Novel inclusion bodies in Betz cells of cortical area 4 of aged rhesus monkeys.

Seven adult rhesus monkeys ranging in age from 5 to 35 years were studied in an ultrastructural investigation of cortical area 4. Some Betz cells of layer 5 contained a perikaryal inclusion body (IB) of a unique and characteristic ultrastructural configuration that has not previously been described. The IBs consist of a regular array of three sets of equidistant parallel sheets and are not membrane bound. The distance between the sheets measures approximately 80 nm. The three sets of sheets intersect at an invariable angle of 60 degrees, forming a regular geometric structure. Each sheet comprises a single layer of parallel and densely packed straight filaments, which are approximately 10 nm thick. Thus the IBs present highly ordered crystalline arrays of filaments. The occurrence and number of IBs may be age related, since they appeared only in monkeys in mid to late adulthood and were most numerous in the oldest monkey.