Synaptic specializations associated with the outer hair cells of the Japanese macaque

Across species the innervation of outer hair cells (OHCs) shows a remarkable similarity. There are, however, notable differences in fine structure. The present work describes the normal synaptic morphology of OHCs in the Japanese macaque (Macaca fuscata), as determined by examination of serial sections with transmission electron microscopy. The nerve endings at the base of OHCs were divided primarily into two groups: vesiculated (efferent) and non-vesiculated (afferent). In addition, we found supranuclear efferent nerve endings and reciprocal synapses in all three cochlear turns. We also found presynaptic bodies in OHCs at the afferent synapse, the branching of afferent nerve fibers and axodendritic synapses between afferent and efferent fibers in the outer spiral bundle and just beneath OHCs. In terms of synaptic structure, the data indicate that that the Japanese macaque is more similar to that of the human than other species examined to date.