulum origin.