Cross-midline interactions between mouse commissural hindbrain axons contribute to their efficient decussation

Area of Science:

• Neuroscience
• Developmental Biology
• Axon Guidance

Background:

• Brain hemispheres communicate via axons crossing the central nervous system midline.
• Formation of these commissures is not fully understood, especially with repulsive signals at the ventral midline.
• Invertebrate studies suggest homologous axon interactions aid midline crossing, but this is unproven in vertebrates.

Purpose of the Study:

• To investigate if hindbrain commissural axons interact with contralateral counterparts at the ventral midline.
• To assess the significance of this reciprocal interaction in midline crossing.

Main Methods:

• Double anterograde axon labeling using lipophilic tracers.
• Utilizing unilateral mechanical barriers (solid, permeable, and axon-permeable) to block axon projections.
• Microscopic observation of axon growth cone apposition and fasciculation.

Main Results:

• Contralateral pioneer axons showed close apposition of growth cones at the midline.
• Fasciculation between contralateral axons was observed, persisting post-crossing.
• Unilateral blocking of axon projections significantly reduced contralateral midline crossing.
• Midline crossing was blocked by permeable barriers but not axon-permeable ones.

Conclusions:

• Reciprocal interactions between contralateral axon partners facilitate midline crossing in the hindbrain.
• These interactions involve physical contact and potentially diffusible molecules.
• Findings suggest a novel mechanism for commissure formation in vertebrates.