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Irregular geometries in normal unmyelinated axons: a 3D serial EM analysis
M M Greenberg1, C Leitao, J Trogadis
1Playfair Neuroscience Unit, Toronto Western Hospital, Ontario, Canada.
Journal of Neurocytology
|December 1, 1990
Summary
Unmyelinated axons are not straight cylinders but have irregular, varicose shapes due to organelles. This finding challenges traditional analysis and suggests new predictors for axonal properties like conduction velocity.
Area of Science:
- Neuroscience
- Cell Biology
- Anatomy
Background:
- Axons are traditionally modeled as simple cylinders for analysis.
- Standard morphometric analysis relies on cross-sectional diameter to predict conduction velocity.
Purpose of the Study:
- To investigate the three-dimensional (3D) geometry of unmyelinated axons.
- To determine if axons conform to a simple cylindrical model.
- To reassess methods for predicting axonal electrical properties.
Main Methods:
- Computer-assisted serial electron microscopy was used.
- Examined the 3D structure of 98 central and peripheral unmyelinated axons.
- Reconstructed axonal shapes from serial sections.
Main Results:
- Virtually all examined unmyelinated axons exhibited irregular, varicose shapes.
- Varicosities contained membranous organelles, including mitochondria, and cytoplasm.
- Mitochondria within varicosities contacted microtubules.
Conclusions:
- Unmyelinated axons are fundamentally varicose structures, not simple cylinders.
- Axonal caliber is dynamic in both space and time.
- Total microtubule count may be a better predictor of conduction velocity than single-section diameter.
- Findings question the validity of 2D morphometric analysis for predicting electrical properties and offer insights into axonal pathology.