Consequences of reduced eye size on the visual and photoperiodic systems in the blind mole rat, Spalax ehrenbergi

Although the subterranean mole rat is considered to be completely blind, thermal and circadian activity rhythms are synchronized by the external light cycle. The subcutaneous eye measures 600-700 μm in diameter and the optic nerve (50 μm diam.) contains 900-1000 unmyelinated fibers only (EM study). In vitro application of HRP to the optic nerve labels a population of 850 ganglion cells (RGC), homogeneously distributed in the retina. Cell soma size distribution is unimodal (6- 16 μm) and RGC’s appear immature with sparsely branched dendritic trees. Intraocular injection of CT-HRP shows that all structures known to be innervated by the retina in other mammals, receive retinal projections in Spalax. The vLGN, dLGN, pretectum, superior colliculus and accessory optic system all receive a sparse bilateral retinal projection, are highly reduced in size, and show a poor cytoarchitectural organization. Injection of HRP in the colliculus labels RGC’s distributed over the entire surface of both retinas, suggesting a lack of topographic organization. Geniculo-cortical connections, revealed by multiple injection of fluorescent tracers in area 17, were also poorly organized. In contrast to other mammals, the suprachiasmatic nucleus (SCN) and the bed nucleus of the stria terminalis receive a significant proportion of retinal projections (> 30%). Histochemical and immunohistochemical studies show that the SCN is organized similar to that of other rodents, and contains cells immunoreactive to VIP, VP, and SOM as well as 5-HT, MET-ENK and NPY immunopositive fibers. These results demonstrate that adaptive evolution in the subterranean niche has led to a mosaic of regressive and progressive features: structures related to form vision, motion analysis, and visuomotor function, are hypoplasic, whereas structures related to photoperiodic light detection and circadian neuroendocrine regulations are relatively hypertrophied and conserve a normal pattern of intrinsic organization.