Axonal microtubules: comparative anatomy in vertebrates, including man

Area of Science:

• Neuroscience
• Cell Biology
• Evolutionary Biology

Background:

• Microtubules are essential cytoskeletal components within axons, playing critical roles in neuronal structure and function.
• Understanding microtubule organization across different vertebrate species can provide insights into conserved cellular mechanisms.

Purpose of the Study:

• To quantify and compare microtubule density in myelinated and non-myelinated axons across various vertebrate classes.
• To investigate the relationship between axon size and microtubule packing.
• To determine if microtubule organization in peripheral axons is evolutionarily conserved.

Main Methods:

• Electron microscopy was used to assess microtubular density in myelinated fibers (myelin excluded) from 11 vertebrate species.
• Microtubule content was analyzed in non-myelinated axons of trout, lizard, finch, and man.
• Microtubule density was correlated with axon cross-sectional area.

Main Results:

• The average microtubular density across all studied species was 20.6 microtubules/µm², with low variability (CV=15.8%).
• In non-myelinated axons of trout, lizard, and finch, microtubule density decreased with increasing axon size.
• Microtubule packing in non-myelinated axons was similar across species, including comparisons with published data for cats and rats.

Conclusions:

• Microtubule packing in vertebrate peripheral axons is a highly conserved feature across evolution.
• Axonal size influences microtubule density, particularly in non-myelinated fibers.
• Conserved microtubule organization suggests fundamental importance for axonal integrity and function.