Embryology of Cochlear Nerve and Its Deficiency

Abstract

Most studies on development of the human auditory system and hearing loss have to date focused on the sensory apparatus, the cochlea. With advancements in magnetic resonance imaging (MRI), there has recently been increasing interest in the subject of cochlear nerve deficiency (CND) and dysfunction. Cochlear nerve deficiency (CND) amongst individuals with congenital sensorineural hearing loss (SNHL) is not as rare as previously thought, with prevalence as high as 18-21 % reported amongst cochlear implant recipients. Cochlear nerve (CN) morphogenesis is a complex process involving cell populations from two disparate progenitors of the otic placode and neural crest cells. In the first trimester, the basic foundation of the auditory pathway is laid down, with the vestibulocochlear ganglion cells delaminating from the otocyst and establishing peripheral and central connections with the developing cochlea and brainstem, respectively. The second trimester is a period of proliferation, growth and myelination. As the number of axons is pruned back closer to the adult level, myelination begins in the intra-cochlear portion of CN and extends proximally. In the third trimester, further maturation of the neuronal connections in conjunction with paralleled development of the cochlea and brainstem leads to emergence of foetal responses to auditory stimuli. Based on the currently available knowledge of the embryological development of CN, various phenotypes of CND are discussed. It is hoped that better understanding of CN ontogenesis will not only lead to further refinement of auditory implant candidacy but also open doors to potential regeneration therapies such as stem cell therapy in the future.