title: Cyclic flexure and laminar flow synergistically accelerate mesenchymal stem cell-mediated engineered tissue formation: Implications for engineered heart valve tissues.
abstract: Bone marrow-derived mesenchymal stem cells (BMSCs) are relatively accessible and exhibit a pluripotency suitable for cardiovascular applications such as tissue-engineered heart valves (TEHVs). Recently, Sutherland et al. [From stem cells to viable autologous semilunar heart valve. Circulation 2005; 111(21): 2783-91] demonstrated that BMSC-seeded TEHV can successfully function as pulmonary valve substitutes in juvenile sheep for at least 8 months. Toward determining appropriate mechanical stimuli for use in BMSC-seeded TEHV cultivation, we investigated the independent and coupled effects of two mechanical stimuli physiologically relevant to heart valves-cyclic flexure and laminar flow (i.e. fluid shear stress)-on BMSC-mediated tissue formation. BMSC isolated from juvenile sheep were expanded and seeded onto rectangular strips of nonwoven 50:50 blend poly(glycolic acid) (PGA) and poly(l-lactic acid) (PLLA) scaffolds. Following 4 days static culture, BMSC-seeded scaffolds were loaded into a novel flex-stretch-flow (FSF) bioreactor and incubated under static (n=12), cyclic flexure (n=12), laminar flow (avg. wall shear stress=1.1505 dyne/cm(2); n=12) and combined flex-flow (n=12) conditions for 1 (n=6) and 3 (n=6) weeks. By 3 weeks, the flex-flow group exhibited dramatically accelerated tissue formation compared with all other groups, including a 75% higher collagen content of 844+/-278 microg/g wet weight (p<0.05), and an effective stiffness (E) value of 948+/-233 kPa. Importantly, collagen and E values were not significantly different from values measured for vascular smooth muscle cell (SMC) -seeded scaffolds incubated under conditions of flexure alone [Engelmayr et al. The independent role of cyclic flexure in the early in vitro development of an engineered heart valve tissue. Biomaterials 2005; 26(2): 175-87], suggesting that BMSC-seeded TEHV can be optimized to yield results comparable to SMC-seeded TEHV. We thus demonstrated that cyclic flexure and laminar flow can synergistically accelerate BMSC-mediated tissue formation, providing a basis for the rational design of in vitro conditioning regimens for BMSC-seeded TEHV.
authors.affiliation: Engineering Tissue Mechanics Laboratory, Department of Bioengineering and the McGowan Institute for Regenerative Medicine, University of Pittsburgh, 100 Technology Drive, Suite 200, Pittsburgh, PA 15219, USA.
authors.forename: George C
authors.initials: GC
authors.lastname: Engelmayr
authors.forename: Virna L
authors.initials: VL
authors.lastname: Sales
authors.forename: John E
authors.initials: JE
authors.lastname: Mayer
authors.forename: Michael S
authors.initials: MS
authors.lastname: Sacks
mesh_terms: D000818:Animals; D001705:Bioprosthesis; D019149:Bioreactors; D018929:Cell Culture Techniques / Q000295:instrumentation / Q000379:methods; D049109:Cell Proliferation; D002478:Cells, Cultured; D042783:Endothelial Cells / Q000166:cytology* / Q000502:physiology*; D004867:Equipment Design; D006350:Heart Valve Prosthesis; D040542:Mechanotransduction, Cellular / Q000502:physiology; D059630:Mesenchymal Stem Cells / Q000166:cytology* / Q000502:physiology*; D044085:Microfluidics / Q000295:instrumentation / Q000379:methods; D011474:Prosthesis Design; D012756:Sheep; D013314:Stress, Mechanical; D023822:Tissue Engineering / Q000295:instrumentation* / Q000379:methods