How thick should a retina be? A comparative study of mammalian species with and without intraretinal vasculature.

In this study we describe a measurement method which closely approximates in vivo retinal thickness. Using this method we examined the laminar organisation of vascular and avascular retinae from placental and marsupial mammals. Thickness measurements on retinal wholemounts show that the avascular retinae of the placental guinea pig (140 microns) and the marsupial brushtail possum (170 microns) are thinner and show less centroperipheral taper than do the vascular retinae of placental cats (250 microns), rats (220 microns) and marsupial quolls (220 microns). In general, limitation in thickness of avascular retinae is borne by most retinal layers, but most particularly by the inner plexiform layer, the synaptic region farthest removed from the choroidal blood supply. Except for the absence of blood vessels, the histological organisation of the brushtail possum's retina resembles closely that of its fellow marsupial, the quoll's. In contrast, intraretinal organisation differs amongst the two avascular retinal species with the guinea pig displaying a much coarser photoreceptor grain.