Scaffold-free tissue formation in microgravity is a new method in regenerative medicine and an important topic in Space Medicine. 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 microgravity, physico-chemical and biological processes can proceed without potential distortions caused by the gravitational force experienced on Earth. In this environment, both sedimentation and buoyancy are negligible factors, facilitating the accurate observation of even minute changes in physical or biological systems, otherwise possibly overlain by gravity-related effects. Previous studies in the area of cell biology have indeed delivered evidence that cellular function and to some extent morphology can be altered by microgravity in comparison with conditions of 1 g on Earth. Thus, microgravity offers advantages in the area of tissue engineering, promoting three-dimensional (3D) self-assembly of various cell types (Ulbrich C et al., 2010; Grosse J et al. 2012). During this process, the cells are floating in the culture medium and behave as in a free-fall condition. Tissue formation under microgravity conditions is currently widely used and various studies investigating 3D growth in microgravity were performed, examining self-assembling of hepatocellular carcinoma cells, breast cancer cells, prostate cancer cells (Chang TT et al. 2009; Ingram M et al., 1997) or thyroid cancer cells (Grosse J et al., 2012; Grimm D et al., 2002; Warnke E et al., 2014; Svejgaard B et al., 2015; Pietsch J et al., 2013). As spaceflight opportunities are rare and expensive, 3D cultivation of cells on Earth can be achieved to some extent using devices constructed to simulate microgravity. In this review, Aleshcheva G and colleagues summarize outstanding results obtained worldwide by using 3 promising devices: Desktop 3D Random Positioning Machine (RPM), 2D Fast- Rotating Clinostat (FRC) or the Rotating Wall Vessel (RWV), (Grimm D et al., 2014). For example, when CD34-positive stem cells, derived from human umbilical cord blood supplemented with vascular endothelial growth factor (VEGF), were cultured in simulated microgravity on a RWV, they exhibited increased growth and differentiated into a vascular endothelial phenotype after day four of the experiment. This was demonstrated by an expression of endothelial markers such as VEGF receptor 2 (FLK1) and, more importantly, by the formation of 3D structures, which resembled vascular tubes (Chiu B et al., 2005). In addition, many different benign and malignant cell types start to form spheroids within only 1 day under continuous cultivation in simulated microgravity. Multi-cellular tumor spheroids mimic small metastases and areas of solid tumors in vivo and they can be used for pharmacological drug testing of chemotherapeutic drugs or for toxicological investigations, thus sparing animal experiments. Finally, cell culture in simulated microgravity find a promising application to grow clinically applicable cartilage tissues to be used in regenerative medicine for patients suffering from osteochondrosis or from injuries with cartilage damage. There is a tremendous clinical interest in neo-cartilage, which may be used for transplantation purposes and it appeared reasonable to analyze the various steps of cellular changes, which occur when chondrocytes transit from 2D to a 3D growth (Aleshcheva G et al., 2016). Research on these disruptive devices for simulated microgravity supports the preparation of spaceflights: in this context, it is possible to estimate the size of 3D structures, length and speed of their aggregation (Pietsch J et al., 2013). Device-specific comparisons and verification in real microgravity within a spaceflight have to be carried out to understand the underlying mechanisms and to compare the results obtained in space with those on Earth.
Fig. 1 from Aleshcheva G. et al., Scaffold-free Tissue Formation Under Real and Simulated Microgravity Conditions; Basic & Clinical Pharmacology & Toxicology, 2016, 119, 26–33. Ground-based facilities: (A) 2D Fast-rotating clinostat (FRC) as described by Eiermann et al. and used by Warnke et al.; (B) 3D Random Positioning Machine (RPM) developed by ADS (Airbus, Defense & Space, former Dutch Space, Leiden, NL) and described by van Loon; (C) Rotating Wall Vessel (RWV) developed by NASA, described by Klaus et al.
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