[Morphological study of early changes in rat bones during exposure to simulated weightlessness].

By histomorphometric methods tibial bones and lumbar vertebrae of rats exposed for 7 days to hypokinesia or head-down suspension were investigated. Both hypokinesia and suspension led to osteoporosis of the tibial metaphyseal spongiosa which was primarily induced by bone growth inhibition and probably by increased bone resorption. No signs of osteoporosis were seen in tibial diaphyses. In contrast to tibial bones, osteoporosis of the spongiosa of lumbar vertebrae was found only in hypokinetic rats. It is concluded that during exposure to simulated microgravity early signs of osteoporosis occur in the tibial spongiosa and that changes in the spongy matter of tubular bones and vertebrae are similar and systemic. It is suggested that an acute stress-reaction in response to simulated microgravity plays a certain role in the development of osteoporosis.