The eye of the african mole‐ratCryptomys anselli: to see or not to see?

Abstract

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.

Magnetic compass in the cornea: local anaesthesia impairs orientation in a mammal

“…Contrary to the putative connection of magnetite-based receptors with the ophthalmic nerve, Cernuda-Cernuda et al (Cernuda-Cernuda et al, 2003) reported findings of retinal crystalloid bodies in the inner photoreceptor segment of the Ansell's mole-rat. The authors interpreted these structures as potential magnetite grains, and suggested the retinal photoreceptors as the magnetite-based structure.…”

Orexinergic neuron numbers in three species of African mole rats with rhythmic and arrhythmic chronotypes

“…It should be noted that Ansel"s and Damaraland mole rats were previously considered to be members of the genus Cryptomys. All three species of mole rat have a reduced visual system, live a subterranean lifestyle and have unusual patterns of circadian rhythmicity (Lovegrove and Papenfus, 1995;Lovegrove and Muir, 1996;Oelschlager et al, 2000;Cernuda-Cernuda et al, 2003;Negroni et al, 2003;Oosthuizen et al, 2003;Gutjahr et al, 2004;Nemec et al, 2004;Nemec et al, 2008). Brains from circadian distinct rhythmic and arrhythmic individuals of each species were examined.…”

Eye and vision in the subterranean rodent cururo (Spalacopus cyanus, octodontidae)

“…The model species to support such claims is the muroid blind mole-rat Spalax ehrenbergi, which has atrophied subcutaneous eyes and retinal deficits (Sanyal et al, 1990;Cooper et al, 1993;Cernuda-Cernuda et al, 2002;David-Grey et al, 2002). On the other hand, recent studies on subterranean African mole-rats (bathyergid rodents) have reported small but well-developed eyes with photoreceptor properties that suggest visual capabilities at higher light levels (Cernuda-Cernuda et al, 2003;Peichl et al, 2004). These findings call for a more differentiating view of visual regression in subterranean species.…”