Scaffold-free Tissue Formation Under Real and Simulated Microgravity Conditions.

Scaffold-free tissue formation in microgravity is a new method in regenerative medicine and an important topic in Space Medicine. In this MiniReview, we focus on recent findings in the field of tissue engineering that were observed by exposing cells to real microgravity in space or to devices simulating to at least some extent microgravity conditions on Earth (ground-based facilities). Under both conditions - real and simulated microgravity - a part of the cultured cells of various populations detaches from the bottom of a culture flask. The cells form three-dimensional (3D) aggregates resembling the organs from which the cells have been derived. As spaceflights are rare and extremely expensive, cell culture under simulated microgravity allows more comprehensive and frequent studies on the scaffold-free 3D tissue formation in some aspects, as a number of publications have proven during the last two decades. In this MiniReview, we summarize data from our own studies and work from various researchers about tissue engineering of multi-cellular spheroids formed by cancer cells, tube formation by endothelial cells and cartilage formation by exposing the cells to ground-based facilities such as the 3D Random Positioning Machine (RPM), the 2D Fast-Rotating Clinostat (FRC) or the Rotating Wall Vessel (RWV). Subsequently, we investigated self-organization of 3D aggregates without scaffolds pursuing to enhance the frequency of 3D formation and to enlarge the size of the organ-like aggregates. The density of the monolayer exposed to real or simulated microgravity as well as the composition of the culture media revealed an impact on the results. Genomic and proteomic alterations were induced by simulated microgravity. Under microgravity conditions, adherent cells expressed other genes than cells grown in spheroids. In this MiniReview, the recent improvements in scaffold-free tissue formation are summarized and relationships between phenotypic and molecular appearance are highlighted.