The evolution of retinal vascularization in mammals. A comparison of vascular and avascular retinae.

There is a bimodal distribution of retinal thicknesses among mammals. Avascular retinae that are totally dependent on the choroid measure less than the theoretical oxygen diffusion maximum of 143 microns. Vascular retinae often measure twice this distance, but their avascular portions do not exceed the calculated diffusion maximum. Avascular retinae also show heavy glycogen deposition in the Müller cells of the inner retina, short photoreceptors, and lack both a tapetum and retinal taper. It is argued (1) that these anatomical features represent adaptations to their nutritional dependence on the choroid, and (2) that the phylogenetic distribution of avascularity (monotremes, many marsupials, and some of the primitive placental mammals) suggests that it is the ancestral condition in mammals. Finally, it is noted that some of the pathologic changes associated with retinal vascular damage in humans, namely thinning, gliosis, and glycogen deposition, are reminiscent of these ancestral patterns for coping with a limited blood supply.