When a limb is immobilized in a cast, when an astronaut experiences zero gravity, or whenever a person lies down, the weight-bearing bones of the body such as those in the spine and leg, are relieved of their burden, a condition known as skeletal unloading. When skeletal unloading persists for several weeks, bones start to deteriorate: the number of bone cells decreases, movement into the bone of such minerals as calcium and phosphorous slows, and production of bone-cell precursors called osteoprogenitor cells diminishes. All these changes result in weakened, brittle bones prone to fracture.
While the bones of children may be able to eventually recover from such changes, adult bones have a harder time of it. Studies of Skylab and Salyut-6 long-term space missions (28-184 days) have found that, not only did astronauts on board the craft lose bone density during their missions, but five years later they had failed to recover to pre-launch bone density levels.
"The big problem that NASA is facing in their plans to send a manned flight to Mars is how to get people there and back without having their skeletons turn to matchsticks," Bikle says. "Yet discovering a way to stop bone loss from skeletal unloading will impact more than just a few astronauts. Anyone who is immobilized in any way for a long period of time can benefit."
To track the mechanism of bone loss, Bikle and his team devised a way of taking the weight off a rat's hindquarters while maintaining normal weight and movement in the animal's front legs. With hindquarters suspended by a freely moving line tethered to the cage lid (much like the pole that connects a bumper-car to the ceiling, allowing it to move anywhere on the floor), the rats moved around, groomed, ate and otherwise managed with just their forelimbs in the same way as their untethered companions during the 14-day trial period. The research team observed the ani
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Contact: Liese Greensfelder
lgreensfelder@pubaff.ucsf.edu
415-476-2557
University of California - San Francisco
23-Feb-2004