Myelinization of the Optic Nerve and its Dependence on Visual Function— A Quantitative Investigation in Mice

Abstract

ABSTRACT Quantitative studies have been made of the number, myelinization and diameter of the fibres of the optic nerve during post-natal development in black mice, using phase-contrast and electron microscopy. The influence of complete darkness was investigated. In mice reared in the light no myelinated fibres are present up to 5 days of age, whereas a few hundred are demonstrable at 10 days. Between 15 and 20 days of age, myelinization is intense, the mean number of such fibres per nerve being about 22,000 at 20 days. After this time there is only a slight increase, i.e., to about 25,000 at 30 days. Up to 30 days, only a few per cent of the fibres are more than 1 μ. in diameter (measured in fixed and embedded specimens). In adult mice, three maxima in fibre diameter are observed; the largest group comprises those with a diameter less than 1 μ. (0·6–0·8 μ.), and the two smaller ones fibres more than 1 μ. in diameter (1·1–1·3 and about 2·0 μ.). The fibres are found to be more closely packed in the central part of the optic nerve than in its peripheral parts, but the coarser fibres are more numerous peripherally. No appreciable number of unmyelinated fibres is visible in mature animals. In mice reared in complete darkness for 20 and 30 days, delay in myelinization of the optic nerve fibres causes a reduction in the number of those myelinized by some 12–13 per cent. The coarser fibres are affected to a greater extent than the thin ones; thus at 30 days the number of those more than 1 p in diameter is reduced by almost 60 per cent. In mice reared in darkness, a retardation is observed in growth of the nuclei of the retinal ganglion cells, as well as in their shape maturation (decrease in eccentricity).