Ganglion Cell Topography and Retinal Resolution in an Irrawaddy Dolphin (Orcaella brevirostris)

The topographic distribution of retinal ganglion cells was investigated in a retinal wholemount of an Irrawaddy dolphin (Orcaella brevirostris). Two zones of increased concentration of ganglion cells were observed-one in the temporal segment and the other in the nasal segment of the retina. The maximal cell concentration was 250 cell/mm(2) in the temporal area and 194 cell/mm(2) in the nasal area. Based on the posterior nodal distance of 11 mm, the resolution was evaluated as 19.8 arc min in the temporal retinal area (the frontal visual field) and 22.4 arc min in the nasal retinal area (the temporal visual field). Among retinal ganglion cells, a group of giant cells that ranged from 42 to 52 gm in diameter were observed; these cells represented 8% of the total population. The obtained data allowed us to suggest that the presence of two areas of concentrated ganglion cells is characteristic of many delphinids inhabiting both clear oceanic waters or turbid river waters, whereas low ganglion cell density is associated with optic properties of the media.