Differentiation of human mesenchymal stem cell spheroids under microgravity conditions

Stem cell-based therapies offer tremendous advantages for the treatment of orthopedic defects. One of the most widely studied stem cells are human mesenchymal stem cells (hMSC). HMSCs display a very high degree of plasticity and are found in virtually all organs, however, the bone marrow contains the highest density [1]. HMSCs serve as renewable source for mesenchymal [2] and potentially epithelial cells and have pluripotent ability of differentiating into several cell lineages, including osteoblasts, chondrocytes, adipocytes, skeletal and cardiac myocytes, endothelial cells, and neurons in vitro upon appropriate stimulation, and in vivo after transplantation [3]. Although the pathophysiologic functions of hMSCs are critically under investigation, the in vitro pluripotency of hMSC suggests a role in tissue regeneration, wound healing, or tissue repair after transplantation [4]. These characteristics make hMSCs good vehicles for autologous transplantation with the genuine benefits for tissue regeneration or cell-based gene therapies [5].HMSCs isolated from the bone marrow have several limitations, however, the most paramount is the limited number of cells easily obtainable. Limited cell number presents further constraints, particularly for autologous transplantation, as the number of cells per area on tissue culture plates requires multiple passaging and potential loss of pluripotency [6]. An additional hurdle is the length of time required to promote lineage-specific differentiation. For example, it is well established that 3 to 4？weeks of in vitro incubation of hMSC monolayers with osteogenic differentiation media is required for calcium accumulation and positive von Kossa staining [7]. Various matrices or scaffolds have been employed to promote differentiation such as porous gelatin, polyethylene terephthalate, or thermo-reversible gelatin polymer [8-10]. The scaffold is a very important component for promoting tissue differentiation because it represents a struct