Experiments in space have given researchers insights into how regeneration works and how space flight could affect human biology, and it starts with the planarian worm.
What are planaria?
Planaria are flat worms that live in freshwater. Most species are light brown and they crawl around on rocks and are generally unremarkable parts of our wetland ecosystem. That is, until you realize they can regenerate any part of their body at any time and are basically immortal.
You can cut off the head of a planarian worm and, lo and behold, a few days later the leftover, headless chunk has grown an entirely new head, complete with remedial brain. Not only that, but they don’t show signs of aging. Because of their regenerative capacity, they can pretty much live forever unless they are eaten or damaged to the point where there is not enough body left from which they could regenerate.
That makes them fascinating for biologists, not only to understand the basic biology of how they are able to regenerate, but also because it’s a holy grail for human medicine and has far-reaching implications into human regeneration and aging. It even unlocks secrets of cancer therapies.
Why send these worms to space?
The group in charge of this mission, led by Michael Levin from Tufts University in Boston, has spent years working on the hypothesis that electrical signalling between cells are the key to planaria’s regenerative capacity.
In space, the physics gets all weird, and that could impact the worms’ development. And, that is exactly what happened.
“In space you have the opportunity to study not only loss of the gravitational field, but also loss of the geomagnetic field. So we were really curious to see how that type of environment and in general the effects of space travel would impact the ability to regenerate,” Levin said.
The planarian worms lived on the International Space Station where one grew a second head. (NASA)
“Not only to learn about important aspects of space medicine for humans and animals who are going into space eventually, but also to understand basic aspects of cell decision-making and pattern formation that would be useful for regenerative medicine and basic science here on earth.”
So they sent up self-contained vials of 15 worms to the International Space Station where the worms lived for five weeks in zero-gravity, and then had them shipped back to Earth to compare the space worms with worms that were treated the exact same here on Earth.
How much did space travel affect the worms?
A lot. It affected both their ability to regenerate and their behaviour, two features very important to planaria’s survival.
It changed their ability to regenerate so much that when they came home from their space flight, one of the worms had two heads.
“Because we were doing regeneration, we had cut some of these worms before they went up. Now, the important thing to realize is that planarian regeneration in the wild has 100 per cent fidelity, so they always regenerate correctly,” Levin said. “We’ve cut many tens of thousands of worms in our lab and when you cut them in plain water without treating them in any special way, they never ever make a mistake and make a head at the tail end.
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“So, in a sample in a tube of 15 worms, we found that one of them came back as double headed, so it literally had its normal head, and then an extra head at its posterior, where the tail should be.”
There’s no indication yet what caused this two-headed worm, whether it was the lack of gravity, some kind of change to the electrical signalling ability of the worm, or something even more mysterious. But it doesn’t bode well for future human space flight and things like having babies in space.
How do you determine the impact of space on worm behaviour?
It turns out you can train planaria on all kinds of behaviours. Levin’s lab even developed the first worm testing and training station. The worms are actually quite smart considering their simplicity, and, amazingly, you can train memories that are remembered even after decapitating them and having their heads grow back. The new head remembers what the old head learned.
What they looked at here in terms of worm behaviour is their preference for light. The normal earth-bound worms preferred the dark 100 per cent of the time, as it’s usually vital for their survival. The worms that experienced space flight weren’t so scared of the light. In fact they spent only about 70 per cent of their time in the dark, and were fine venturing out the rest of the time.
Now, what the significance of this finding is is still unclear, and what other behaviours were affected by space flight are still being tested, but it’s an interesting start into more research.
How does this help us understand the impact of space flight on humans?
Obviously, we don’t regenerate like planaria and grow new heads on demand, but we do quite a bit of regeneration on a much smaller scale. Our bodies regenerate tissues like skin and bone all the time. We have always known that space flight can increase the risk of cancer because of the mutations from all the radiation that is present outside the Earth’s atmosphere.
This study adds to our worries that it wasn’t mutation from radiation that caused these effects in the worms, it was something subtler. The findings open the door to new ways of looking at how space affects the cell.
No one ever expected space flight to affect the worms in this way, and there’s no reason why humans aren’t bound by the same biology, and could therefore could see similar effects. The task now is to figure out where to look for those effects in the human body.