Changes with age in the morphology of the cochlear nerve in rats: light microscopy.

Cochlear nerve morphology was examined in a series of rats ranging in age from young adulthood to advanced age in order to assess the extent of fiber loss and the nature of degenerative changes with age. The animals were perfused via the aorta with mixed aldehydes. Blocks including the cochlear nerves were removed, embedded in Araldite, and sectioned in a plane transverse to the longitudinal axis of the nerve. Analysis of the material included counts of normal and degenerating fibers and of glial cells, maps of fiber packing densities, and measurements of the cross-sectional area of the nerve. The median number of normal fibers in the young adult animals (2-3 months) was 21,218. This number was reduced by 21% at 26.5 months and by 24% in the oldest group (35-36 months). The number of degenerating myelin sheaths was first seen to be significantly increased at 6 months, reached a peak at 26.5 months, and declined at 35-36 months. There was an age-related increase in the cross-sectional area of the nerve, amounting to about 60% at 26.5 months and to about 50% at 35-36 months. Fiber packing density decreased evenly with age over the area of the nerve. The increased cross-sectional area and decreased fiber packing density appeared to be related to increases in the thickness of myelin sheaths and in the area occupied by interneural elements.