Ultrastructural study of Betz cells in the primary motor cortex of the human brain.

The ultrastructure of Betz cells in the 5th layer of the primary motor cortex of 17 neurologically and psychiatrically normal control individuals was studied. Normal-appearing Betz cells showed a wide range of features including novel electron-dense inclusion bodies (Bunina-like bodies) resembling Bunina bodies characteristic of amyotrophic lateral sclerosis (ALS), accumulations of neurofilaments (10 nm in diameter), bundles of filaments (20-25 nm in diameter) thicker than neurofilaments, lamellar structures, lamellar bodies and structures similar to Hirano bodies. Among these 'abnormal' features, the presence of Bunina-like bodies may be an age-related nonspecific degenerative change, since they appeared more frequently in elderly individuals. The presence of these abnormal features--particularly the Bunina-like bodies--in the Betz cells of normal human brains must be considered in the assessment of the pathognomonic significance of such structures in ALS and other neurological diseases that affect the motor cortex.