An electron microscopic analysis of the optic nerve of the eastern chipmunk (Tamias sibiricus asiaticus): total fiber count and retinotopic organisation.

On the basis of light and electron microscopic observations of the cross-section, the total count of optic nerve fibers in the eastern chipmunk (Tamias sibiricus asiaticus) was estimated to be 5.65 x 10(5). Almost all the fibers were myelinated and only 0.9% were unmyelinated. Axon diameters of myelinated fibers showed a skewed unimodal distribution ranging from 0.2 to 3.6 microns (mean, 0.83) with the peak at 0.6-0.7 microns. Myelin sheath thickness positively correlated with the axon diameter. In the cross-section fiber density was highest in the naso-dorsal to the central part and lowest in the temporo-ventral part. Large axons are concentrated in the temporo-ventral part of the optic nerve, corresponding to the localisation of large ganglion cells in temporal crescent of the retina. A gross retinotopic organisation was verified among the optic nerve fibers in the cross-section after localised lesions by laser beam in the nasal and temporal periphery. However, when the retinal lesion was made in or around the central high density area, a non-retinotopic arrangement of the optic nerve fibers was suggested.