Pathfinding of olfactory neuron axons to stereotyped glomerular targets revealed by dynamic imaging in living zebrafish embryos.

In the vertebrate olfactory system, sensory neurons with common odorant specificities project to specific glomeruli in the olfactory bulb. How do olfactory sensory neurons find their glomerular targets? To address this question, we have visualized the genesis of the peripheral olfactory system in living zebrafish embryos. Dye labelings reveal that a primordial yet stereotyped map of glomeruli is apparent during embryogenesis. By labeling a small number of cells with an ectopically expressed green fluorescent protein reporter, we can observe the dynamic growth behaviors of individual olfactory neuron growth cones as they project to their glomeruli. We find that olfactory axons extend directly to their partner glomeruli, suggesting that these cells' growth cones rely upon pathfinding cues to reach their targets.