Date of Completion

5-3-2018

Embargo Period

4-25-2018

Keywords

Biomaterials, tissue engineering, scaffolds, anisotropy, freeze casting, differentiation, osteochondral, multi-layered, zonal

Major Advisor

Dr. Mei Wei

Associate Advisor

Dr. Harold Brody

Associate Advisor

Dr. George Rossetti

Associate Advisor

Dr. David Rowe

Associate Advisor

Dr. Wendy Vanden Berg-Foels

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Osteochondral tissue is a biphasic material comprised of articular cartilage integrated atop subchondral bone. Damage to this tissue is highly problematic, owing to its intrinsic inability to regenerate functional tissue in response to trauma or disease. Further, the function of the tissue is largely conferred by its compartmentalized zonal microstructure and composition. Current clinical treatments fail to regenerate new tissue that recapitulates this zonal structure. Consequently, regenerated tissue often lacks long-term stability. To address this growing problem, we propose the development of tissue engineered biomaterials that mimic the zonal collagen orientation and composition of osteochondral tissue. First, a unidirectional freeze casting platform was developed that facilitated the fabrication of highly-aligned porous collagen scaffolds to serve as the superficial zone of the multidirectional scaffold. Subsequently, a novel lyophilization bonding process, which seamlessly bonds scaffolds with different orientations and compositions together, forming a monolithic multidirectional collagen-based scaffolds that mimicked the structure and composition of the superficial, transition, calcified cartilage, and osseous zones of osteochondral tissues. The microstructure of multidirectional scaffolds was characterized and the ability of these scaffolds to promote osteochondral differentiation of progenitor cells was assessed using a novel transgenic multi- reporter cell platform. Zonal osteogenic differentiation of bone marrow stromal cells was confirmed by transition of pre-osteoblast marker BSP-GFPtopaz to late osteoblast/early osteocyte marker DMP1-RFPmCherry. Conventional histological and gene expression analysis corroborated fluorescence data. Zonal chondrogenesis of triple transgenic articular chondrocytes harboring reporters for fibrocartilage (Col3.6-GFPtopaz), articular cartilage (Col2a1-GFPcyan), and hypertrophic (Col10a1-RFPmCherry) lineages was assessed and corroborated in a similar manner. Though the majority of fluorescent cells (65%) remained of articular origin, there was a progression (28%) towards a pre-hypertrophic phenotype. Combined with fluorescent reporter information, gene expression, histological, and immunohistochemical analyses, subtle differences in zone-specific cellular phenotype and newly-formed tissue were identified. Upon ectopic implantation in mice for 8 weeks, pre- hypertrophic cell populations terminally matured into mineralized tissue. Together, these data suggest that multidirectional zonal scaffolds hold promise for clinical use in directing zonal osteochondral repair by articular chondrocytes.

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