Quantitative analysis of the opossum's optic nerve: An electron microscope study

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Abstract
An electron microscope examination of the optic nerve of the opossum, Didelphis marsupialis aurita, indicates a total count of 74,700 fibers of which approximately 20% are unmyelinated. The diameters of myelinated fibers ranged from 0.5–7.0 μm (mode at 1.25 μm) and those of the unmyelinated fibers ranged from 0.25–2.25 μm (mode at 0.75 μm). Myelinated and unmyelinated fibers are uniformly distributed throughout the nerve. Myelin sheath structure is similar to that described in other species and are formed by 2 to 17 myelin rings (mode at 6). There is a good correlation (r = 0.92) between axon and myelin areas. The relation between myelin thickness and axons diameter shows a wide distribution with a peak at 0.77.