Morphogenesis of the lateral semicircular canal in the zebrafish inner ear

The study of the morphogenetic processes underlying organogenesis has always proved to be challenging, as many aspects need to be considered (e.g. cell movement and genetic interactions). This is particularly interesting in the case of very complex organs, such as the inner ear, which hosts both the auditory and vestibular apparatus, the latter being the organ of balance. This is composed by three semicircular canals, which detect angular acceleration of the head and send information to the brain to allow the maintenance of posture and stabilisation of gaze. A failure in the activity of these canals has been linked to various kinds of disease, such as the Branchio-Oto-Renal syndrome and Menière’s disease. This project is focused on the development of the lateral (horizontal) canal, as it has been proposed to be the last to have evolved and to be the most affected by inner ear malformations. In the zebrafish, the lateral canal development requires the formation of an epithelial pillar, between 48 and 72 hours post fertilisation (hpf), which is the hub of the canal. Previous studies have described the ventral pillar formation as an epithelial fusion between two cell populations (ventral bulge and ventral projection), but the cell movements required to achieve that have never been described in detail. By taking advantage of light-sheet microscopy, I have demonstrated that, upon fusion, other cell rearrangements need to take place to develop the ventral pillar. In particular, after the fusion event, the cells exhibit complex movements and a dynamism that is not characteristic of an epithelial state. In addition, I built on previous knowledge regarding the inner ear phenotype of the otx1 and eya1 mutants to analyse a possible genetic interaction between these two genes and how their mutation affects the ventral pillar development. Here, I show that these two mutants exhibit reciprocal phenotypes, with respect to the ventral pillar, and that this correlates with changes in the expression pattern of specific markers of the ventral otic epithelium. In conclusion, this study sheds light on the cell movements and genetics underlying the development of the ventral pillar and, more broadly, provides new ways of analysing the morphogenetic processes that take place during organogenesis.