A model of the proposed roles of Sema6A and PlexinA as gate-keepers at the motor exit point (MEP). (a) In wild-type mice, boundary cap cells (BCCs) express Semaphorin 6A (Sema6A) and motor neurons express members of the class A plexins. Motor neuron cell bodies are caged in the spinal cord. (b) Knock down of Sema6A (Sema6A-/-) leads to a lack of clustering of BCCs and ectopic migration of motor neuron cell bodies at the MEP. (c) Interactions between BCCs expressing Sema6A and motor neurons expressing PlexinA activate both reverse and forward signals to induce formation of BCC clusters and motor neuron caging, respectively. MIC, MICAL3, an intermediary of semaphorin signaling.