Intracytoplasmic eosinophilic inclusions in the neurons of the central nervous system

This study reports the histological, ultrastructural, and immunocytochemical characteristics of intracytoplasmic eosinophilic inclusion bodies occurring in various types of neurons of the human central nervous system. By light microscopy, the inclusions were brightly eosinophilic, slightly birefringent, and sharply demarcated; they were found in the thalamus in 92% of the cases, in the substantia nigra in 88%, in the locus coeruleus in 45%, and rarely in the spinal cord. Ultrastructurally, the inclusions were composed of assemblies of parallel, alternating dark and light rectilinear profiles. The dark profiles corresponded to thin filaments (microfilaments) that measured 5.5-6.0 nm in diameter. A second set of dense lines crisscrossed the first at right angles. In sections perpendicular to their long axis, the filaments were distributed in a tetragonal lattice pattern in which individual elements occupied the angles of a square. Immunocytochemical preparations for actin were negative. Due to their high rate of occurrence in nonpathological brains, it is thought that the inclusions represent a normal but as yet unidentified cytoplasmic organelle.