As the space shuttle Endeavour prepares for its return to earth, researchers at Harvard Medical School eagerly await the data of an experiment they designed to test the effects of weightlessness on bone and muscle tissue.

The cells being used for the experiment, which is funded by the National Aeronautics and Space Administration (NASA) and the National Institutes of Health (NIH), are on board Endeavour.

"It was amazing to see [the launch] and know that you had an experiment on board," says Dr. Louis Gerstenfeld, one of the researchers. "This is an opportunity to study an experiment that cannot be done on earth."

Scientists already know that bone cells lose mass when they must live under the influence of the small amount of gravity in space, says Gerstenfeld who is associate professor of orthopedic surgery at Boston Children's Hospital. But the reasons behind the loss of mass are unknown, he says.

The experiment could have practical applications for life on earth, researchers say.

Dr. William J. Landis, associate professor oforthopedic surgery at Boston Children's Hospital,says he hopes the experiment will provide insightinto the causes for the loss of bone and muscletissue in patients who are bed-ridden or sufferingform osteoporosis. The bones of patients withosteoporosis become weak and break easily.

NASA, which has talked about building spacestations and even a permanent base on the moon,has also expressed interest in what the experimentsays about the effects of extended periods ofweightlessness on astronauts.

Landis says the Medical School is fortunate tohave the opportunity to use space as itslaboratory.

"We feel extremely gratified that we wereselected by NASA as one of three projects toparticipate," Landis says. "We feel really proudto represent the hospital and the Medical School."

Dr. Emily Morey-Holton, payload scientist atNASA's Ames Research Center in California,oversees all three science projects on Endeavour.The experiments on this mission are "the firstpeer-reviewed cell cultures by NIH that haveflown."

Morey-Holton says the experiment will give"insight into factors involved in bone formation."

"Landis and Gerstenfeld have one of the bestcharacterized bone cell culture systems forlooking at factors that influence cell maturationand mineralization, "Morey-Holton says.

Landis says the experiment used two sets ofskull bones taken from embryonic chicks. The bonesare first separated and purified, then placed incartridges that allow the cells to thrive byproviding them with essential nutrients.

One set of bones remains on earth as control.The other is sealed an placed in the mid-decklocker in the shuttle cargo bay. By comparing thetwo sets of cells, researchers can study theimpact of gravity on bone cells.

Landis and Gerstenfeld began working on theproject six years ago to address, in Gerstenfeld'sworlds, "why bone loses mineral mass on a cellularand biochemical level."

"The work has been absolutely exhausting interms of involvement of detail," Landis says."When the shuttle launched on Saturday, I think weall felt a terrific feeling of relief."

In spite of two launch delays for an experimentwhere time is critical, the shuttle successfullyleft earth on April 9. It is scheduled to returntoday.

The team, which is headed by Landis andGerstenfeld also includes Dr. Cameron Owen,Post-doctoral fellow, and technicians KarenHodgens and Diana Berkery