A quantitative morphological study of interstrain variation in the developing rat optic nerve.

Groups of pigmented (Black and White Hooded Lister) and albino (Sprague Dawley) rats were killed at 7, 15, and 25 postnatal days of age. Their optic nerves were embedded in resin suitable for both light and electron microscopy. Quantitative stereological procedures were used to estimate total fibre number and the degree of myelination in the optic nerves at the various ages. At 7 days of age, both albino and pigmented rats had about 220,000 optic nerve fibres. By 15 days, both strains showed a reduction of some 30,000 fibres. This fibre loss continued in both strains after 15 days of age, but more rapidly in the pigmented strain. At 25 days of age, pigmented rats had 72,371 +/- 7,244 fibres, whilst albino rats had 102,681 +/- 4,138 fibres (P less than .01). More than 99.5% of optic nerve axons in both strains were unmyelinated at 7 days of age. By 25 days of age about 90% of all remaining fibres were myelinated in both strains. The mean diameter of the myelinated axons was estimated to be between 0.59 and 0.65 micron in all animals, there being no significant age or strain differences. In contrast, the mean diameter of nonmyelinated axons increased significantly with age. This increase was greater for albino than pigmented rats, such that by 25 days of age the respective values were 0.49 micron and 0.42 micron (P less than .05).