Conservation of vestibular afferent segregation within the otolith endorgans at the striola across vertebrate classes

Hair cells are the basic units of transduction in the vestibular apparatus. An intriguing feature of the hair cells in the utricle and saccule is the reversal of their morphological polarity at the striola: the kinocilia of all hair cells lie toward the striola in the utricle, and away from it in the saccule. In a previous study we showed that, in mice, the striola demarcates afferent fibers projecting to the brainstem and cerebellum. We used retrograde transport of lipophilic tracers to determine whether the striola in the chick also separates afferent domains projecting to the brainstem and cerebellum. Here, we found that in the utricle, afferents projecting to the cerebellum originate lateral to the striola, whereas afferents to the vestibular nuclei in the brainstem originates medial to it. In the saccule, afferents projecting to cerebellum originate from an area superior to the striola, whereas afferents projecting to the vestibular nuclei are inferior to it. In the lagena, the cerebellar afferents originate predominantly anterior to the striola, whereas the afferents to the vestibular nuclei originate predominantly posterior to it. As in mice, these findings indicate that, the striola demarcates not only the reversal of hair cell polarity, but also the segregation of afferents to the brainstem and cerebellum. This implies a conservation of primary macular afferent pathways in several classes of vertebrates.