Visualization of individual axons in excised lamprey spinal cord by magnetic resonance microscopy

The direct visualization of axons within their native tissue environment by magnetic resonance (MR) microscopy is presented for the first time in the excised larval sea lamprey spinal cord. A home-built transverse radio frequency coil of 1.5 mm diameter was used in conjunction with a commercial 400 MHz MR microscopy system, implementing both 2-D and 3-D imaging pulse sequences. Images having nominal voxel sizes of 9x9x500 and 9x9x125 microm(3), respectively, in the lamprey spinal cord were obtained, resolving individual Mauthner and Müller axons. Furthermore, architectural changes associated with axonal degeneration were visualized in the spinal cord of one animal, excised 8 weeks after hemisection of the cord. Although the lamprey previously has not been the subject of MR microscopy investigations, these results demonstrate the method's potential for imaging this axon system, which is an important model of spinal cord injury and regeneration.