Scientists have found skin abnormalities among three mice that recently spent 91 days aboard the International Space Station. The results bolster mounting evidence suggesting animals (humans included) aren't built for life in space.

After returning from space, scientists dissected the three mice and distributed their tissue and organs among different teams of researchers. Scientists studying the animals' skin found the dermis, a subsurface layer of the skin, to be 15 percent thinner than a control group's on Earth.

Collagen, which is the main structural protein in connective tissue, makes up a significant portion of the dermis. Young mice (and people) are constantly producing new collagen, but that production slows with age. Further analysis of the space-aged mice skin showed that microgravity boosted both the destruction and production of collagen. But collagen production couldn't keep pace.

"The final balance [of collagen] is negative, meaning that the degradation is probably more active than the synthesis," Betty Nusgens, a biologist with the University of Liege in Belgium, told Space.com. "So this is a progressive loss of collagen in the skin, like in aging people."

Researchers aren't sure exactly why microgravity has this effect on skin regeneration, but it holds with previous studies of human skin in space.

"We know that the cells that make collagen in the skin -- called fibroblasts -- these cells are responsive to mechanical stress and to stress relaxation, so [the] cell can respond to the loss of gravity in space," Nusgens added. "Knowing that, we are going to send fibroblasts alone [to] the [International Space Station] next December."

Researchers also found that the mice's hair growth was altered by microgravity. Hair typically grows in spurts, but space hair growth is all acceleration.

"Hair grows and then stops growing -- they rest and then they fall, this is the cycle," Nusgens told BBC News. "The cycle of the hair [in the space mice] was disturbed."

In space, there was no time for rest. Hair just kept on growing. As with the thinning skin, Nusgens and her colleagues don't have an explanation.

Skin layers and hair follicles are just a couple of the body parts and tissue samples being investigated by researchers back on Earth. More information on the health effects of life in space is expected soon. Researchers at NASA are anxious to better understand how to better protect astronauts from the health risks of lengthy stays in space.