Comparative Retinal Pigment Epithelium and Photoreceptor Ultrastructure in Nocturnal and Fossorial Rodents: The Eastern Woodrat, Neotoma floridana, and the Plains Pocket Gopher, Geomys bursarius

In fossorial plains pocket gophers (Geomys bursarius) the eyes are extremely small and the cells of the retinal pigment epithelium (RPE) have extensive, highly developed apical microvilli and numerous melanolysosomes. The neural retina consists of both cone (up to 25%) and rod photoreceptor cells. The cones often are clustered and surrounded by rods. Ultrastructurally, the cones differ from rods in position and appearance of nuclei, overall shape of cell body, disc membrane, and presence of pedicle-type synaptic terminals. By way of comparison, in nocturnal eastern woodrats (Neotoma floridana) the RPE cells have poorly developed apical microvilli and relatively few melanolysosomes. The neural retina consists almost entirely (99%) of abundant and densely packed rod photoreceptors characterized by unusually long outer segments with a high number of discs. Numerous significant quantitative differences between RPE and photoreceptors of Neotoma and Geomys serve to underscore further the dissimilarities of the retinas of these rodents. Although G. bursarius and other species of gophers live in dark or semi-dark burrow systems and have eyes that are reduced in size, the RPE and neural retina is characterized by an intermediate (diurnal-nocturnal) structural pattern suggestive of good visual acuity. The lack of any obvious specialization in RPE or photoreceptor morphology is striking and could imply that the generalized eye in gophers is representative of the eyes found in their ancestors. If so, gophers probably evolved from ground-dwelling diurnal species.