Cell positioning and axon pathfinding establish neural circuits in the developing nervous system. Previous fate-mapping studies in zebrafish have revealed that olfactory sensory neuron (OSN) precursors along the anterior neural plate converge - via the action of unknown factors - to form the olfactory placode. Miyasaka et al. now report on p. 2459 that, in zebrafish, Cxcl12/Cxcr4 chemokine signalling mediates both the olfactory placode assembly and the correct projection of differentiated OSN axons to the olfactory bulb. In odysseus (ody) zebrafish mutants (in which Cxcr4b receptor function is lost), olfactory placode assembly is perturbed, olfactory neurons displace ventrally and OSN axons fail to exit the placode, and instead accumulate near the placode-telencephalon border. The misexpression of the Cxcl12 ligand (ubiquitously and in mosaics) in zebrafish embryos also perturbs placode formation. The role of Cxcr4 signalling in zebrafish has some similarities to its role in mice, where it regulates motor axon projections. Perhaps, the authors suggest, Cxcl12/Cxcr4 chemokine signalling creates a favourable environment for initial axonal trajectories in vertebrates.
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