Experiencing weightlessness for prolonged periods of time may cause skin to become thinner, according to a very small study of mice that spent three months aboard the International Space Station.
Space missions carry a lot of health risks
Scientists already know that prolonged space missions carry health risks for astronauts. Prolonged weightlessness can increase a person’s risk of experiencing kidney stones, for instance. And previous studies have shown that astronauts are at risk for bone and muscle mass loss. But, unlike bones and muscles, the impact of reduced gravity on skin has never been thoroughly investigated in either animals or astronauts.
"To my knowledge, there is one ongoing study on the skin of a limited number of astronauts having spent a few months on the ISS, and one publication dealing with skin in mice having spent 10 days in microgravity," says Betty Nusgens, a connective tissue researcher at the University of Liege in Belgium, and a co-author of the study published today in Microgravity. And so far, only one astronaut, Thomas Reiter, has had his skin examined after a prolonged orbital mission. In short, studying how skin reacts in Space is pretty new.
In the study, six mice — known as "astromice" — were housed in a controlled environment on the International Space Station for three months. That may not seem like a long time, but three months for a mouse is equivalent to about nine to 10 years in humans. During the same period, another set of mice were housed on Earth, so that they could act as a comparison group. The study become much smaller, however, after three of the animals aboard the ISS died. The mice that died were frozen until they could be examined. Back on Earth, scientists killed three mice that corresponded to the ones that died aboard the ISS. They, too, were frozen and examined once all the animals were back on our planet.
significant reduction in the thickness of the dermis
In just three months, the astromice experienced a significant reduction in the thickness of the dermis, the middle layer of the skin, compared with the mice that stayed on Earth. This change resembled what happens to mammals in old age, the researchers say. In addition, genes involved in muscle contraction and development became more active in the muscle that underlies the skin — a type of muscle that has largely become functionless in humans over evolutionary time. Finally, a microscopic analysis showed that hairs that should have been in the final "resting" phase of the hair growth cycle remained in the "active" growth phase for far longer than normal.
"The effects on hair cycle were totally unexpected," Nusgens says. This type of change might seem minor, but the cells that enable hair growth are stem cells — cells that have the ability to turn into different types of specialized cells depending on where they are located in the body. If the stem cells involved in hair growth were affected by the space environment, then it’s possible that other stem cells, in other parts of the body, may have been as well. "This would deserve more attention in the future because of the critical significance of stem cells in almost any aspect of biology," Nusgens says.
"The effects on hair cycle were totally unexpected."
The fact that only three mice survived their stay in space dampens the study’s impact considerably. If more mice had been included in the study from the start, it would be a lot easier to argue that the results from the skin analysis aren’t just a fluke. "Even though we have multiplied the various analyses on several pieces of skin to increase the number of ‘technical’ replicates, the number of ‘biological’ replicates is unfortunately too low and does not fit with the statistical guidelines that are used in biology in ‘on Earth’ experiments," Nusgens says. And because the experiment is so unique, it’s "very unlikely" that researchers will be able to replicate it any time soon, she says.
That’s why the study should be regarded as a "warning signal to the space policy makers" who plan long-term missions. The study can’t be used to make any definitive pronouncements about the effects of reduced gravity on skin, but it can spur more research. Skin is the largest organ in the human body after all.