Ganglion Cell and Displaced Amacrine Cell Density Distribution in the Retina of the Howler Monkey (<i>Alouatta caraya</i>)

<div><p>Unlike all other New World (platyrrine) monkeys, both male and female howler monkeys (<i>Alouatta sp.</i>) are obligatory trichromats. In all other platyrrines, only females can be trichromats, while males are always dichromats, as determined by multiple behavioral, electrophysiological, and genetic studies. In addition to obligatory trichromacy, <i>Alouatta</i> has an unusual fovea, with substantially higher peak cone density in the foveal pit than every other diurnal anthropoid monkey (both platyrrhines and catarrhines) and great ape yet examined, including humans. In addition to documenting the general organization of the retinal ganglion cell layer in <i>Alouatta</i>, the distribution of cones is compared to retinal ganglion cells, to explore possible relationships between their atypical trichromacy and foveal specialization. The number and distribution of retinal ganglion cells and displaced amacrine cells were determined in six flat-mounted retinas from five <i>Alouatta caraya</i>. Ganglion cell density peaked at 0.5 mm between the fovea and optic nerve head, reaching 40,700–45,200 cells/mm². Displaced amacrine cell density distribution peaked between 0.5–1.75 mm from the fovea, reaching mean values between 2,050–3,100 cells/mm². The mean number of ganglion cells was 1,133,000±79,000 cells and the mean number of displaced amacrine cells was 537,000±61,800 cells, in retinas of mean area 641±62 mm². Ganglion cell and displaced amacrine cell density distribution in the <i>Alouatta</i> retina was consistent with that observed among several species of diurnal Anthropoidea, both platyrrhines and catarrhines. The principal alteration in the <i>Alouatta</i> retina appears not to be in the number of any retinal cell class, but rather a marked gradient in cone density within the fovea, which could potentially support high chromatic acuity in a restricted central region.</p></div

Retinal area before (N = 4) and after histological procedure (N = 6), retinal shrinkage due to histological procedure (N = 4), and totals for ganglion cells and displaced amacrine cells (N = 5).

<p>Totals for ganglion cells and displaced amacrine cells were estimated by integration of isodensity contours. For the “average” retina, the isodensity contours were drawn from mean density values of N = 6 retinas and plotted in the map of retina AC 02LM. AC, <i>Alouatta caraya</i>. R, right retina. L, left retina. M, retina from a male howler monkey.</p><p>Retinal area before (N = 4) and after histological procedure (N = 6), retinal shrinkage due to histological procedure (N = 4), and totals for ganglion cells and displaced amacrine cells (N = 5).</p

Displaced amacrine cell density in New World monkeys.

<p>Comparison of displaced amacrine cell density in two different New World monkeys: <i>Alouatta</i> (red lines, this study) and <i>Aotus</i> (green lines, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Silveira5" target="_blank">[38]</a>). <i>Alouatta</i> is diurnal and trichromat while <i>Aotus</i> is nocturnal and monochromat. (<b>A–B</b>) Nasotemporal horizontal meridian. (<b>C–D</b>) Dorsoventral vertical meridian. Displaced amacrine cell counts were converted to cells per square millimeter and eccentricity values were taken as distances from the foveal pit. Left panels (<b>A</b> and <b>C</b>) show displaced amacrine cell density for the central 3 mm of eccentricity, whilst right panels show values for the entire horizontal and vertical meridians (<b>B</b> and <b>D</b>). The displaced amacrine cell density is higher in the diurnal <i>Alouatta</i> than in the nocturnal <i>Aotus</i> in the central retinal region and most of retinal periphery.</p

Dimensions of the howler monkey optic disk (papila) along the dorsoventral (DV) and nasotemporal (NT) dimensions (N = 5) and the distance from the center of the optic disk to the foveal pit (N = 6).

<p>AC, <i>Alouatta caraya</i>. R, right retina. L, left retina. M, retina from a male howler monkey.</p><p>Dimensions of the howler monkey optic disk (papila) along the dorsoventral (DV) and nasotemporal (NT) dimensions (N = 5) and the distance from the center of the optic disk to the foveal pit (N = 6).</p

Ganglion cell isodensity maps for another <i>Alouatta</i> retina.

<p>(<b>A</b>) Isodensity contours for the retina AC 02 Left Male. (<b>B</b>) Isodensity contours for the central retinal region. Conventions were the same of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone-0115291-g004" target="_blank">Fig. 4</a>. Similarly to other retinas studied in this work, ganglion cell isodensity contours were slightly elongated in the nasal direction. In addition, in this retina there was also an elongation of the central isodensity contours along the dorsoventral meridian.</p

Relation between total of ganglion cells and retinal area in different species of primates.

