Freeze-fracture ultrastructure of developing and adult non-myelinated ganglion cell axolemma in the retinal nerve fibre layer

Abstract

The ultrastructure of non-myelinated ganglion cell axolemma in the retinal nerve fibre layer from developing and adult rats was examined by freeze-fracture electron microscopy. The axolemma of fibres from neonatal (2–8 days) rats had a moderate density (510–556/μm²) of intramembranous particles (IMPs) on the P-fracture face, while there was a low density (101–146/μm²) of particles on the E-fracture face. Particle density on the P-face increased with development, such that by 28 days the density of IMPs was 1281/μm². Adult fibres had a high (1741/μm²) density of particles on the P-face. On the E-face, the density of IMPs did not change substantially throughout development, and remained less than 225/μm² at all ages. Mean particle diameters were compiled for P- and E-fracture faces at the various developmental ages and were greatest in fibres from adult animals. P-face particle density of non-myelinated axons in the retinal nerve fibre layer changed at approximately the time that myelination occurred in distal (optic nerve) segments of these axons. The alteration in membrane structure during development of non-myelinated axons in the retinal nerve fibre layer suggests that conduction properties may also change with development.