Imagine spooking a tortoise, by making a loud noise or knocking on its shell. The startled reptile will hide in the shell, pulling back its head and legs. Where the tortoise’s scaly legs used to be, there will now be an empty space right by the opening of the shell. The retracted leg is inside the shell.
Nerves of the spinal cord behave in a similar way. If a nerve fiber is cut, it degrades and pulls back from the site of injury, leaving behind its now-empty “shell.” This “shell” consists of a fatty substance called myelin, which is wrapped around nerves to insulate them and to help them signal faster. But what goes on in this empty space in the shell after the injured nerve retracts? Scientists have discovered that, very quickly after a spinal cord injury, the leftover myelin shell, called the “dieback zone,” is invaded by immune cells.
Immune cells play a huge role in the aftermath of a spinal cord injury. They clean up debris from the crushed tissue, help the wound heal, and, in some animals, affect regeneration of the nerve. In a recent study, scientists have observed, for the first time, immune cells within the myelin shell of a well-characterized nerve fiber in a goldfish.
Looking into the nerve
The nerve that the scientists chose to study is called the Mauthner cell. This cell is a thick nerve fiber that runs from the brain throughout the entire spinal cord. This nerve helps fish and tadpoles to escape from danger. To see this nerve in action, try to catch a tadpole the next time you are at a lake – the tiny amphibian will scurry away so quickly you won’t even see it disappear.
Because of its large size and important role in behavior, the Mauthner cell nerve fiber has been studied extensively for several decades. Now, scientists have managed to catch on camera its interactions with immune cells after an injury.
A team of scientists at Williams College and Marine Biological Laboratory at Woods Hole, Massachusetts, studied what goes on inside the Mauthner cell and its myelin sheath as the nerve pulled back from the injury site, leaving behind its shell of myelin. The researchers cut the nerve in the goldfish spinal cord, waited from one to 41 hours, and collected the spinal cord. They stained the tissue with a dye that colors cells purple and then examined it under a transmission electron microscope. With this technique, the research team got a good look inside the injured Mauthner cells.
As soon as one hour after the injury, the researchers saw that there were many cells at the wound site where the nerve had been cut. At 12 hours after the injury, there were cells within the myelin “dieback zone” and also on the tip of the injured, retracting Mauthner cell nerve fiber. The researchers then set out to identify those cells.
It can be hard to identify cells just by looking at their pictures. But because transmission electron microscopy produces very fine-detailed images, the researchers were able to figure out the identities of some cells from their shapes. In particular, they labeled some of these cells as microglia.
Microglia are a special subset of macrophages, which are immune cells that patrol the body looking for invading pathogens. But unlike the broad population of macrophages, microglia only exist in the central nervous system, which consists of the brain and the spinal cord
If the central nervous system is injured, microglia are critical for cleaning up post-injury debris. In this study, the scientists saw that some of the microglia cells were even carrying bits of myelin, which had probably broken off the Mauthner cell fiber when it was injured.
Granulocytes were another type of immune cell that the scientists saw near the injury site. This type of white blood cell contains small granules, by which the researchers were able to identify these cells under the microscope. Like microglia, granulocytes were found not just at the wound site, but in the leftover myelin and even on the tip of the retracting nerve.
The scientists identified that granulocytes at the site of the spinal cord injury were neutrophil-like. They based this conclusion off other studies that had described what neutrophil-like granulocytes look like in fish.
A double-edged sword
Unlike humans, some fish and amphibian species are really good at regenerating their body parts, including the spinal cord. Scientists don’t know why that is, but suspect that this incredible ability at least partially has to do with the immune system.
Some immune cells promote healing and regrowth, while others tell the injured nerve to close off and lose the connection with the rest of the body. In species that can repair their spinal cord nerve, the pro-regenerative effects of the immune system clearly outweigh the anti-regenerative ones. Goldfish, which were used in this study, belong to those species.
We don’t know yet what makes immune cells decide if the injured nerve fiber should regenerate or not. But now we have snapshots of immune cells inside the “dieback zone,” interacting with an injured nerve and its myelin sheath. These snapshots can help scientists decipher what specific immune cells do during a nerve injury and continue to unravel the mystery of spinal cord regeneration, in the hopes that, one day, humans can also regenerate their spinal cords.
Journal article: Koganti, L., Liu, J., DeMajewski, A., Agostini, M. A., Wong, T. W., Faber, D. S., & Zottoli, S. J. (2020). Invasion of microglia/macrophages and granulocytes into the Mauthner axon myelin sheath following spinal cord injury of the adult goldfish, Carassius auratus. Journal of morphology, 281(1), 135-152. https://onlinelibrary.wiley.com/doi/full/10.1002/jmor.21086
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