Resumo

PURPOSE. Differently from all other Platyrrhini, males and females howler monkeys are regular trichromats. Previous studies have shown that the howler monkey has a well developed fovea and a very high cone density in the foveola. In the present work, the density distributions of ganglion cells (GC) and displaced amacrine cells (DAC) were determined in six retinas from five different Alouatta caraya. METHODS. The animals were deeply anesthetized and perfused transcardially. The eyes were removed, the retinas were prepared as whole, flat-mounts, and stained with cresyl violet using the method of Nissl. The criteria to distinguish GC from DAC were refined by inspecting a collection of capuchin monkey retinas retrogradely labeled after horseradish peroxidase or biocytin deposits in the optic nerve and couterstained with cresyl violet. RESULTS. Ganglion cell density peaks at 0.5 mm from the fovea, reaching 40,700 – 45,200 cells/mm2 (mean values along different meridians, n = 5). From the foveal slope, ganglion cell density declines following initially a very steep gradient down to 3 mm of eccentricity, and then a less steep gradient towards retinal periphery, reaching 200 cells/mm2 in the nasal, and 75 – 150 cells/mm2 in the temporal, dorsal, and ventral retinal borders. Ganglion cell density decreases towards the retinal periphery at approximately the same rate along the temporal, dorsal, and ventral meridians, but is 1.3 – 6.5 times higher in the nasal region when compared to the temporal region at the same eccentricities (1 – 13 mm from the fovea, respectively). Displaced amacrine cell density distribution peaks between 0.5 - 1.75 mm from the fovea, reaching 2,050 – 3,100 cells/mm2 (mean values along different meridians, n = 5). From the foveal slope, displaced amacrine cell density declines following initially a steep gradient down to about 5 mm of eccentricity, and then a less steep gradient towards retinal periphery, reaching 500 – 550 cells / mm2 in the retinal borders. No consistent asymmetries between different retinal quadrants were observed for displaced amacrine cell density. At retinal periphery, displaced amacrine cells outnumber ganglion cells. Isodensity maps were draw for the two cell classes and then used to obtain the total number of ganglion cells and displaced amacrine cells by integration. The means and standard deviations for the retinal area and total numbers of cells were: retinal area 641 ± 62 mm2 (573 – 736 mm2, n = 6); ganglion cell total number 1,133,525 ± 79,208 cells (1,034,015 – 1,231,000 cells, n = 5); displaced amacrine cell total number 536,917 ± 61,794 cells (478,685 – 639,500 cells, n = 5). For future reference, mean isodensity maps for ganglion cells and displaced amacrine cells were drawn based on the mean density values at retinal locations. The ganglion cell and displaced amacrine cell totals were respectively 1.078.968 e 519.970 for these mean maps. CONCLUSION. Ganglion cell density distribution in the howler monkey retina is consistent with that observed among several species of diurnal Anthropoidea, both Platyrrhini and Catarrhini.