Fig. 7. A model for trochlear axon guidance. Trochlear cell bodies (green) arise in a cluster in ventral anterior r1. Extending axons (green arrows) are exposed to a number of both positive and negative chemotropic signals within the neural tube. Initially, a strong repulsive signal from the floor plate (netrin 1; dark blue) initiates a dorsal trajectory. Additional repulsive signals are present in posterior r1 (Sema3F; light blue). In combination with an attractive signal from the isthmus (Fgf8, red), a net positive signal towards the dorsal isthmic region is produced.

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