Fig. 1.

Schematic drawing of the spinal commissural projections and the series of guidance decisions controlling their trajectory before and after midline crossing. Commissural axons are initially repelled by BMP repellents emanating from the RP towards the ventral spinal cord. They are attracted to the FP by combinations of Netrin and Shh activity. Interactions between commissural growth cones and FP glial cells mediated by cell adhesion molecules (IgSFCAMs) regulate FP entry. After crossing, commissural axons lose their sensitivity to Netrin and acquire responsiveness to FP/midline repellents, including Slits, Ephrins, and Semaphorins. This novel sensitivity expels commissural axons away from the FP. A growth-promoting property of the FP SCF also contributes to stimulate commissural axons’ exit. Finally, responsiveness to antero-posterior gradients of repulsive and attractive cues such as Shh and Wnt4 specify commissural axon rostrocaudal turning