An embryonic development-inspired, cell-based and scaffold-free method to engineer single fibers for tendon

With the primary function of transmitting force from muscle to bone, tendon is an essential component of human motion. With increased activity in an aging population, tendon injuries are on the rise. Such injuries can severely limit biomechanical function. The combination of a poorly developed vascular network and a high mechanical stress environment limit tendon healing potential. The reduced intrinsic healing capacity necessitates surgical augmentation or complete replacement to restore function. However, current clinical treatment (e.g., natural tissue repairs and synthetic grafts) have been associated with numerous problems, including mechanical inferiority, limited supply of auto/allografts, and donor site morbidity. As an alternative, engineered tissues for tendon replacement would significantly improve function and quality of life for patients. To date, current tissue engineering approaches have not resulted in a clinically accepted tendon tissue replacement. Looking toward developmental biology may offer strategies to more effectively guide tissue engineering solutions to enhance and accelerate in vitro tendon tissue formation.;The overall goal of this thesis was to create and characterize a novel tendon tissue engineering approach guided by embryonic tendon development. Using this approach we sought to provide a foundation for recreating the fiber-level structure and mechanical properties required for autologous cell-based tendon replacements. To accomplish this objective, my doctoral thesis was comprised of three specific aims. The first aim focused on developing and characterizing a scaffold-free method to first guide cell growth into forming a single fiber, and then deliver a precise dynamic tensile strain to the fiber as it developed. This method of directed cellular assembly offered a unique approach to engineer tendon, as no three-dimensional extracellular matrix or scaffold was initially incorporated in the fiber structure, rather the matrix synthesis and tissue structure was determined solely by the cells that form the fiber. In the second aim, the early stages of fiber development were investigated to determine if aspects of embryonic tendon development were captured by the developing fibers. The third aim evaluated the influence of dynamic tensile stimulation on the mechanical properties of the single fibers. The results demonstrated that cells could be guided to form engineered single fibers, on the scale of primary single tendon fibers. These fibers lay the foundation for the future creation of patient-specific, autologous cell-based tendon replacements, with fiber-level architectural fidelity. Due to the scaffold-free nature of this approach, the fibers demonstrated key aspects of embryonic tendon development; they were highly aligned and highly cellular, with cells in direct cell-to-cell contact. The single fibers also responded to the application of cyclic tensile strain with increased mechanical properties, demonstrating a mechanism for enhancing tissue formation. Tissue engineering strategies, inspired by embryonic development, may provide unique insight into accelerated maturation of engineered replacement tissue, and offer significant advances for regenerative medicine applications in tendon and other collagenous soft tissues.