Postnatal development of the inner ear efferent innervation in mammals

Efferent innervation of the inner ear is extensively studied but the whole model revealing the development of efferent synapses is not clear yet. In mammals the lateral and medial olivocochlear systems are known as the source of efferent fibers. The lateral olivocochlear system innervates the ipsilateral cochlea, terminating on the dendrites beneath the inner hair cells (IHCs), the dendrites being spiral ganglion neuron compounds. The medial olivocochlear system is involved in forming synapses directly on the outer hair cells (OHCs). To reach the final targets efferent axons use the afferent fibers as a scaffold. Efferent synaptogenesis occurs just before the onset of hearing. At P0 in rats we observed synaptic-like contacts lacking typical features. At P3 the synapses were immature. At P4-P5 efferent contacts with IHCs were clearly defined. At P6-P7 the efferent terminals were larger with distinct synaptic vesicles. During maturation, at P8-P10, the number of efferent synapses at the base of the ICHs reduced alongside with a decrease in the synaptic cisternae. After P12 efferent terminals formed axodendritic synapses below IHCs and large axosomatic synapses on OHCs. The innervation of OHCs underwent two stages, i.e. transitional with simultaneous innervation of IHCs and OHCs and a final OHC-targeted innervation. These results support the idea for a waiting period of efferent innervation before its final establishment in adult organ of Corti. We also summarize the role of neurotrophic factors, specific neurotransmitter systems, their receptors and transporters for refinement of cochlear efferent innervation. Biomedical Reviews 2013; 24: 33-48.