[Study of the peripheral nerve fibers during aging in the rat].

An histological study of the peripheral nervous fibres has been performed at various anatomical levels during aging: the spinal ganglion, the dorsal and the ventral nerve roots, the spinal and the sciatic nerves. During aging the various alterations occurring in a peripheral nerve can be summarized as following. In the myelinated fibres, the axoplasm was progressively invaded by several inclusions: glycogen granules, granulo-filamentous bodies and lipofuscins. The crystalloid networks arising from the cytoskeleton were mainly localised in the intraganglionic fibres. Among the axoplasmic organelles, the mitochondria were the most affected. The myelinic sheath split, became dystrophic and then was totally disrupted. The inner schwann cell compartment was invaded by several inclusions like Hirano bodies and dense residual deposits. Further, macrophages phagocytosed the axon and the myelin sheath. In the non-myelinated fibres, the alterations were less important and less precocious. When these results are analysed from a chronological point of view it is established that the alterations appear at the same time in each observed level but their amount differ from each other. In the 24-month-old-rats, the ventral root and the sciatic nerve present many dystrophies whereas in the spinal ganglion and in the dorsal root they are less numerous. From these results, it can be suspected that the motor fibres are more vulnerable during aging. Moreover, the myelinated fibres of large diameter are the first affected. Furthermore, only the ventral root and the sciatic nerve show typical regeneration pictures at 32 months.