Maturation of the Structure and Function of the African Naked Mole-Rat Inner Ear

African naked mole-rats (Heterocephalus glaber) are a highly vocal and long-lived subterranean species with elevated auditory thresholds, poor frequency selectivity, and a limited ability to localize sound compared to other rodents. The poor hearing in naked mole-rats (NMRs) suggests an under-developed auditory system, specifically the cochlea, which is responsible for encoding and relaying auditory signals to the brain. To examine the patterns of innervation and physiology of the NMR cochlea techniques included: immunofluorescence and quantitative image analysis, physiological assessments of hair cell and auditory nerve function, and structural examination of the organ of Corti, the sensory epithelium in the cochlea. NMRs were compared to mice (Mus musculus), gerbils (Meriones unguiculatus), and Damaraland mole-rats (Fukomys damarensis), another subterranean rodent. The results of the study indicate that in comparison to mice and gerbils, there are alterations in afferent and efferent innervation as well as in the patterns of developmental refinement in both naked and Damaraland mole-rats. However, there were noticeable differences in these alterations between naked and Damaraland mole-rats. In both naked and Damaraland mole-rats, inner hair cell afferent ribbon density was reduced, whereas outer hair cell afferent ribbon density was increased. Naked and Damaraland mole-rats also showed reduced lateral and medial efferent terminal density. Developmentally, NMRs showed reduced and prolonged postnatal reorganization of afferent and efferent innervation. However, Damaraland mole-rats showed no evidence of postnatal reorganization. A restricted cochlear amplifier and altered organization of the organ of Corti and outer hair cell morphology compared to other rodents was also observed. Overall, these results provide more insight into the mechanisms that enhance frequency sensitivity and sound localization, maturation and aging of the auditory system, as well as the evolutionary adaptations occurring in response to subterranean environments.