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Comparing Hypergravity and Hydrostatic Pressure Loading on Osteoblast Cells.   J.S. Alwood1, R.K. Globus2, N.D. Searby2. 1Aeronautics and Astronautics, Stanford University, Stanford, CA, 2NASA Ames Research Center, Moffett Field, CA.

   How does gravity affect the mechanical and biological response of bone cells? Previous work from our lab showed that hypergravity decreases cytoskeleton height and increases prostaglandin E2 production in osteoblast cells. Hypergravity increases the amount of hydrostatic pressure that the cell experiences. We propose to develop and test models of osteoblast morphological changes under hypergravity and hydrostatic pressure loading, with the aim of determining the stress and strain fields within the cell. We then aim to test our model using biological assays of osteoblasts under the two different types of loading conditions. We propose the following models to evaluate the forces exerted on cellular constituents (nucleus, structural filaments, cytoskeleton and contacts with the extracellular matrix). Force diagrams characterize the different loading conditions. Hypergravity loads the cell surface and contacts with the extracellular matrix via a combination of shear and compressive forces. Hypergravity also directly affects the cell’s internal constituents by making them heavier. Increased hydrostatic pressure also provides shear and compressive forces to the cell surface, although the shear magnitude is less than in hypergravity. Unlike hypergravity, the cell’s internal constituents do not become heavier under hydrostatic loading and will experience different stresses and strains. To test the models empirically, an apparatus was built to apply increased hydrostatic pressure to cell cultures. Two different pressure chambers allow real-time microscopy or proteomic/biochemical analyses of cells. Results from the models and experimental analyses will help determine the contribution of hydrostatic pressure to hypergravity and provide insight into how mechanical forces influence cell behavior. (Supported by the NASA Graduate Student Research Program grant NNA04CK68H and NASA DDF grant 02-02)