Morphometry of axon cytoskeleton at internodal regions of rat sciatic nerve during aging

Nerve endings undergo a lifespan morphofunctional modulation which is reported to be markedly impaired with aging. Neurone structural remodelling is in charge of processes occurring in the nerve cell soma, however the axonal transportation of organelles and molecules by cytoskeletal elements plays a very important role in the morphological rearrangements taking place at peripheral compartments.

To assess the involvement of axonal ultrastructure in the reported age-related decline of slow axoplasm flow mechanisms, we carried out a morphometric study of axon cytoskeleton in aging.

Female Wistar rats (3, 12 and 30 months of age) were anesthetized and perfused with saline followed by a fixation solution (glutaraldehyde 5% + formalin 2% in 0.1 cacodylate buffer pH 7.4). The excised sciatic nerves were processed according to conventional electron microsopic procedures. Axons sectioned perpendicularly to their longitudinal axis at the internodal region (mean axoplasm area: 18.25-26.5 microm(2)) were sampled by a systematic random procedure. The overall number of neurofilaments (No.Nfs) and microtubules (No.Mts) per total axoplasm area analysed, the numeric density (number/microm(2) of axoplasm area) of neurofilaments (NaNfs) and microtubules (NaMts), the myelin thickness, the number of myelin lamellae and the R proportion [No. Nfs/(No.Nfs + No.Mts)] were the parameters measured by computer-assisted semiautomatic methods.

No.Nfs, NaNfs, myelin thickness and the number of myelin lamellae did not change between 12 and 30 months of age, while a significant increase of these parameters was found in a comparison with younger rats. No.Mts and NaMts were significantly increased at 12 vs. 3 as well as at 30 vs. 12 months of age, respectively. R proportion did not show any difference due to age.

The present findings support that the dynamic condition of the axonal cytoskeleton appears to be preserved at a high extent in aging. Thus, the intra-axonal defective spacing of cytoskeletal elements (e.g. neurofilaments), rather than their number, is proposed to contribute to the decline of the slow axonal transport of organelles and molecules reported in aging.