<p>Data were grand means across different studies (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone-0115291-t003" target="_blank">Table 3</a>). The following catarrhines and platyrrhines were represented in the plot: <i>Saguinus midas niger</i> (1), <i>Saimiri sciureus</i> (2), <i>Cebus apella</i> (3), <i>Macaca mulatta</i> (4), <i>Papio anubis</i> (5), <i>Alouatta caraya</i> (6), <i>Cercopithecus aethiops sabeus</i> (7), <i>Aotus trivirgatus</i> (8), and <i>Homo sapiens</i> (9). Only data from diurnal species of monkeys were considered in the linear regression fitting (blue circles). Data from the single nocturnal species (<i>Aotus</i>, red circle) as well as data from humans (green circle) were excluded from the analysis. There was a good correlation for diurnal monkeys between the total of ganglion cells estimated by counting retinal ganglion cell bodies or optic nerve axons and retinal area. <i>Alouatta</i> had the number of ganglion cells that it was expected by its retinal area (see the text for further comments).</p

Foveal region of an <i>Alouatta</i> retina.

<p>Retinal flat mount AC 02 Left Male stained by the method of Nissl using cresyl violet as stain. Some retinal pigment remained attached to the region surrounding the foveal pit. The arrow points towards the location of the optic nerve head, which is out of the field. The retinal raphé is located on the opposite side in relation to the fovea and is indicated by the convergence of retinal vessels.</p

Comparison between the distributions of ganglion cells and cones in the central region of <i>Alouatta</i> and <i>Cebus</i> retinas.

<p>Trichromatic <i>Alouatta</i> differs from dichromatic male <i>Cebus</i> essentially by having high cone density in the foveola. The plots show that this occurs at expenses of a decrease in the number of cones in the remaining of the central region of the <i>Alouatta</i> retina. Other aspects of the retinal distribution of cones and ganglion cells are essentially similar in the trichromatic <i>Alouatta</i> and dichromatic <i>Cebus</i>. (<b>A</b>) Density of cones <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Franco1" target="_blank">[33]</a> and ganglion cells (this study) across the horizontal meridian of the <i>Alouatta</i> retina. (<b>B</b>) Density of cones <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Franco1" target="_blank">[33]</a> and ganglion cells <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Silveira4" target="_blank">[34]</a> across the horizontal meridian of the <i>Cebus</i> retina. Only data from male dichromats were plotted. (<b>C</b>) Cumulative number of cones in the retinas of <i>Alouatta</i> and <i>Cebus</i>. Data from the original study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Franco1" target="_blank">[33]</a> were used to estimate the total number of cones in ring of retina centered in the foveola. Then, the cumulative number of cones in circular areas was estimated by adding the number of cones in successive rings. (<b>D</b>) Cumulative number of ganglion cells in the retinas of <i>Alouatta</i> and <i>Cebus</i> obtained from ganglion cell densities estimated in this study and the original study of Silveira and colleagues <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone.0115291-Silveira4" target="_blank">[34]</a>. The cumulative number of ganglion cells in circular retinal areas was estimated by adding the number of ganglion cells in successive rings centered in the foveola.</p

Density peak of retinal ganglion cells in catarrhines and platyrrhines estimated by using different techniques. Data were rounded to the thousands. Estimates from retinal area are also listed.

<p>*This work.</p><p>Density peak of retinal ganglion cells in catarrhines and platyrrhines estimated by using different techniques. Data were rounded to the thousands. Estimates from retinal area are also listed.</p

Ganglion cell isodensity maps for an “average” <i>Alouatta</i> retina.

<p>Isodensity contours in cells/mm² were drawn from mean values from every retinal location that was measured and using the map of retina AC 02LM as template. (<b>A</b>) Isodensity contours for the whole retina. (<b>B</b>) Isodensity contours for the central retinal region. Conventions were the same of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone-0115291-g004" target="_blank">Figs. 4</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115291#pone-0115291-g005" target="_blank">5</a>. Ganglion cell isodensity contours were elongated in the nasal direction reflecting the higher ganglion cell density in the nasal quadrant, a feature present in different degrees in all six retinas studied. The “average” retina also had a small elongation of the central isodensity contours along the dorsoventral meridian, reflecting the presence of this feature in half of the retinas (see text for details).</p