Which guidance molecule/s is/are responsible for preventing commissural axons of the spinal cord from crossing the midline more than once during development?单项选择题
A
a) Slit
B
b) Netrin
C
c) Semaphorin
D
d) SynCAM
E
e) A and C

Question

Which guidance molecule/s is/are responsible for preventing commissural axons of the spinal cord from crossing the midline more than once during development?

BlackTom AI · Accepted Answer

Slit proteins are secreted repulsive midline cues that, acting through Robo receptors on commissural axons, prevent those axons from crossing the spinal cord midline more than once.

Relevant knowledge points:
- Commissural axon guidance at the midline is coordinated by opposing cues: Netrins (attractive) draw axons toward the midline via DCC/Unc receptors, while Slit proteins provide a repulsive signal via Robo receptors to push axons away after crossing.
- Regulation of receptor responsiveness is critical: before crossing, commissural axons are insensitive to Slit (low Robo activity), allowing approach; after crossing, Robo is upregulated (or de-inhibited), making the axon responsive to Slit and preventing re-entry.
- Drosophila Comm protein and vertebrate regulatory mechanisms temporally control Robo trafficking/sensitivity.

Step-by-step reasoning:
1. Pre-crossing: commissural growth cones express receptors (e.g., DCC) that detect Netrin secreted at the midline; this attraction directs axons to cross.
2. During approach/crossing: Robo-mediated sensitivity to Slit is suppressed (e.g., by Commissureless in flies), so Slit repulsion does not block initial crossing.
3. Post-crossing: Robo receptors are expressed/activated on the axon surface; encountering Slit at the midline now produces repulsive signaling.
4. Result: repulsion by Slit–Robo pushes axons away from the midline and prevents them from turning back and re-crossing.

Why Slit (a) is correct:
- Genetic and functional studies in flies and vertebrates show loss of Slit or Robo leads to axons failing to stay lateral and sometimes recrossing the midline, demonstrating Slit’s key role in preventing re-crossing.

Why the other options are incorrect:
- b) Netrin is primarily an attractive cue that draws commissural axons to the midline, not the main signal preventing re-crossing.
- c) Semaphorins are repulsive/attractive guidance cues in many contexts but are not the principal mechanism enforcing one-time midline crossing.
- d) SynCAM is a synaptic adhesion molecule, not a midline guidance repellent.
- e) A and C is wrong because semaphorin is not required for preventing re-crossing; Slit alone is the canonical repulsive signal.

Conclusion: Slit (option a) is the correct answer. This explanation confirms and supports the provided answer.

Which guidance molecule/s is/are responsible for preventing commissural axons of the spinal cord from crossing the midline more than once during development?单项选择题

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