Role of pioneer neurons and neuroblast behaviors on otic ganglion assembly

Cranial ganglia are aggregates of sensory neurons that mediate distinct types of sensation. It is little understood how individual neurons coalesce, distribute and shape the ganglion. The statoacoustic ganglion (SAG) displays several lobes spatially arranged to properly connect with hair cells of the inner ear. To investigate the cellular behaviors involved in the 3D organization of the SAG, we use high resolution confocal imaging of single cell labeled zebrafish neuroblasts (NB), photoconversion, photoablation and genetic perturbations. We find that otic NB delaminate out of the otic epithelium in an EMT-like manner, rearranging apical polarity and primary cilia proteins. We also show that, once delaminated, NB migrate directionally and actively, requiring RhoGTPases. Interestingly, cell tracking of individual delaminated NB reveals that NB migrate and coalesce around a small population of pioneer SAG neurons. These pioneer SAG neurons are not from otic placode origin and populate the coalescence region before otic neurogenesis begins. Upon ablation of these cells, migratory pathways of delaminated NB are disrupted and, consequently, SAG shape is affected. Altogether, this work shows for the first time the role of pioneer SAG neurons in orchestrating SAG development. Summary StatementLittle is known how cranial sensory ganglia organize in 3D. We unveil the repertoire of cellular behaviours underlying statoacoustic morphogenesis and its dependence on relevant pioneer neurons.