This research story really opens a brand new window on how the spinal cord works, or doesn’t work. A team led by Frank Bradke at the Max Planck Institute for Neurobiology has found a way to see inside a living cord. It’s a “a leap forward in regeneration research,” says the Institute.
Ever see the spinal cord, maybe in a lab, or even a photo? It’s opaque, the color of uncooked calamari; there is no sign of any kind of nerve fibers or wiring. It’s hard to imagine just looking at it how the cord sends or receives information, or what happens when it’s damaged.
A conventional microscope cannot penetrate the surface -- the light just scatters. So, in order deconstruct and make sense of spinal cord tissue scientists have used various types of stain. Until recently, that meant that the tissue had to be removed from the animal, certainly a limitation.
Now, for the first time, the group from Germany has used a kind of solvent (tetrahydrofuran) to “clear” the tissue, thus providing a penetrating view into its basic cellular structures. This technique, along with a special florescence method called ultramicroscopy, developed by Hans Ulrich Dodt from the Technical University of Vienna, allows scientists to see into a living spinal cord and create three-dimensional images of individual regenerating axons. They were also able to characterize the response of astrocytes.
This news is from a study published in
Nature Medicine on Christmas day called “Three-dimensional imaging of the unsectioned adult spinal cord to assess axon regeneration and glial responses after injury.”
From a
press release from the Institute:
Spinal cord tissue is opaque due to the fact that the water and the proteins contained in it refract light differently. Thus, the scientists removed the water from a piece of tissue and replaced it by an emulsion that refracts light in exactly the same way as the proteins. This left them with a completely transparent piece of tissue. "It's the same effect as if you were to spread honey onto textured glass", Ali Ertürk, the study's first author adds. The opaque pane becomes crystal clear as soon as the honey has compensated for the surface irregularities.
From the paper abstract:
In large spinal cord segments, we imaged fluorescently labeled cells by 'ultramicroscopy' and two-photon microscopy without the need for histological sectioning. We found that more than a year after injury growth-competent axons regenerated abundantly through the injury site. A few growth-incompetent axons could also regenerate when they bypassed the lesion. Moreover, we accurately determined quantitative changes of glial cells after spinal cord injury. Thus, clearing CNS tissue enables an unambiguous evaluation of axon regeneration and glial reactions. Our clearing procedure also renders other organs transparent, which makes this approach useful for a large number of preclinical paradigms.
