Transplantation of Ears Provides Insights into Inner Ear Afferent Pathfinding Properties

Numerous tissue transplantations have demonstrated that otocysts can develop into normal ears in any location in all vertebrates tested thus far, though the pattern of innervation of these transplanted ears has largely been understudied. Here, expanding on previous findings that transplanted ears demonstrate capability of local brainstem innervation and can also be innervated themselves by efferents, we show that inner ear afferents grow toward the spinal cord mostly along existing afferent and efferent fibers and preferentially enter the dorsal spinal cord. Once in the dorsal funiculus of the spinal cord, they can grow toward the hindbrain and can diverge into vestibular nuclei. Inner ear afferents can also project along lateral line afferents. Likewise, lateral line afferents can navigate along inner ear afferents to reach hair cells in the ear. In addition, transplanted ears near the heart show growth of inner ear afferents along epibranchial placode-derived vagus afferents. Our data indicate that inner ear afferents can navigate in foreign locations, likely devoid of any local ear-specific guidance cues, along existing nerves, possibly using the nerve-associated Schwann cells as substrate to grow along. However, within the spinal cord and hindbrain, inner ear afferents can navigate to vestibular targets, likely using gradients of diffusible factors that define the dorso-ventral axis to guide them. Finally, afferents of transplanted ears functionally connect to native hindbrain vestibular circuitry, indicated by eliciting a startle behavior response, and providing excitatory input to specific sets of extraocular motoneurons.