Which axon guidance cues guide commissural axons crossing the midline?

Which axon guidance cues guide commissural axons crossing the midline?

• A. Netrin-1 attraction and Slit–Robo signaling repulsion
• B. Netrin-1 only
• C. Slit–Robo signaling only
• D. Nogo signaling

Answer: (A)

Guidance of commissural axons across the midline relies on a balance of attraction to reach the midline and repulsion to move away after crossing. The floor plate secretes netrin-1, which attracts growing axons toward the ventral midline by signaling through receptors like DCC on the axons. Once the axons cross, Slit ligands interact with Robo receptors to generate repulsion, helping steer the axons away from the midline and preventing recrossing. A coordinated switch—initial netrin-1–mediated attraction to reach the midline, followed by Slit–Robo–mediated repulsion after crossing—is essential for proper midline crossing. Nogo signaling is not involved in this midline-crossing process. So both cues together explain how commissural axons are guided across the midline.

Guidance of commissural axons across the midline relies on a balance of attraction to reach the midline and repulsion to move away after crossing. The floor plate secretes netrin-1, which attracts growing axons toward the ventral midline by signaling through receptors like DCC on the axons. Once the axons cross, Slit ligands interact with Robo receptors to generate repulsion, helping steer the axons away from the midline and preventing recrossing. A coordinated switch—initial netrin-1–mediated attraction to reach the midline, followed by Slit–Robo–mediated repulsion after crossing—is essential for proper midline crossing. Nogo signaling is not involved in this midline-crossing process. So both cues together explain how commissural axons are guided across the midline.