Human mesenchymal stem cell position within scaffolds influences cell fate during dynamic culture

Cell-based tissue engineering is limited by the size of cell-containing constructs that can be successfully cultured in vitro. This limit is largely a result of the slow diffusion of molecules such as oxygen into the interior of three-dimensional scaffolds in static culture. Bioreactor culture has been shown to overcome these limits. In this study we utilize a tubular perfusion system (TPS) bioreactor for the three-dimensional dynamic culture of human mesenchymal stem cells (hMSCs) in spherical alginate bead scaffolds. The goal of this study is to examine the effect of shear stress in the system and then quantify the proliferation and differentiation of hMSCs in different radial annuli of the scaffold. Shear stress was shown to have a temporal effect on hMSC osteoblastic differentiation with a strong correlation of shear stress, osteopontin, and bone morphogenic protein-2 occurring on day 21, and weaker correlation occurring at early timepoints. Further results revealed an approximate 2.5-fold increase in cell number in the inner annulus of TPS cultured constructs as compared to statically cultured constructs after 21 days. This result demonstrated a nutrient transfer limitation in static culture which can be mitigated by dynamic culture. A significant increase (P < 0.05) in mineralization in the inner and outer annuli of bioreactor cultured 4 mm scaffolds occurred on day 21 with 79 ± 29% and 53 ± 25% mineralization area, respectively, compared to 6 ± 4% and 19 ± 6% mineralization area, respectively, in inner and outer annuli of 4 mm statically cultured scaffolds. Surprising lower mineralization area was observed in 2 mm bioreactor cultured beads which had the highest levels of proliferation. These results may demonstrate a relationship between scaffold position and stem cell fate. In addition the decreased proliferation and matrix production in statically cultured scaffolds compared to bioreactor cultured constructs demonstrate the need for bioreactor systems and the effectiveness of the TPS bioreactor in promoting hMSC proliferation and differentiation in three-dimensional scaffolds.