Number and distribution of retinal ganglion cells in anubis baboons (Papio anubis)

The number of retinal ganglion cells in Papio anubis was determined from light microscopic observations of wholemounted and vertically sectioned retinal tissue and electron microscopic examination of optic nerve cross sections. The total number of ganglion cells ranged from 1.41 to 1.81 million (mean 1.58 million, n = 6, SD = 169,927) per retina. The distribution of ganglion cells in cresyl violet stained wholemounts was also examined. Isodensity contours were almost circular perifoveally, but became horizontally elongated outside of the central retina, providing strong evidence for a visual streak. Ganglion cell somata within the streak were found to be significantly smaller than those outside of the streak in comparing regions of equal density. Finally, the distribution of blood vessels within the retina formed a watershed pattern with its crux centered on the ridge of this horizontally oriented high-density zone. Combined, these features indicate that anubis baboons possess a visual streak specialization as reported for lagomorphs, felines, and several primate species. Further, the visual streak appears more pronounced in anubis baboons than in any other primate species studied to date, with the possible exception of Homo sapiens, a similarly ground-dwelling/foraging and secondarily terrestrial species.