The impact of simulated and real microgravity on bone cells and mesenchymal stem cells.

How microgravity affects the biology of human cells and the formation of 3D cell cultures in real and simulated microgravity (r- and s-µg) is currently a hot topic in biomedicine. In r- and s-µg, various cell types were found to form 3D structures. This review will focus on the current knowledge of tissue engineering in space and on Earth using systems such as the random positioning machine (RPM), the 2D-clinostat, or the NASA-developed rotating wall vessel bioreactor (RWV) to create tissue from bone, tumor, and mesenchymal stem cells. To understand the development of 3D structures, in vitro experiments using s-µg devices can provide valuable information about modulations in signal-transduction, cell adhesion, or extracellular matrix induced by altered gravity conditions. These systems also facilitate the analysis of the impact of growth factors, hormones, or drugs on these tissue-like constructs. Progress has been made in bone tissue engineering using the RWV, and multicellular tumor spheroids (MCTS), formed in both r- and s-µg, have been reported and were analyzed in depth. Currently, these MCTS are available for drug testing and proteomic investigations. This review provides an overview of the influence of µg on the aforementioned cells and an outlook for future perspectives in tissue engineering.