Contrasting Effects of Vasculogenic Induction Upon Biaxial Bioreactor Stimulation of Mesenchymal Stem Cells and Endothelial Progenitor Cells Cocultures in Three-Dimensional Scaffolds under in Vitro and in Vivo Paradigms for Vascularized Bone Tissue Engineering

Abstract

Clinical translation of bone tissue engineering approaches for fracture repair has been hampered by inadequate vascularization required for maintaining cell survival, skeletal regeneration, and remodeling. The potential of vasculature formation within tissue-engineered grafts depends on various factors, including an appropriate choice of scaffold and its microarchitectural design for the support of tissue ingrowth and vessel infiltration, vasculogenic potential of cell types and mechanostimulation on cells to enhance cytokine expression. Here, we demonstrated the effect of biomechanical stimulation on vasculogenic and bone-forming capacity of umbilical-cord-blood endothelial progenitor cells (UCB-EPC) and human fetal bone marrow-derived mesenchymal stem cell (hfMSC) seeded within macroporous scaffolds and cocultured dynamically in a biaxial bioreactor. Dynamically cultured EPC/hfMSC constructs generated greater mineralization and calcium deposition consistently over 14 days of culture (1.7-fold on day 14; p<0.05). However, in vitro vessel formation was not observed as compared to an extensive EPC-vessel network formed under static culture on day 7. Subsequent subcutaneous implantations in NOD/SCID mice showed 1.4-fold higher human:mouse cell chimerism (p<0.001), with a more even cellular distribution throughout the dynamically cultured scaffolds. In addition, there was earlier evidence of vessel infiltration into the scaffold and a trend toward increased ectopic bone formation, suggesting improved efficacy and cellular survival through early vascularization upon biomechanical stimulation. The integrative use of bioreactor culture systems with macroporous scaffolds and cocultured osteogenic and vasculogenic cells promotes maturation of EPC/hfMSC-scaffold grafts necessary for vascularized bone tissue engineering applications.