The statoacoustic ganglion (SAG) is derived from neuroblasts that delaminate from the otic placode during development. Proper formation of the SAG is crucial for the establishment of appropriate connections with the sensory regions of the inner ear. However, imaging this process is challenging and the cell behaviours underpinning SAG morphogenesis are unclear. Here, Aitor Baňón and Berta Alsina develop ‘CRISPR Eraser’ zebrafish embryos to follow neuroblast delamination and migration in real time. By injecting an anti-GFP guide RNA into a neuroblast reporter line, they trigger a loss of GFP expression in most neuroblasts and leave a small GFP-positive population that can be imaged at high contrast. Using this system, the authors show that delaminating neuroblasts exhibit cell shape changes consistent with an epithelial-to-mesenchymal transition. Following delamination, neuroblasts migrate in a RhoGTPase-dependent manner and converge at a common ‘coalescence region’ where they form part of the SAG. This coalescence region is populated by pioneer SAG neurons, and ablating these neurons disrupts neuroblast migration, thus reducing the number of neuroblasts that go on to form the SAG and ultimately producing SAG morphological defects. Overall, this work identifies a population of pioneer neurons as crucial players in SAG morphogenesis via their role in modulating neuroblast recruitment.
