Neurofilamentous axonal swellings as a normal finding in the spinal anterior horn of man and other primates.

To define the nature and extent of axonal swellings in the normal spinal anterior horn, we studied the spinal cords of patients five days to 83 years of age from a general autopsy population. Axonal swellings were routinely found in the anterior horn of the cervical and lumbosacral spinal cord. The swellings measure 5-50 micron in diameter and are most numerous at the anterior edge of the anterior horn. They first appear about five months of age and appear to increase in number until about 20 years of age, with no increment thereafter. Ultrastructurally, they are filled with neurofilaments and surrounded by a thin myelin sheath. Most are probably aberrant components of motor axons. Identical axonal swellings, in the same anatomical site, were found in the spinal cords of cynomolgus and rhesus monkeys. On the basis of their natural history and morphologic features, they should be distinguished from the neuroaxonal dystrophy of aging. The largest of them resemble the neurofilamentous axonal swellings of early onset motor neuron disease but occur in much smaller numbers. Moreover, location on the proximal axon could not be demonstrated for any of these swellings. An awareness of this normal phenonemon is essential for the interpretation of axonal swellings in the spinal cord.