The eye of the african mole-rat Cryptomys anselli: to see or not to see?

In an attempt to clarify its possible physiological role, we studied the eye of the Zambian mole rat Cryptomys anselli by light, electron and confocal microscopy using conventional staining as well as immunolabelling with rod and cone cell markers. The small eyes of Cryptomys are located superficially and display all features typical of sighted animals: iris, pupil and well-developed lens, separating the anterior chamber and the vitreous. The retina shows a well stratified organization and the folds described in blind subterranean or nocturnal mammals were not observed. The major population of the photoreceptor cells in the Cryptomys retina consists of rod cells, again with a morphology quite similar to that found in sighted animals. The relatively short outer segments contain numerous well-stacked disks and show a strong rod-opsin as well as transducin immunoreaction. Synapses were evident in the spherules, the round basal processes of the rod cell, but they lacked the precise organization reported for sighted mammals. Cone cells were present as well, as indicated by peanut lectin staining, but no immunolabelling with polyclonal M/L-opsin antisera was detectable. The presence of cone cells was also suggested by some basal processes at the outer plexiform layer which displayed several synaptic active sites and irregular contours. While the other retinal layers also showed an organization typical of sighted mammals, there were signs of less tightly preserved morphology as well. Displaced rods and amacrine and/or ganglion cells were observed, and some sparse rod spherules penetrated into the inner nuclear layer. A major reduction was observed in the number of ganglion cells, estimated from the number of axons in the optic nerve, that was very low (approximately 1000 per retina on average) relative to sighted mammals. The data we have suggest a slow, ongoing loss of cells with ageing. Apoptotic nuclei, mainly corresponding to photoreceptor cells and ganglion cells, were detected in young individuals, and an overall reduction in the thickness of the retina was observed in older animals. The morphological data presented here allow some first speculations on the physiological role of the Cryptomys eye and will hopefully trigger detailed studies on the chronobiology and the anatomy of the retinal projections and of the visual cortex of this remarkable species.