A Regional Ultrastructural Analysis of the Cellular and Synaptic Architecture of the Mouse Vestibular Periphery, With...

The mouse utricular macula is increasingly being used as a model preparation to study the vestibular periphery because we can generate transgenic mice to investigate molecular details of development and function. Yet, detailed knowledge of its synaptic innervation is lacking or inconsistent. Accurate ribbon synapse numbers and location are needed to quantitatively model quantal transmission in the mouse, as has recently been done for non-quantal transmission in the Type I vestibular HC (Govindaraju et al. 2023). We investigated this at the ultrastructural level, as we have done previously in the chinchilla and squirrel monkey. The same investigative methods that we used in those previous studies (dissector and transmission electron microscopy [TEM]) were used to confirm recent confocal and TEM studies of the synaptic ribbons contained in the two types of vestibular HCs, Type I (enveloped by a large calyceal, or chalice-shaped, terminal) and Type II (contacted by more conventional synaptic boutons). Because vestibular function varies depending on specific regions in the sensory epithelium (central/striolar, peripheral/extrastriolar), the present study examined the different regions and found both regional and cell-type variations. Synaptic ribbon numbers were higher in Type II than in Type I HCs in both the utricular macula and the crista ampullaris. Previous work in chinchilla crista ampullaris had a gradient of synaptic ribbons in Type I HCs, being more numerous in the central zone versus the periphery. In the mouse crista (present study), the opposite was true; ribbon numbers were slightly higher in the periphery. For comparison to the mouse utricle, we also collected new data from the chinchilla utricular macula in this study. Finally, a variety of ribbon shapes were present in the vestibular epithelium, ranging from spheroid to elongated and intermediate forms. The reasons for these observed variations in shapes are unknown. These data should inform future functional and modeling studies of the vestibular sensory epithelium.