Intraretinal axon diameters of a New World primate, the marmoset (Callithrix jacchus)

Purpose : Previously, measurements of retinal ganglion cell axon diameter have been used to make inferences about the physiology and clinical pathology of the visual pathway. However, few of these studies were able to unequivocally relate axon diameter to retinal ganglion cell type and other associated measurements. In this and our previous study we have examined intraretinal axon diameters to determine if differences in axon diameter may help to explain conduction velocity measurements found previously. Methods : Individual retinal ganglion cells of a New World primate, the common marmoset (Callithrix jacchus) were injected iontophoretically with 2% Lucifer yellow and 4% neurobiotin. Labelled cells were visualized by horseradish peroxidase immunohistochemistry and diaminobenzidine and then retinae were mounted vitreal side up on a glass slide. Cell measurements were made with the aid of a camera lucida attachment and computer‐aided morphometry. Axons were photographed under ×100 oil immersion and measured at a final magnification of ×4600. Results : A sample of 62 parasol cells, 22 midget cells, 16 hedge cells and 11 small bistratified cells were analysed. Dendritic field diameter of the different cell classes showed only moderate (non‐significant) increases with eccentricity. Only the parasol cells demonstrated a significant increase in mean axon diameter with eccentricity. When the parasol class was examined more closely, it was found that only parasol cells of the superior, inferior and temporal retina (SIT group) showed significant positive correlations between different cell parameters (mean axon diameter, soma diameter, dendritic field diameter, eccentricity). Soma and dendritic field diameters of the SIT group were significantly larger than those of the nasal parasol cells. However, mean axon diameters of the SIT cells were not significantly different from nasal parasol cells. Axon diameters of nasal parasol cells were very variable and overlapped those of the midget and hedge cell classes to a large extent. Conclusions : The present data show that for marmoset parasol cells there may not be a clearly defined distinction between nasal and superior, inferior and temporal parasol cells on the basis of axon size. Of particular interest in the present analysis is the clear separation of superior, inferior and temporal parasol cells and nasal parasol cells when comparing soma and dendritic field diameters which is not reflected in the distribution of axon diameters. We suggest that changes in diameter along the length of an axon, differences between retinal quadrants and the variability between cells may be related to minimization of spatiotemporal dispersion necessary for accurate perception of motion within the visual world.