Background: It has been suggested that retinal ganglion cells (RGCs) of Tupaia can be subdivided into three classes that correspond to the X, Y, and W classes in the cat. Estimates of these classes as determined by electrophysiological experiments and by histological studies of the retina are at variance. Because the RGC classes differ in axon diameter, this parameter could serve as a reliable criterion for the evaluation of RGC classes and their proportions.
Methods: An electronmicroscopic analysis of four optic nerves was carried out. The density of axons and their diameters was recorded from cross sections in the middle of the nerves. Based on a theoretical model, axons were classified according to the three known RGC classes. In addition, we investigated how axons of different size are distributed within the nerve.
Results: On average, the total number of axons is 570,000. More than 99% of axons are myelinated. Axon diameters can be 0.2-3.6 microns, and fiber diameters can be 0.3-4.6 microns. The frequency distributions of axon and fiber diameter are positively skewed and multimodal. Our analysis revealed three distinct axon diameter groups with the following mean axon diameters and proportions: the small or S group: 0.55 micron, 70%; the medium-sized or M group: 0.88 micron, 20%; the large of L group: 1.39 microns, 10%. Density and axon diameter plots produced consistent patterns, according to which axons of different sizes were distributed in the optic nerve. Thick axons are located dorsotemporally and centrally in the nerve. The average diameter value decreases toward the periphery. The smallest values are found at the ventronasal border of the nerve.
Conclusions: The observed axon diameter groups probably arise from functionally distinct RGC types. There is evidence that these groups correspond to functional W, X, and Y RGC classes. Our study also provides the first evidence of the existence of a topographic order of fibers within the nerve of the tree shrew.