During the past four years of investigation at the University of Pennsylvania, under the sponsorship of NASA, our objective has been to determine the mechanisms responsible for changes in osteoblast behavior under simulated microgravity conditions by employing experimental and numerical/analytical methods. Our hypothesis has been that simulated microgravity affects cell behavior through (1) buoyancy induced motions of intracellular structures, (2) simultaneous modulation of the fluid shear at the plasma membrane, and (3) alteration in the intra- and extracellular mass-transport processes. We have conducted in vitro experiments with osteoblasts in the NASA designed rotating wall vessels (RWVs HARV, STLV) in order to assess the effects of both gravity and hydrodynamic shear stresses on the function of bone cells, focusing on changes in growth rate, morphology, metabolism, and the production of functionally important proteins. We have successfully grown three-dimensional rodent bone-like tissue structures employing surface-modified, hollow bioactive glass beads as microcarriers in the HARV. We have examined the morphology of such 3-D rat marrow stromal cell cultures and the expression of bone-related markers. Evaluation by scanning electron microscopy has revealed the presence of 3-D multicellular aggregates consisting of multiple cell-covered beads bridged together. Mineralized extracellular matrix has been observed in these aggregates. Using immunocytochemistry, confocal scanning microscopy, and antibodies for osteopontin and collagen type I, we have found that these bone-related proteins were strongly expressed. Simultaneously, we have modeled our observations using mathematical formulations and numerical schemes with HARV and STLV geometries. The governing equations include the conservation laws for the fluid and Newton's law for the microcarriers. We have employed the Runge-Kutta-Nystrom method to solve the equations that govern the motion of the microcarrier particles. For the single particle problem the liquid motion corresponds to solid body rotation.For any particles system, the liquid motion is suitably modified to reflect the volume fraction of the particles. Collisions between microcarriers and with the walls of the vessel have been accounted for by introducing a fictitious external short range contact force on the colliding particles normal to the contact area. This contact force is activated when the gap between two approaching particles or the particle and the wall is less than a prescribed critical distance. The corresponding unsteady, two dimensional mass transfer problem has been solved using ADI finite difference procedures. The resulting linear sets of algebraic equations were then solved by the Thomas algorithm and an algorithm employing the Sherman-Morrison formula. In this way we evaluated the microcarrier trajectories, the local and average shear stresses at the microcarrier surface, and the local and global mass transport of oxygen and the nutrients in the RWV. Our numerical predictions of trajectories have been very well confirmed by photographs taken in a rotating frame during experimentation. Predictions of mass transport rates have, on an average, been noted to be in the range observed in experiments. In our future studies, we plan to use this information to not only formulate optimal culture conditions which expeditiously produce three-dimensional bone-like tissue matrix but make inroads into the fundamental understanding of signaling and mechanotransduction mechanisms at play in bone cells subjected to shear in micro/normal gravitational fields. With this information, we envision that the bioreactor technology can be employed for the in vitro synthesis of bone tissue using cells aspirated from the patient's marrow, an appealing idea to correct skeletal disorders such as osteoporosis, fracture healing, and craniofacial abnormalities. Studies such as these will also significantly impact the astronaut bone loss problem, an unresolved and critical issue for extended manned spaceflight and the establishment of space colonies.