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Effects of Microgravity on Human Mesenchymal Stem Cell Differentiation.  A.M. Frias1,2, J. Chamberlain1, C.Porada1, E.D. Zanjani1, J.M.S. Cabral2, G.Almeida‑Porada1   1Dept Animal Biotech, UNR, USA and 2 CEBQ, Instituto Superior Tйcnico, Portugal

   We have previously reported that Stro-1 is able to identify populations of human Mesenchymal Stem Cells (MSCs) in different organs. Here, we investigated whether microgravity (G) can alter the "stemness" of MSCs in such a way as to hinder the ability of these cell populations to differentiate into various cells/lineages. As the effects of G on bone loss are well known, we investigated whether alterations in MSC's differentiative pathways could lead to osteoporosis. Thus, we compared the ability of MSCs to differentiate into adipocytes and bone under regular or G culture conditions. We used Ca2+ content, Von Kossa staining and tetracycline incorporation to determine osteocytic and Oil-Red-O staining for adipocytic differentiation, respectively.  Adipocytes and osteocytes were obtained within 9 and 12 days, in regular cultures.  Under G conditions, adipocytic differentiation occurred in 12 days, similar to that of regular cultures. However, osteocytic differentiation under G conditions took 4 times longer than cultures grown under normal G. Since MSCs give rise to both osteoblasts and adipocytes, it is possible that an imbalance in these differentiative pathways may lead to impaired osteogenesis, contributing to osteoporosis. We also investigated whether G affected MSC differentiation into other cell lines. To this end, we grew MSCs for 5 days under G conditions and transplanted them into fetal sheep at a concentration of 1x106 cells/fetus and compared the in vivo differentiative potential of these cells with control sheep transplanted with MSCs grown in regular cultures. Flow cytometric evaluation of the recipients at 60 days post-transplant for the presence of human-derived blood cells revealed that although MSCs grown under G conditions generated multilineage blood cells, they did so at a lower level than their regular culture counterpart, suggesting that G impairs either MSCs engraftment or in vivo differentiation. (Supported by NASA NAG9-1340)