A regional specialization in the opossum's retina: quantitative analysis of the ganglion cell layer.

The distribution of ganglion cells in the opossum's retina was determined from flat-mounted preparations stained with cresyl-violet. The retinal area is 109 mm2 (SD = 16 mm2). Maps of ganglion cell density were made from retinae of seven animals. In all maps iso-density lines were approximately concentric, showing a slight elongation towards the nasal region. Cell density varied from 400 cells/mm2 at the extreme periphery to 2,900 cells/mm2 in the region of highest count, the are centralis. The center of this region lies 1.85 mm (26.3 degree) temporal to the center of the optic nerve head. The average total number of ganglion cells is 77,384 (SD = 10,173). Based upon soma diameter histograms ganglion cells were classified into three groups, showing at area centralis peaks at 7 micrometer, 12 micrometer and 15 micrometer respectively. Cell soma diameter ranged from 6 micrometer to 21 micrometer, larger values being observed at the periphery.