Semaphorin and neuropilin co-expression in motoneurons sets axon sensitivity to environmental semaphorin sources during motor axon pathfinding.

Class III semaphorins (SemaIIIs) are intercellular cues secreted by surrounding tissues to guide migrating cells and axons in the developing organism. This chemotropic activity is crucial for the formation of nerves and vasculature. Intriguingly, SemaIIIs are also synthesized by neurons during axon pathfinding, but their function as intrinsic cues remains unknown.