Sep. 8, 2004 Philadelphia, PA – Sometimes it is the extremes that point the way forward. Researchers at the University of Pennsylvania School of Medicine have induced nerve fibers – or axons – to grow at rates and lengths far exceeding what has been previously observed. To mimic extreme examples in nature and learn more about neuronal physiology, they have mechanically stretched axons at rates of eight millimeters per day, reaching lengths of up to ten centimeters without breaking. This new work has implications for spinal cord and nerve-damage therapy, since longer implantable axons are necessary for this type of repair.
In the present study, the team, led by Douglas H. Smith, MD, Professor of Neurosurgery and Director of the Center for Brain Injury and Repair, placed neurons from rat dorsal root ganglia (clusters of nerves just outside the spinal cord) on nutrient- filled plastic plates. Axons sprouted from the neurons on each plate and connected with neurons on the other plate. The plates were then slowly pulled apart over a series of days, aided by a precise computer-controlled motor system. "By rapid and continuous stretching, we end up with huge bundles of axons that are visible to the eye," says Smith. The axons started at an invisible 100 microns and have been stretched to 10 centimeters in less than two weeks. Smith and colleagues report their findings in the cover story of the September 8, 2004 issue of the Journal of Neuroscience.
"This type of stretch growth of axons is really a new perspective," says Smith. Despite the extreme growth in length, the axons substantially increased in diameter as well. Using electron microscopy, they confirmed this growth by identifying a fully formed internal skeleton and a full complement of cellular structures called organelles in the stretched axons. "Surprisingly, the axon appears to be invigorated by this extreme growth," says Smith. "It doesn't disconnect, but forms a completely normal-appearing internal structure."
These extreme rates of growth are not consistent with the current understanding of the limitations of axon growth. "Proteins necessary to sustain this growth are somehow correctly brought to sites along the axon faster than conceivable rates of transport," notes Smith. The team suggests two possible mechanisms to explain this: increasing transport to a very fast rate or making the necessary proteins at the site, proximal to the growing axons. Smith believes that this form of growth commonly occurs in nature. "For example, it can be inferred that axons in a blue whale's spine grow more than three centimeters a day and in a giraffe's neck at two centimeters a day at peak growth."
The team also found that they had to condition the axons to grow in an extreme way. "Although they can handle enormous growth, you can't just spring it on them," explains Bryan Pfister, PhD a post-doctoral fellow in Smith's lab and coauthor of the study. "If we ramp up the stretch rate too fast, the axons will snap." From this the team surmises that in nature animals must grow at a metered pace, which allows for constant feedback and conditioning.
It has been well established that axons initially grow out from neurons and follow a chemical stimulus to connect with another neuron. However, once the axon has reached its target a relatively unknown form of stretch-growth must ensue as the animal grows. Mechanical changes in the growing brain, spine, and other bones are the starting point for natural stretch-growth in axons. "We know that it's not tension on the neuron itself, but tension on the axon," says Smith. "It's deformation, a pulling on the axon." At this point, it is unclear what receptors and cell signaling pathways are involved to get the process started, but from this and previous studies the investigators do report that the signal is from a mechanical stimulus along the length of the axon as opposed to a chemical stimulus. "The stretch is coming from the whole body growing," explains Smith. "For example, the growing spine bones in the whale likely exert mechanical forces on the axons in the spinal cord."
The researchers conclude that this is a genetic program for growth that has been conserved throughout animal species, but just hasn't been studied in depth. By revealing the mechanisms of extreme-stretch growth, the team is currently applying this knowledge to develop nerve constructs to repair nerve and spinal cord damage. "To find that tension is actually good for your nerves for both growth and repair may not be such a long stretch," says Smith.
Penn colleagues on the paper are: Akira Iwata and David F. Meany. This research was funded by the National Institutes of Health.
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