Recent studies have shown that human mesenchymal stem cells (hMSCs) exhibit significant developmental sensitivities to microenvironmental cues when grown in 3-D scaffolds (Grayson et al., 2006; Zhao et al., 2006; Zhao et al., 2007).   Biomaterial components and architecture, oxygen tension, as well as dynamic stresses were found to synergistically direct the fate of hMSCs in 3-D constructs.  Among these microenvironmental parameters, dynamic stresses and pressure gradients are crucial regulators of 3-D hMSC tissue development, which impart forces on the cells and affect cell behavior by transporting solutes and shaping the extracellular distribution of key signaling proteins.  Dynamic stresses also drive the small but essential fluid flows, called interstitial flow, through the extracellular matrix (ECM).  Interstitial flow not only helps to transport nutrients throughout the tissue, but also is an important morphoregulataor in tissue development, maintenance and remodeling (Rutkowski and Swartz, 2007).  Our previous study has found that hMSCs actively respond to shear stresses at a range of 1x10^-5 to 1x10^-4 Pa, orders of magnitude lower than those observed on planar surfaces (Zhao et al., 2007).  However, the role of interstitial flow on 3-D hMSC tissue formation requires further investigation.  In the current study, we examined the effects of interstitial flow on the developmental characteristics of hMSCs grown in 3-D poly (ethylene terephthalate) (PET) matricesby operating the modular perfusion bioreactor system in either transverse or parallel perfusion modes.  The two perfusion modes have different hydrodynamic characteristics but identical volumetric velocity of 0.2 mL/min, residence time, and initial cell seeding population.  Specifically, hMSCs under transverse perfusion were subjected to convective transport at a velocity of 6.5mm/s, whereas the majority of cells were dominated by diffusion under the parallel flow mode.  Results showed that the phenotype and tissue-morphogenesis patterns of hMSCs were significantly regulated by interstitial flow generated under the transverse perfusion mode at a time-dependent manner.  A 1.6 times higher proliferation rate, higher CFU-F formation, and stem cell gene expression at day 20 were observed in parallel perfusion in comparison with those under transverse perfusion.  The interstitial flow also upregulated osteogenic differentiation potential at day 20 as measured by the osteonectin gene expression and calcium deposition in the matrices.  In addition, ECM of hMSCs was patterned differently under the two perfusion modes, in which transverse perfusion directed a graded ECM distribution along the flow direction, as opposed a denser and more uniform spatial patterning of ECM on both sides of 3-D constructs dictated by parallel perfusion flow mode.  These results demonstrated the important role of interstitial flow as a key microenvironmental component that significantly modulates 3-D hMSC construct development in bioreactor systems.  

Reference:

Grayson WL, Zhao F, Izadpanah R, Bunnell B, Ma T.  Effects of hypoxia on human mesenchymal stem cell expansion and plasticity in 3D constructs.  Journal of cellular physiology, 2006;207: 331-339.

Rutkowski JM, Swartz MA.  A driving force for change: interstitial flow as a morphoregulator.  Trends in Cell Biology, 2007;17: 44-50.

Zhao F, Grayson WL, Ma T, Bunnell B, and Lu WW. Effects of hydroxyapatite in 3-D chitosan-gelatin polymer network on human mesenchymal stem cell construct development. Biomaterials, 2006;27:1859-1867.

Zhao F, Chella R and Ma T. Effects of shear stress on 3-D human mesenchymal stem cell construct development in a versatile perfusion bioreactor system: experiments and hydrodynamic modeling. Biotechnology and Bioengineering, 2007;96:584-595.