Quantitative electron microscopic analysis of the optic nerve of the turtle, Pseudemys

It is estimated by means of electron microscopy that the optic nerve of the turtle Pseudemys scripta elegans contains 394,900 fibers of which approximately 80% are myelinated. The total fiber count agrees well with counts obtained from electron microscopic studies on other turtle species. There are, however, differences among these species in the percentage of myelinated fibers in the optic nerve. The axon diameter distribution of the myelinated fibers (excluding myelin) has a mode at 0.87 micrometer while that of the unmyelinated fibers has a mode at 0.42 micrometer. Both distributions are unimodal and are of a similar form in all areas of the nerve sampled. The total fiber count reported here agrees well with previous reports of ganglion cell counts in Pseudemys and the characteristics of the fiber distributions are comparable to those reported for nonreptilian vertebrates.