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Hypergravity and Hydrostatic Pressure Loading Resulted in Different Modeled Stress and Strain but the Same Osteoblast Functional Response.  J.S. Alwood¹, E.A. Almeida², R.K. Globus² and N.D. Searby². ¹Aeronautics and Astronautics, Stanford University, Stanford, CA, ²NASA Ames Research Center, Moffett Field, CA.

   Reduced mechanical stress during microgravity exposure is known to result in bone loss, yet the role gravity (acceleration) plays in mechanotransduction at the cellular level is unclear. In this study, we computationally and experimentally investigated the respective contribution of hydrostatic pressure (HP) and hypergravity to cellular mechanical stress to understand gravity’s role at the cellular level. A 1000-element axisymmetric finite element model of a subconfluent cell attached to a substrate was developed, and its elastic response was analyzed using ANSYS software. For experiments, MC3T3-E1 preosteoblast cells were plated on collagen coated dishes and allowed to adhere for 1 hour in α-MEM supplemented with 1% FBS, then centrifuged at 10-g (98 m/s²) or loaded in a pressure chamber at 1.2 kPa for 24 hours and compared with unloaded cells (1-g and 0.12 kPa). Cell number was determined by DNA quantity. For our experimental load values, the finite element model indicated maximum displacement and von Mises stress/strain due to HP was three orders of magnitude greater than corresponding acceleration values, yet hypergravity resulted in a unique stress/strain contour compared to HP. Maximum stress occurred at the peripheral cell-substrate contact under HP and below the nucleus during acceleration. Reduction of HP to a stress magnitude on the order of gravity resulted in maximum stress occurring simultaneously at the peripheral cell-substrate contact and below the nucleus. Experimentally, both hypergravity and HP increased cell number by 10-15% compared to unloaded controls, with no statistical difference in cell number between hypergravity and HP. In conclusion, modeling suggested the hypergravity elastic response was dominated by HP and increased cell numbers due to centrifugation are reproduced by HP. The effect of gravity on cellular mechanotransduction appeared to be less relevant than the external HP. (Supported by NASA GSRP grant NNA04CK68H and NASA DDF grant 02-02)