Photograph by George Trevis.

In adult vertebrates, the epibranchial ganglia relay sensory information from the oropharyngeal cavity to the hindbrain. During development, the sensory neurons that populate these ganglia migrate inwards from the epibranchial placodes — thickenings of the surface ectoderm that lie in clefts between the pharyngeal arches. The signals that guide the placodal cells to their target ganglia are unknown, but as reported in Science, a new study by Begbie and Graham indicates a role for another migratory cell population — the neural crest.

Using the lipophilic dye DiI to label migrating epibranchial placodal cells in chick embryos, the authors noticed that the trajectories of these cells bore a striking resemblance to certain neural crest migration routes in the hindbrain. By labelling neural crest and epibranchial placodal cells with different markers, they showed that the migrating placodal cells were closely apposed to the neuroglial crest, a late-migrating population of cells that gives rise to neurons and glia.

To test whether these cells have a role in epibranchial neuronal guidance, the authors ablated the neural crest by removing the entire hindbrain neuroepithelium. In these embryos, the placodal cells failed to migrate away from the surface epithelium, although they were still able to differentiate and extend axons. Ablation of specific regions of the hindbrain had more localized effects; for example, ablation of rhombomere 4 blocked only the migration of placodal cells destined for the geniculate ganglion.

To confirm that their results were caused by loss of the neural crest, and not the neural tube itself, the authors removed the neural tube only after the neural crest had emerged. In this case, the placodal cells were able to migrate towards their usual targets, although loss of the hindbrain neuroepithelium still led to defects in axonal pathfinding.

These data represent compelling evidence that ablation of the neural crest prevents neuronal migration from the epibranchial placodes. This gives rise to a new model, in which the neuroglial neural crest lays down tracks as it emerges from the hindbrain neural tube, and these tracks then facilitate the migration of epibranchial neuronal cells in the opposite direction. Interestingly, this might also help to explain the observation that, in jawless fish, the hindbrain neural crest splits into streams. Although such streaming was initially thought to be an adaptation for patterning the jaws, it is now possible that it had an ancestral role in placodal innervation of the hindbrain.