A group of three mice have returned alive from a record-breaking 91 days aboard the International Space Station, the longest space trip of any animal besides us. What they experienced can help us understand the effects of long-term space travel on the body. And that might help protect humans in space and back on Earth
AGE-OLD PROBLEM
Might our space mice have stumbled across a way to prevent thyroids from ageing? The thyroid produces hormones that control metabolism and sensitivity to other hormones, but ageing can slow thyroid function, leading to impaired cognition and weight gain.
Earth-bound mice have smaller follicles in the middle of their thyroids, and larger ones around the edge. But Maria Masini at the University of Genoa in Italy and her colleagues found that the follicles in the thyroids of the space travellers were all the same size. The space mice also had more receptors for a thyroid-stimulating hormone, suggesting they maintained a higher level of thyroid activity in adulthood (PLoS One, DOI: 10.1371/journal.pone.0035418).
So space flight seems to slow the ageing of the thyroid. Further research should reveal how follicle size contributes and could potentially be used to prevent thyroid ageing on Earth.
BLOOD TEST
Astronauts can suffer from what's called space flight anaemia, characterised by a 10 per cent drop in the number of red blood cells and volume of blood plasma.
To find out the cause, Bruno Berra at the University of Milan, Italy, and his colleagues analysed the level of thiobarbituric acid reactive substances (TBARS) in the mice's red blood cells.
Cells exposed to radiation produce chemicals called reactive oxygen species (ROS), which can damage fat, proteins and DNA. TBARS are a by-product of ROS and damage cell walls.
TBARS were significantly higher in the cells of mice that had been in space compared with those in mice back home on Earth (PLoS One, DOI: 10.1371/journal.pone.0032361).
Antioxidants can help undo some of the damage by donating an electron to each ROS molecule so that the chemical is no longer reactive. The team therefore suggests that an antioxidant-rich diet may help counter some of the adverse effects of radiation in space.
SPERM SHOCK
Reproductive organs are particularly vulnerable to the damaging effects of radiation in space, says Joseph Tash at the University of Kansas in Lawrence, especially if there is a solar flare. "It releases particles that cause a huge amount of radiation damage," he says.
Maria Masini at the University of Genoa in Italy found that the number of sperm present in the testes of mice on the space flight dropped by about 90 per cent, though the mice appeared to retain a limited ability to make mature sperm (PLoS One, DOI: 10.1371/journal.pone.0035418).
Radiation might not be the only factor affecting fertility - microgravity may also interfere with sperm production, says Tash. In mammals, the testes need to stay cool to function, which is why they are suspended away from the body in the scrotum. Take away Earth's gravity and suddenly the testes are a lot closer to the body - and potentially a lot warmer. Stress is also known to affect sperm production.
Although the effect of space travel on human fertility is still unclear, male astronauts should be offered the option to preserve healthy semen samples before they go into space, says Tash. Female fertility in space remains a mystery.
BODY BUILDING
Suspended in microgravity, astronauts don't get to stretch their legs as they would on Earth. As a result, they experience muscle loss. Studies on astronauts in space for just 5 to 11 days show that slow-twitch muscles, which help maintain posture, are particularly vulnerable. Astronauts can lose about 30 per cent of these muscle fibres. Fast-twitch muscles, which are involved in short bursts of power, appear relatively unaffected.
But if space-travelling mice are anything to go by, posture-maintaining muscles might adapt to the effects of space after a couple of weeks and stop deteriorating. Stefano Schiaffino at the Venetian Institute of Molecular Medicine in Padua, Italy, and his team found that mice on this mission lost a similar amount of slow-twitch muscle fibre as did mice on shorter missions (PLoS One, DOI: 10.1371/journal.pone.0033232)."It seems there are some initial changes due to stress and microgravity, but the situation stabilises," says Schiaffino.
This could be good news for astronauts preparing for longer missions, says Alan Hargens at the University of California, San Diego. Muscle is a flexible tissue, he says. "Maybe after a few weeks in space the muscle has adapted to microgravity."
While only slow-twitch muscles seemed to be significantly affected by space travel, both slow and fast-twitch muscles showed signs of stress. Fast-twitch muscles must somehow be protected, says Schiaffino. Once they have figured out how, the team hope to offer slow-twitch muscles similar protection.
BONING UP
Space can seriously weaken astronauts' bones. Weight-bearing bones are little used in microgravity and gradually break down without being rebuilt, as they would be on Earth.
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