Effect of ageing on the nerve fibre population of rat optic nerve.

The age-related changes in the number and density of optic nerve fibres were studied in 3-month-old (young), 12-month-old (adult), 20-month-old (senescent) and 30-month-old (aged) male Wistar rats. Two-micrometer-thick resin-embedded transverse sections of the optic nerve of animals of the different age groups were stained with toluidine blue and examined under a light microscope at low (X5) and high (X500) magnification. The optic nerve cross-sectional area and the number of nerve fibres with diameters shorter or longer than 1 micron were assessed by means of computerized image analysis. Optic nerve cross-sectional area is constant in young and adult rats, but is increased in senescence. The number of optic nerve fibres with a diameter less than 1 micron is decreased by about 18 and 12% in 20-month-old rats versus 3- and 12-month-old rats, respectively. The number of these nerve fibres is decreased by about 43, 39 and 30% in 30-month-old rats versus 3-, 12- and 20-month-old animals, respectively. In contrast no age-dependent changes were observed in nerve fibres with diameters greater than 1 micron. The present data suggest that age-related impairment of nerve cell population described by other authors in rat retina and visual cortex occurs also at the level of the optic nerve.