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

An electron microscope examination of the optic nerve of the opossum, Didelphis marsupialis aurita, indicates a total count of 74,7000 fibers of which approximately 20% are unmyelinated. The diameters of myelinated fibers ranged from 0.5-7 mum (mode at 1.25 mum) and those of the unmyelinated fibers ranged from 0.25-2.25 mum (mode at 0.75 mum). 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.