Development of hydrodynamically engineered cartilage in response to insulin-like growth factor-1 and transforming growth factor-beta1: formation and role of a type I collagen-based fibrous capsule
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Articular cartilage which covers the surfaces of synovial joints is designed to allow smooth contact between long bones and to absorb shock induced during joint movement. Tissue engineering, a means of combining cells, biomaterials, bioreactors and bioactive agents to produce functional tissue replacements suitable for implantation, represents a potential long-term strategy for cartilage repair. The interplay between environmental factors, however, gives rise to complex culture conditions that influence the development of tissue-engineered constructs. A fibrous capsule that is composed of abundant type I collagen molecules and resembles fibrocartilage usually forms at the outer edge of neocartilage, yet the understanding of its modulation by environmental cues is still limited. Therefore, this dissertation was aimed to characterize the capsule formation, development and function through manipulation of biochemical parameters present in a hydrodynamic environment while a chemically reliable media preparation protocol for hydrodynamic cultivation of tissue-engineered cartilage was established. To this end, a novel wavy-wall bioreactor (WWB) that imparts turbulent flow-induced shear stress was employed as the model system and polyglycolic acid scaffolds seeded with bovine primary chondrocytes were cultivated under varied biochemical conditions. The results demonstrated that tissue morphology, biochemical composition and mechanical strength of hydrodynamically engineered cartilage were maintained as the serum content decreased by 80% (from 10% to 2%). Transient exposure of the low-serum constructs to exogenous insulin-like growth factor-1 (IGF-1) or transforming growth factor-β1 (TGF-β1) further accelerated their development in comparison with continuous treatment with the same bioactive molecules. The process of the capsule formation was found to be activated and modulated by the concentration of serum which contains soluble factors that are able to induce fibrotic processes and the capsule development was further promoted by fluid shear stress. Moreover, the capsule formation in hydrodynamic cultures was identified as a potential biphasic process in response to concentrations of fibrosis-promoting molecules such as TGF-β. Comparison between the capsule-containing and the capsule-free constructs, both of which had comparable tissue properties and were produced by utilizing the WWB system in combination with IGF-1 and TGF-β1, respectively, showed that the presence of the fibrous capsule at the construct periphery effectively improved the ability of engineered cartilage to integrate with native cartilage tissues, but evidently compromised its tissue homogeneity. Characterization of the fibrous capsule and elucidation of the conditions under which it is formed provide important insights for the development of tissue engineering strategies to fabricate clinically relevant cartilage tissue replacements that possess optimized tissue homogeneity and properties while retaining a minimal capsule thickness required to enhance tissue integration.