The retina of Spalax ehrenbergi: Novel histologic features supportive of a modified photosensory role

PURPOSE. The retina of the blind mole rat Spalax ehrenbergi was compared with other vertebrate photosensitive organs in an attempt to correlate its histologic organization with a presumptive nonvisual photoreceptor role. METHODS. The eyes of eight adult animals were analyzed by light and electron microscopy, using conventional staining and immunolabeling with antibodies against phototransduction proteins and calretinin. RESULTS. Rods accounted for most of the photoreceptor cells in the Spalax retina, although their morphology is dissimilar to that of sighted mammals, in that they contained only rudimentary outer segments. The latter showed strong rod-opsin and transducin immunoreactions. The phagosomes in the retinal pigmentary epithelium were also rod-opsin positive. Synapses were evident at the photoreceptor cells pedicles. Occasionally, several synaptic active sites were present, suggesting cone cell origin; however, cone-opsin was not immunodetected in the study samples. Synaptic ribbon fields, sometimes distant to the active sites, resembled those found in the vertebrate pineal. The other retinal layers were somewhat less organized than in sighted mammals. Some cells were displaced and the calretinin-positive inner plexiform layer had no sublayers. Calretinin immunolabeling was found in horizontal, amacrine, and ganglion cells. Folding of the retina produced rosette-like images similar to those reported before in the retina of nocturnal mammals and in the avian pineal gland. CONCLUSIONS. These data suggest that the retina of the mole rat has undergone evolutionary restructuring to a photoreceptive pineal-like organization. This supports the thesis that the photoreceptor cells of this unique organ have been reprogrammed during the subterranean adaptation of Spalax, from their original visual function to mediating photoperiodic regulation.