EFFECTS OF GRAVITATIONAL CHANGES ON THE BONE SYSTEM IN-VITRO AND IN-VIVO

Spaceflight data obtained on bone cells, rodents, and humans are begin ning to shed light on the importance of gravitational loading on the s keletal system. The space environment is a relevant model to explore t he bone cell response to minimal strains. However, whether there is a direct effect of gravity on the cell rather than changes related to la ck of convection forces in cell cultures performed in microgravity is unknown. In vitro studies carried out using osteoblastic cell cultures in space show changes in cell shape, suggesting that cell attachment structures as well as cytoskeleton reorganization might be involved. V aluable information is expected from in vitro models of an increase or decrease in mechanical stress in order to identify the different path ways of mechanoreception and mechanotransduction in the osteoblastic l ineage. Results obtained from both humans and rodents after spacefligh ts indicated that bone mass changes are site specific rather than even ly distributed throughout the skeleton, thus emphasizing the need to p erform measurements at different bone sites: weight-and non-weight-bea ring bones, and cancellous and cortical envelopes. Bone mass measureme nts and biochemical parameters of bone remodeling are currently under evaluation in cosmonauts. Histomorphometric studies of bones from rats after space missions of various periods provided the time course of t he cancellous bone cellular events: transient increase in resorption a nd sustained decrease in bone formation. The underlying bone loss occu rred first in weightbearing banes and later in less weight-bearing bon es. During the postflight period, time required to recover the lost bo ne was greater than the mission length. Thus, the postflight period de serves more attention than it is currently receiving. On earth, the ra t tail-suspension model is currently used to mimick spaceflight-induce d bone loss. Data from the model confirmed the impairment of osteoblas tic activity and showed an alteration in osteoblast recruitment with s keletal unloading. However, this model needs to be further validated, (C) 1998 by Elsevier Science Inc. All rights reserved.