Afferent neurons transmit information from both external and internal origin into the central nervous system (CNS). Sensory organs are connected at the periphery to these neurons, which in turn project into specific regions in the CNS. In sensory organs, such as the vertebrate ear, which receive auditory and vestibular stimuli, establishing precise connections with central targets is necessary for discrete, simultaneous, and efficient processing. However, it is not clear how afferents of the inner ear establish central projections with their target nuclei in the hindbrain. Transplantation of ears in Xenopus laevis offer a method through which the navigational properties of inner ear afferents can be experimentally tested. Specifically, grafting of ears to novel locations allow us to assess the pathfinding capabilities of afferents following ectopic placement.
In transplanting ears adjacent to the spinal cord, we found that despite variable entry points along the dorsal-ventral axis, afferents projected dorsally, similar to projections of native ears in the hindbrain. Furthermore, these afferents were able to reach the hindbrain and project into vestibular nuclei. Late stage transplantations to the spinal cord revealed ear afferent fasciculation with afferents of the lateral line, indicating an alternative navigational route. Similarly, ventral transplantations to the heart region demonstrated ear afferent projection with the vagus nerve. These results collectively suggest that inner ear afferents are molecularly guided to reach their targets in the CNS once they are in proximity to it. However, they also display a capability to project along existing nerves both within the central and peripheral nervous systems. These results provide new information into how inner ear afferents navigate to connect with the CNS.