Critical Reviews in Oral Biology & Medicine, Vol 12, 252-261, Copyright © 2001 by International & American Associations for Dental Research
The impact of microgravity on bone metabolism in vitro and in vivo
P. M. Loomer
Division of Periodontology, School of Dentistry, University of California, San Francisco 94143-0650, USA. loomer@itsa.ucsf.edu
Exposure to microgravity has been associated with several physiological
changes in astronauts and cosmonauts, including an osteoporosis-like loss
of bone mass. In-flight measures used to counteract this, including
intensive daily exercise regimens, have been only partially successful in
reducing the bone loss and in the process have consumed valuable work time.
If this bone loss is to be minimized or, preferably, prevented, more
effective treatment strategies are required. This, however, requires a
greater understanding of the mechanisms through which bone metabolism is
affected by microgravity. Various research strategies have been used to
examine this problem, including in vitro studies using bone cells and in
vivo studies on humans and rats. These have been conducted both in flight
and on the ground, by strategies that produce weightlessness to mimic the
effects of microgravity. Overall, the majority of the studies have found
that marked decreases in gravitation loading result in the loss of bone
mass. The processes of bone formation and bone resorption become uncoupled,
with an initial transitory increase in resorption accompanied by a
prolonged decrease in formation. Loss of bone mass is not uniform
throughout the skeleton, but varies at different sites depending on the
type of bone and on the mechanical load received. It appears that the
skeletal response is a physiologic adaptation to the space environment
which, after long space flights or repeated shorter ones, could eventually
lead to significant reductions in the ability of the skeletal tissues to
withstand the forces of gravity and increased susceptibility to fracture.
