The Distribution and Size of Retinal Ganglion Cells in Cheirogaleus medius and Tarsius syrichta: Implications for the Evolution of Sensory Systems in Primates

Sensory specializations, particularly in the visual system, have been crucial factors in the evolution of brain and behavior in primates (Allman, 2000). The entire output of the retina is channeled through the retinal ganglion cells whose axons form the optic nerve connecting the eye and brain. We have mapped the density of retinal ganglion cells in flattened retinal whole mounts in the dwarf lemur, Cheirogaleus medius, and the Philippine tarsier, Tarsius syrichta. Figure 1 illustrates these maps, which reveal that the ganglion cell density is much lower throughout the retina in dwarf lemur than in the tarsier. In Figure 2, the data for cross sections near the horizontal meridian are depicted for the dwarf lemur and tarsier from our data compared with similar cross sections from the galago (DeBruyn et al., 1980) and mouse lemur (Dkhissi-Benyahya et al, 2001). These data indicate that the retinal ganglion cell densities as a function of eccentricity are similar in tarsier, galago and mouse lemur, and that the ganglion cell densities in dwarf lemur are much lower, particularly near the center of the retina. In Figure 3, we have added comparable cross sectional data for the diurnal rhesus macaque to illustrate the enormous increase in ganglion cell density associated with higher acuity in diurnal anthropoids. This point is further documented in Table 1, which shows peak retinal ganglion cell density and total ganglion cell number for 12 primate species. We have also measured ganglion cell diameters in the central and peripheral retinae of dwarf and mouse lemurs and tarsiers and combined our data with measurements previously made by Stone and Johnston (1981) (see Figure 4). Stone and Johnston found that retinal ganglion cells tend to be much larger in the periphery than in the center for the cat and the three diurnal anthropoids they examined (squirrel monkeys, rhesus macaques, and humans). There is also a strong tendency for peripheral ganglion cells to be larger than central cells in the nocturnal owl monkey for both the M and P subpopulations (Yamada et al., 2001). The peripheral cells are only slightly larger than the central cells in tarsiers and galagos, are about the same in mouse lemurs, and are slightly smaller in dwarf lemurs. The large size of the peripheral ganglion cells suggests that they may have larger, more rapidly conducting axons than those in the center. The large peripheral cells may thus serve to transmit …