Functional development of the visual system in normal and protein-deprived rats. VIII. Post-natal development of optic nerve axons.

Protein deprivation results in a persistent impairment of transmission of impulses from the specific cortical projection within the visual cortex. In order to evaluate changes in subcortical structures during pre-weaning development, a study was made on number, calibre and myelination of optic nerve axons in control (C) and protein-deprived (PD) rats 5, 12, 20 and 30 days of age. Protein deprivation was induced by giving rat mothers a diet containing 7% protein by weight (control diet 14%, during gestation and lactation. The cross-sectional area of the nerve was measured on a digitizer. Between 3000 and 4000 axons collected from 7-13 field areas sampled by a random, systematic procedure from a cross-shaped area of the nerve were counted and measured. Between 5 and 12 days after birth, the number of axons was reduced by 50% in C rats. The total number of optic nerve axons was not significantly different in PD compared to C rats, indicating that protein deprivation does not affect the formation or naturally occurring nerve cell death of retinal ganglion cells. At all ages examined there were significant reductions in the number of axons larger than 0.52 micron as well as the number of myelinated axons. The rates of growth/maturation and myelination of axons in C rats and PD rats suggested that the reductions seen in PD rats up to 20 days of age may represent a developmental delay of approximately 4 days. At 30 days, a delay or a distortion of development may present. The retarded development of optic nerve axons is discussed in relation to delays, distortions and deficits during visual system maturation in protein-deprived rats.