A morphometric study of age changes in the rat optic nerve.

Age-related changes of the optic nerve fibres were studied in 3-month-old (young), 12-month-old (adult) and 24-month-old (aged) male Sprague-Dawley rats. The optic nerve was harvested with particular care from the intracranial portion. Cross sections from the optic nerve of animals of different age groups were stained with toluidine blue and examined under a light microscope at low and high magnification. Other sections were stained for the demonstration of glial cells using the method described by Holzer. A third group of sections were stained by the immunohistochemical method to detect glial fibrillary acidic protein, which is a marker for localising and characterising astrocytes. All these morphological results were subjected to the quantitative analysis of images and to statistical analysis of the values to identify significant morphometric data. Biochemical dosages of proteins were also performed on homogenised fragments of the optic nerve. Our results demonstrate that the following age-related changes can be observed: (1) an increase in meningeal membranes, (2) an increased number of astrocytes, (3) an increase in areal density of glial fibrillary acidic protein immunoreactivities, (4) an increased thickness of the entire optic nerve and an increased area of the nerve, (5) a decreased number of nerve fibres and (6) a decrease in the nerve fibre/meningeal membrane ratio from 3:1 to 1:1. Moreover, the amount of protein does not change with age. The rat optic nerve, therefore, appears sensitive to aging processes.