Phenotypical plasticity of vascular smooth muscle cells-effect of in vitro and in vivo shear stress for tissue engineering of blood vessels.

Vascular smooth muscle cells (vSMCs) can switch between a contractile (differentiated) and a synthetic (dedifferentiated) phenotype. Synthetic, proliferative vSMCs are observed during embryogenesis, wound repair, and tissue engineering. The potential of isolated vSMCs to reverse this phenotypic modulation depends strictly on culture conditions. Previous studies have demonstrated that applied shear stress is an important signal for vSMC phenotype. The objective of this study was to determine whether applied shear stress is capable of triggering re-differentiation of vSMCs in tissue-engineered aortas. vSMCs were isolated from ovine arteries. Cells were cultured statically or exposed to two- (2D) and three-dimensional (3D) shear stress after seeding on a tubular matrix. For 3D in vivo testing, grafts were seeded additionally with endothelial cells and implanted in the descending aorta. Particular attention was paid to the expression pattern of vSMC markers, cell ultra-structure, matrix remodeling activity, and proliferative activity. Cultured vSMCs de-differentiated during static in vitro culture, but 2D and 3D in vitro shear stress promoted re-expression of vSMC markers. During in vivo culture, vSMCs progressed toward a fully differentiated phenotype. Cells were expressing markers of differentiated vSMCs and resembled a morphologically contractile vSMC phenotype. Matrix remodeling activity and proliferative activity decreased. This study demonstrates the phenotypic plasticity of vSMCs and their ability to return to a differentiated phenotype under shear stress conditions. These results are crucial for tissue engineering of blood vessels, because they indicate for the first time the in vitro potential to regain physiological functionality of isolated vSMCs.

Tissue engineering of aortic tissue: dire consequence of suboptimal elastic fiber synthesis in vivo.

OBJECTIVE: To study autologous tissue engineered blood vessels (TEBV) in the descending aorta of juvenile sheep. METHODS: Autologous vascular smooth muscle cells (vSMC) and endothelial cells were obtained from ovine carotid arteries. vSMC were seeded on bioresorbable scaffolds and dynamically cultured for 14 days. Following endothelialization an additional external ovine small intestinal submucosa wrapping was applied. Constructs were implanted in the descending aorta of juvenile sheep and removed after 1, 3, 6, 12 and 24 weeks for evaluation with histological, microscopical and biochemical techniques. RESULTS: Up to 3 months after implantation, grafts were fully patent, without any signs of dilatation, occlusion or intimal thickening. Scanning electron microscopy revealed a confluent luminal endothelial cell layer. In contrast, the 6 months graft displayed significant dilatation and partial thrombus formation. Histology displayed layered tissue formation resembling native aorta. Extracellular matrix (ECM) stains, immunostaining and Transmission Electron Microscopy (TEM) revealed alternating layers of vSMC and extracellular matrix consisting of collagen, elastin and glycosaminogycans. Compared to native aorta, the elastin content of the TE grafts was significantly reduced. CONCLUSION: In this study, we report for the first time, the implantation of a TEBV in the descending aorta in a large animal model. TEBV were fully functional for up to 3 months. At 6 months the graft remained functional but significantly dilated, most likely caused by an insufficient elastic fiber synthesis. Hence, future studies need to focus on the stimulation of elastin synthesis in TEBV.