Fish traveling aboard the International Space Station in 2014 experienced a near-immediate reduction in bone density upon encountering the microgravity environment of orbit. This is according to research published recently in Scientific Reports by a team of biologists at Tokyo Institute of Technology who conducted remote imaging experiments on newly-hatched medaka fish launched into space.
The general findings are concerning but not all that surprising. The dramatic effects of microgravity on bone density have been observed in human astronauts aboard the ISS, where bone deterioration begins after about 20 days in orbit in a process resembling the sort of osteoporosis more often associated with old age. The mechanisms behind this, however, are still being explored, both for the sake of long-term space travel and for treating osteoporosis here on Earth's surface. And so we have medaka fish, whose process of skeletogenesis is similar to our own.
"Under microgravity, there are several changes in the animal body, such as fluid shift, increase in blood pressure, and dizziness," Akira Kudo, the study's lead author, and colleagues explain. "In particular, bone mineral density is decreased under microgravity; but it is unclear how osteoblasts or osteoclasts respond early in orbit."
To better understand these biological effects of "microgravitational stress," there are two varieties of cell that need to be observed: osteoclasts and osteoblasts. The prior are responsible for breaking down bone tissue, a key role in repairing and maintaining bones, while the latter secrete the matrix used in bone formation.
The researchers were able to observe the activity of these cells live from a remote lab at Tsukuba Space Center using fluorescence microscopy. Basically, they created transgenic fish that would glow under different wavelengths of light. The fish were actually hatched at Baikonur Cosmodrome, the Soyuz rocket launch site in Kazakhstan, after being transported from Japan as eggs by Kudo and co.
The fish spent the first six weeks of their strange lives at the launch site before being embedded in a special gel for their voyage aboard Soyuz flight TMA-10M. They then spent the next two months being reared aboard the ISS. The first eight days of that stay were spent underneath a fluorescence microscope as researchers on Earth observed the fish's bone cells misbehaving in real-time. These observations were compared to an Earth-bound control group of medaka fish.
Gene expression markers for both varieties of bone cell increased significantly from the control group of medaka on Earth. On the very first day, this increase was seen in osteoclasts, where it persisted for up to eight days. In osteoblasts, the increase came four days following arrival at the ISS. The osteoclast increase was seen in two bone-specific genes: osterix and osteocalcin. Normally, these appear in fish development at different phases, with osterix appearing several days before osteocalcin. Here, they appeared simultaneously, offering a new clue into the fundamental mechanism behind spaceflight-related bone loss, and, perhaps, the widespread but poorly treated affliction of old-age, osteoporosis.
Of course, we're still just talking about lab-bred fish on a space station, so obviously more research is needed if we're to ever apply this knowledge to humans. But tucked into the paper, Kudo and his team tease at the possibility of their research opening up a whole new scientific field: "gravitational biology."