Sensory organs, such as the inner ear, send information about the outside world to the central nervous system (CNS) through afferent neurons and in turn, the CNS sends information back to certain sensory organs through efferent neurons to modulate the incoming signal. Though how these afferent and efferent neurons navigated with their processes to the hindbrain or hair cells, respectively, is not clear. By transplanting ears to other locations, or adding ears, we can effectively create a novel ear to ask how the CNS adapts to a new sensory system, complete with efferent innervation of hair cells and afferent innervation into the CNS. In addition, by removal of the existing ear, we can ask what influence an established sensory system has on CNS development.
Transplantation of Xenopus laevis ears caudally to the trunk to replace a somite or to the orbit to replace the eye resulted in the innervation of hair cells of the transplanted ear by spinal motor neurons or by oculomotor and trochlear motor neurons, respectively. The ability to be innervated by any motor neuron is a unique property associated with inner ear hair cells as other tissues normally receiving motor innervation were not innervated by all motor neurons when transplanted. Projections of inner ear afferents into the CNS when the ear was transplanted to the orbit were inconsistent, but occasionally projected into the vestibular nucleus along the trigeminal nerve, suggesting that there may be molecular guidance of inner ear afferents if they projected by chance near the vicinity of the vestibular nucleus. The eye, which is developmentally related to the ear, uses both molecular targeting to the CNS and once there, projections from the two eyes are refined through activity-based mechanisms. Transplantation of an additional ear rostral to the native ear in Xenopus laevis in either the native orientation or rotated 90 degrees with respect to the native ear showed that axons from the two ears project to the vestibular nucleus, likely using molecular cues. Furthermore, axons from the natively-oriented transplanted ear overlap with axons from the native ear, and in contrast, axons from the rotated transplanted ear segregate from those of the native ear. The latter is likely due to differential activity between the two ears and suggests that the ear uses similar mechanisms as the eye for axon guidance. The effect of ear removal has been well studied on populations of hindbrain neurons, but less at the single-cell level. Removal of an ear demonstrated the dependence of the ear for the development and/or survival of a target cell of the ear, the Mauthner cell, but only for a critical time in development. Furthermore, ear ablation resulted in the reduction of the number of dendritic branches in surviving Mauthner cells and an increase in dendritic branching when an extra ear was transplanted rostral to the native ear, suggesting a relationship between sensory afferent input and dendritic development of a target neuron.
Together these results show that the nervous system can adapt to a novel sensory system, but with limitations, especially in sensory afferent guidance. In addition, perturbations of an established system have consequences on the development of target neurons dedicated for that system.