Membrane specialization and axo-glial association in the rat retinal nerve fibre layer: freeze-fracture observations

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Abstract
The ultrastrucrure of non-myelinated ganglion cell axolemma within the retinal nerve fibre layer of adult rats was examined by thin section and freeze-fracture electron microscopy. Most of the axolemma within the nerve fibre layer does not exhibit any membrane specializations; intramembranous particles are partitioned with a density of ∼1750 μm−2 on the P-fracture face and ∼225 μm−2 on the E-face of the non-specialized axolemma. The nerve fibres also exhibit specialized foci of axolemma, at which the axons are abutted by the tips of blunt, radially oriented processes from Müller cells. At such sites of axo-glial association, an electron-dense undercoating is present beneath the axon membrane. Freeze-fracture analysis revealed a substantial increase in the density of E-face particles (>500 μm−2) at sites of association between the tips of blunt glial processes and the axon. These findings demonstrate that non-myelinated axolemma of the retinal nerve fibre layer can exhibit spatial heterogeneity, with patches of node-like membrane at regions of specialized association with glial cell processes. On the basis of their morphological similarity to nodes of Ranvier, we suggest that these specialized axon regions represent foci of inward ionic current.