What are the challenges for axon guidance in setting up the olfactory system?
An incredible example of precise axonal guidance is the axonal convergence of olfactory sensory neurons (OSNs) expressing a given odorant receptor (OR) onto spatially invariant glomeruli. Evidence suggests that both guidance molecules and ORs play integral roles in the process. Each OSN expresses a single type of OR. In mice, the genome encodes around 1000 OR genes. ORs are 7-transmembrane G-protein coupled receptors. There are about 1 mil OSNs, each of which express 1 of the 1000 OR genes. Therefore, on average, there are 1000 neurons expressing the same OR in the olfactory epithelium. Neurons expressing the same OR are scattered in one of the zones in the epithelium.
Upon reaching the olfactory bulb, axons usually converge on one or two glomeruli. There are around 1800 glomeruli, each of which has a topographically-fixed, invariant location in the olfactory bulb. This means that somehow the axons sort, converge, and target the same glomerulus in the olfactory bulb with incredible precision. Researchers have also found that swapping OR with B-adrenergic receptors also enables convergence of axons onto the glomeruli. Pre-sorting of ORNs in the bulb depends on the level of cAMP generated by odorant (Sakano's model).
An incredible example of precise axonal guidance is the axonal convergence of olfactory sensory neurons (OSNs) expressing a given odorant receptor (OR) onto spatially invariant glomeruli. Evidence suggests that both guidance molecules and ORs play integral roles in the process. Each OSN expresses a single type of OR. In mice, the genome encodes around 1000 OR genes. ORs are 7-transmembrane G-protein coupled receptors. There are about 1 mil OSNs, each of which express 1 of the 1000 OR genes. Therefore, on average, there are 1000 neurons expressing the same OR in the olfactory epithelium. Neurons expressing the same OR are scattered in one of the zones in the epithelium. Upon reaching the olfactory bulb, axons usually converge on one or two glomeruli. There are around 1800 glomeruli, each of which has a topographically-fixed, invariant location in the olfactory bulb. This means that somehow the axons sort, converge, and target the same glomerulus in the olfactory bulb with incredible precision. Researchers have also found that swapping OR with B-adrenergic receptors also enables convergence of axons onto the glomeruli. Pre-sorting of ORNs in the bulb depends on the level of cAMP generated by odorant (Sakano's model). 
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