Translating Mechanisms of Tendon Development to Improve Adult Tendon Repair

Tendon and ligament injuries often interfere with normal activities of daily living. As current surgical techniques are not uniformly successful, tissue engineers seek to develop tissue engineered constructs to accelerate and improve musculoskeletal soft tissue healing. Our Functional Tissue Engineering Lab has decades of experience in using adult progenitor cell-based therapies to improve the mechanics of repair tissue. However, even the best repairs to date do not achieve minimum mechanical design limits of matching tendon tangent stiffness beyond peak in vivo loads. As tendons are living tissues, composed of cells and extracellular matrix, we propose that advances in the design of tissue engineered constructs require establishing biological design criteria. In an effort to identify possible biological design parameters, our lab collaborates with developmental biologists to understand normal tendon formation.;Based on these collaborations we used the FTE paradigm to establish a set of biological design criteria for tendon repair. A successful repair should exhibit 1) scleraxis-expressing cells in 2) an aligned collagen matrix that integrates with 3) a regionalized tendon-to-bone insertion site. This dissertation establishes a foundation for translating mechanisms of tendon development to modulate these biological design criteria in vitro and in vivo.;Aim 1 identifies what construct materials, chemical and mechanical signals stimulate tenogenesis (first two biological design criteria) across species in vitro. We found that, compared to collagen-based constructs, fibrin-based materials enhance expression of tenogenic markers and exhibit improved collagen alignment and mechanics for up to two weeks in culture. We also found that transforming growth factor beta (TGFB) signaling is a potent inducer of tenogenic markers in embryonic and adult progenitor cells, and it can be used to increase collagen production and construct mechanics. In an effort to translate these findings to larger, more clinically relevant animal models, another study investigated the effects of TGFB stimulation in rabbit progenitor cell-based constructs. However, rabbit cells did not respond as expected, highlighting the need for more translational research efforts.;On the basis of in vitro findings showing embryonic tendon progenitors produce stiffer constructs than adult progenitors, Aim 2 compares the effects of these cell sources on repair of a central patellar tendon defect model. Results revealed that embryonic progenitors exhibit improved integration with the defect at 2 weeks, but neither cell source improves healing mechanically. Therefore, future studies are warranted to identify how progenitor cell source affects the host healing response.;Aim 3 includes mechanistic research to understand the role of a novel marker (Indian hedgehog, Ihh) on insertion site development and maturation, which might be used to achieve our third biological design criteria. Findings showed that Ihh regulates the mineralization of the fibrocartilaginous insertion and inhibiting this pathway during development causes tendon biomechanical deficiencies. Given its important role in insertion maturation, Ihh is a promising candidate for improving tendon integration at the insertion site after injury.;Findings from these aims contribute to understand how to regulate biological design parameters in our tissue engineered constructs and evaluate their effects on tendon repair.