The distribution of photoreceptors, dopaminergic amacrine cells and ganglion cells in the retina of the North American opossum (Didelphis virginiana).

Quantitative light microscopic techniques have been applied to photoreceptors, ganglion cells and dopaminergic amacrine cells in the retina of the North American opossum Didelphis virginiana. This marsupial has a retina with a central areal specialization where ganglion cells reach a maximum density of 2900/mm2 and cone photoreceptors are concentrated. However, the retina is very much rod dominated and there is a ring of peak rod density approximately 2 mm around the area centralis (a.c.). At the a.c. the cone to rod ratio is 1:50 while in peripheral retina it is 1:120. Dopaminergic amacrines, revealed by induced fluorescence (Fa/Glu) techniques are extremely numerous in opossum retina reaching a peak density at the a.c. and distributed in a high density plateau covering much of tapetal superior retina. Opossum retina is compared to mammalian retinas, particularly cats, with the aim of understanding the evolution of mammalian retinal circuits.