Introduction: Inner ear vestibular organs (utricle, saccule, and semicircular canals) provide the brain with information about motion, head position, and spatial orientation. The crista ampullaris is an important sensory organ of rotation located inside the semicircular canals. Wide morphological heterogeneity characterizes synaptic architecture within both the rodent and mammalian crista ampullaris. A cross-species approach to studying this heterogeneity in synaptic ribbons—vesicle-associated structures critical for transmission between presynaptic hair cells and post-synaptic afferent neurons—has not been undertaken. The chinchilla offers a well-studied, mammalian model of vestibular synapses, and thus can be used comparatively with the mouse model to investigate how synaptic ribbon morphological heterogeneity varies across species. The purpose of this study was to investigate and define differences in synaptic ribbon morphological distributions between mouse and chinchilla.
Methods: Mouse and chinchilla ampullaris were studied using whole-mount staining cryosectioning, respectively. Airyscan methods were used to image individual afferent calyces and synaptic ribbons. Ribbons were statistically analyzed to compare surface area, volume, and sphericity distributions between mouse and chinchilla.
Results: Regarding surface area and volume distributions, it was observed that ribbons on either size-extreme (very small ribbons and large ribbon-clusters) were missing in the chinchilla. Sphericity distributions were not significantly different between mouse and chinchilla.
Conclusion: The chinchilla and the mouse crista synaptic ribbons exhibit similar sphericity distributions, but starkly different size distributions. Large ribbon-clusters are likely associated with more vesicles, and thus with higher exocytotic capacity, suggesting some compensatory mechanism for increasing exocytotic capacity at the ribbon-synapse in the chinchilla.