Retinal topography of the harp seal Pagophilus groenlandicus

The total number, size, topographic distribution, and cell density of ganglion cells were studied in retinal wholemounts of the harp seal Pagophilus groenlandicus. Ganglion cell size varied from 10 to 60 mum. A distinct group were large ganglion cells more than 30-35 mum in diameter which were similar to alpha-cells known in terrestrial mammals. The number of alpha-cells constituted 5.3-5.9% of the total ganglion cell population. The cell size distribution was bimodal, with the second mode composed of alpha-cells. The topographic distribution of ganglion cells showed a definite area of high cell density similar to area centralis of terrestrial carnivores. This area was located in the temporal retinal quadrant, 7-8 mm (16-18 degrees ) from the optic disk. In this area, the peak cell densities in two wholemounts were 2,500 and 1,650 (mean 2,075) cells/mm(2). With a posterior nodal distance of 25.5 mm (underwater), this density corresponded to 495 and 327 (mean 411) cells/deg(2). These values predict a retinal resolution of 2.7-3.3' (11.1-9.0 cycles/deg) in water and 3.6-4.4' (8.3-6.8 cycles/deg) in air. Topographic distributions of alpha-cells was qualitatively similar to that of the total ganglion cell population, but the density of alpha-cells constituted only a few percent (mostly 3-7.5%) of the total ganglion cell density.