Science and technology | Space physiology

Some skin off their noses

A bid to discover microgravity's long-term effects on skin turns up intriguing results

TO SHED the surly bonds of Earth and dwell in microgravity is also to sign up for a puffy appearance; the body's fluids redistribute themselves quite differently in the weightless environment of space. But research published this week suggests that astronauts may be subject to more deleterious effects.

Much has been made of the physiological changes that microgravity can induce in astronauts. The loss of bone density and muscle mass that occurs when the body is free from its fight with gravity has been a constant concern since the earliest days of space outposts. Astronauts now have strict regimens of exercise aboard the International Space Station (ISS), strapped down to treadmills and the like.

However, there has been a paucity of work to understand what effects space travel might have on the skin—the body's largest organ and in many ways its first line of defence against pathogens. Back in the 1990s, when "space station" meant the Russian one, Mir, a study of 19 crew members suggested that the most common injuries were minor skin lesions, and many complained of itchiness and dryness. Such complaints could be easily ascribed to the station's low humidity.

Little more thought was given to the issue until Thomas Reiter, a German astronaut, was subjected to ultrasound skin tests before and after a six-month stint on the ISS. Those showed significant changes in his skin elasticity, but the result added just one data point to earlier, anecdotal reports.

Results published this week add a smidgen of intriguing data to the debate. They come courtesy of six brave mice who endured a 91-day trip to the ISS as part of the "Mice Drawer System" (pictured), a self-contained habitat for the creatures that went up with space shuttle Discovery in 2008. An otherwise identical habitat was maintained on Earth.

A wide array of research groups have spent the intervening time examining the divvied-up remains of the mice (three of the "astromice" died during the experiment—a fact that is itself not a great advertisement for space travel—and the remaining ones were sacrificed afterward).

Betty Nusgens, of the University of Liège, in Belgium, and her colleagues were granted access to skin samples from both sets of mice. With just three astromice to study and six with which to compare them, it is difficult to draw solid conclusions from what they found. But what they found, published this week in the journal npj Microgravity, suggests that the astromice suffered profound changes to their skin during their sojourn.

The team found that the skin of the astro-mice was 15% thinner than that of their grounded counterparts. What is more, the hair follicles of all the astromice were in what is known as the anagen, or growing, phase. Five of the six Earthbound mice showed follicles in the telogen, or resting, phase. Because hair growth tends to thicken the skin temporarily, it is possible that the astromice's skin would otherwise have been even thinner.

The team went on to examine a number of genes known to be implicated in skin and hair regulation, finding differences in the relative activity of more than 400 genes across the two groups. But in the absence of more data, these differences are largely in the statistical noise. Moreover, as Dr Nusgens says, "mice are not humans": they have a layer of muscle beneath the skin, for example, that has been lost in the human lineage. What would happen to a human aloft for an analagous period of time (a few years, say) is still very much an open question.

Results should be coming soon from Skin-B, an experiment being run aboard the ISS that aims to quantify several different parameters of the skin of the station's inhabitants. That will help, but it is the much longer-term effects of spaceflight that are a concern amid ongoing discussions of trips to Mars and beyond. The more that researchers look for detrimental physiological changes in animals and humans that have been to space, the more they seem to find. Many more mice may yet have to give their lives before the full effects of surly-bond-shedding are known.

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