Ventral root axonopathy and its relation to the neurofibrillary degeneration of lower motor neurons in aluminum-induced encephalomyelopathy.

The injection of metallic aluminum (Al) into the cerebrospinal fluid of adult rabbits induces neurofibrillary degeneration of lower motor neurons. We studied the ventral roots and the corresponding motor neurons of Al-treated animals to clarify the modality and extent of reaction of the axon in relation to the severity of perikaryonal involvement. Moreover, the involvement of dorsal root ganglion cells was compared to that of lower motor neurons. Rabbits received 0.15 ml of a 1% Al slurry intracisternally and were perfused through the heart with aldehydes at 14-62 days after injection. Spinal cords and roots were embedded in Epon and examined morphologically and by morphometric techniques. An axonopathy was observed in the ventral roots, characterized by neurofilamentous axonal swellings and myelin attenuation in several size classes of axons. Results obtained from axons traced in serial sections indicate that there may be a unifocal or a multifocal axonopathy. Dorsal root ganglion cells showed milder changes by comparison with motor neurons and their axons in the ventral roots. The most severe axonopathy was associated both with an incidence of 66-81% of motor neurons showing neurofibrillary degeneration and with a rapidly progressing motor weakness. These findings are related in the discussion section to the pathological expression of human neurological disorders in which the lower motor neurons are selective targets.