Regenerating axons and their growth cones observed by scanning electron microscopy

Area of Science:

• Neuroscience
• Regenerative Medicine
• Cell Biology

Background:

• Schwann cells are crucial for peripheral nerve regeneration.
• Understanding axon behavior during regeneration is vital for therapeutic development.
• Previous methods limited direct visualization of regenerating axons in situ.

Purpose of the Study:

• To visualize the morphology and arrangement of regenerating axons in a predenervated nerve graft model.
• To investigate the interaction of regenerating axons with Schwann cell debris.
• To characterize the structure of growth cones during peripheral nerve regeneration.

Main Methods:

• A predenervated rat sciatic nerve segment was grafted.
• Schwann cells were ablated via freeze-thaw cycles.
• Tissue underwent KOH-collagenase digestion to remove non-cellular components.
• Regenerating axons were visualized using scanning electron microscopy.

Main Results:

• Regenerating axons (2-3 microns and <1 micron diameter) were observed navigating Schwann cell and myelin debris columns.
• Thick axons maintained a straight contour; thin axons exhibited intermittent swellings.
• Growth cones (2-5 microns) at axon tips displayed fusiform to polygonal shapes with limited filopodia.

Conclusions:

• This study provides a detailed ultrastructural view of regenerating axons in a nerve graft devoid of intact Schwann cells.
• Regenerating axons utilize Schwann cell debris as a substrate for guidance.
• The observed growth cone morphology offers insights into axonal navigation and extension mechanisms.