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To advance the emerging field of bioengineered prevascularized tissues, we investigated factors that control primary vascular network formation in scaffold-free, high-density cell suspension-derived tissues. Fabricating primary vascular networks in a scaffold-free system requires endothelial cells (ECs) and extracellular matrix (ECM)-producing cells that act together to elaborate a permissive matrix. We report findings on the effects to vascular patterning induced by altering the ratio of human endothelial to human fibroblast cells. Analysis revealed that a 1:4 ratio of ECs to fibroblasts resulted in the synthesis of an ECM permissive for organization of primary vascular networks that recapitulated the pattern of primary vascular networks observed in vivo. Importantly this work highlighted the significance of tension in the organization of vascular networks in prevascularized tissues. To our knowledge our in vitro studies are the first to demonstrate the formation of two distinct vascular patterns in an initially homogenous culture system. Specifically, we demonstrate that within our constructs, vascular networks formed with distinct directional orientations that reflect self-assembly-mediated tension. Further, our studies demonstrate that treatment of prevascularized tissues with matrix-promoting factors such as transforming growth factor beta 1 (TGFβ1) increases tissue strength without altering vascular network patterning. Together, the ability to generate prevascularized tissues from human cells in scaffold-free systems and the ability to enhance the strength of the constructs with matrix-promoting factors represent advances to the potential translational utility of prevascularized tissues both as subcutaneous implants and in surgical scenarios requiring the application of tension to the tissue construct.

Work described herein characterizes tissues formed using scaffold-free, non-adherent systems and investigates their utility in modular approaches to tissue engineering. Immunofluorescence analysis revealed that all tissues formed using scaffold-free, non-adherent systems organize tissue cortical cytoskeletons that appear to be under tension. Tension in these tissues was also evident when modules (spheroids) were used to generate larger tissues. Real-time analysis of spheroid fusion in unconstrained systems illustrated modular motion that is compatible with alterations in tensions, due to the process of disassembly/reassembly of the cortical cytoskeletons required for module fusion. Additionally, tissues generated from modules placed within constrained linear molds, which restrict modular motion, deformed upon release from molds. That tissue deformation is due in full or in part to imbalanced cortical actin cytoskeleton tensions resulting from the constraints imposed by mold systems is suggested from our finding that treatment of forming tissues with Y-27632, a selective inhibitor of ROCK phosphorylation, reduced tissue deformation. Our studies suggest that the deformation of scaffold-free tissues due to tensions mediated via the tissue cortical cytoskeleton represents a major and underappreciated challenge to modular tissue engineering.

Tracheal injury is a rare but complex problem. Primary tracheal reconstructions are commonly performed, but complications such as tension and inadequate vascular supply limit the length of surgical resection. The objective of the present study was to determine whether a hydrated, decellularized porcine tracheal extracellular matrix showed the potential to serve as a functional tracheal replacement graft. Porcine tracheas were decellularized and evaluated to characterize their biochemical composition and biomechanical behavior. Hydrated decellularized tracheal matrix (HDTM) grafts (>5 cm) were implanted heterotopically beneath the strap muscle and wrapped in the omentum in a canine model for 2 and 8 weeks followed by histologic and mechanical analysis. HDTM patches (2 x 3 cm) were also used in a patch tracheoplasty model. The repair site was evaluated bronchoscopically and radiographically, and the grafts were analyzed by histologic methods to evaluate epithelialization and persistence of the cartilage rings. The present study showed that HDTM maintains mechanical characteristics necessary for function under physiologic loading conditions even after 8 weeks of heterotopic implantation. After orthotopic implantation, the grafts were shown to support development of a columnar, pseudostratified, ciliated epithelium, but the cartilage structures showed histologic evidence of degradation and limited new cartilage formation. The results of the study showed tracheal ECM scaffolds support the formation of site-specific epithelium and provide sufficient mechanical integrity withstand physiologic pressures in the short-term. However, for long-term success, it appears that pre-implantation to allow vascularization or preseeding of the graft with chondrocytes will be necessary., (Copyright 2010 Elsevier Ltd. All rights reserved.)