UC San Diego

Physiological changes in response to microgravity are some of the main concerns that must be taken into account prior to a space mission. Bone remodeling may occur in spaceflight as a response to unloading of the lower limbs and head-ward fluid shifts. While unloading results in significant loss of bone mass and density in the lower limbs of animals exposed to microgravity, increased fluid flow to the head may elicit the opposite effect. In bones that normally do not bear weight such as the skull, it has been hypothesized that adaptation to microgravity can induce growth. This paper discusses the various physiological effects of microgravity on astronauts and new data on remodeling of the skull in space. Mice were sent to space on the 15-day STS-131 mission, and upon their return, characteristics of their calvaria and those of ground controls were evaluated by micro-computed tomography (micro-CT) and biomechanical analysis. Micro-CT analysis indicated significantly greater bone volume over total volume in the spaceflight group, 1.904 ± 0.842 mm³, compared to 1.758 ± 0.122 mm³ for that of the control group (p<0.05). Likely due to the short duration of spaceflight, there was no significant difference in the other parameters, cortical thickness and tissue mineral density. Micro-indentation was conducted on the calvaria to determine stiffness. Taken over several consistent points on each specimen, the elastic modulus in the spaceflight group was significantly greater, 10.5 ± 1.9 GPa, compared to 9.3 ± 2.1 GPa in the control group. From this, we concluded that exposure to microgravity causes adaptive growth in calvarial bones