UC San Diego

Articular cartilage is a connective tissue that normally provides a load-bearing, low friction, wear-resistant tissue located at the ends of long bones through its depth -varying cell and matrix properties. When damaged, articular cartilage possesses limited capacity for self- repair. Cartilaginous tissue has been engineered to recapitulate the normal zonal organization of cartilage. The effects of such tissues as treatment for focal cartilage defects in vivo have not been determined. In addition, experimental studies have shown that implanted cells may not be retained in the defect site, and that host cells may also enter the defect site, both of which may modulate the efficacy of defect repair. This dissertation aims to establish cytotracking methods for chondrocytes and to use these methods to assess the cellular mechanisms of articular cartilage defect repair in vivo after implantation of cartilaginous tissue that had a stratified population of cells resembling normal cartilage, or tissue that had a traditional, mixed population of cells. Stratified and mixed cartilage constructs were formed with chondrocytes from the superficial and middle/deep zones of articular cartilage, fluorescently labeled with tracking dyes, PKH26 and CFSE. The dyes were strongly retained by chondrocytes, even when induced to proliferate at varying rates and were tracked through at least 7 cycles of cell division in vitro. The dyes were found to have minimal effects on chondrocyte function, including proliferation and secretion of PRG4. Implant-derived cells were localized and recovered after implantation for 1 week however, stratification of cartilaginous tissues was not maintained in vivo suggesting contributions from both implant and host cells to the population of the defect. After 4 weeks in vivo, persistence of implant-derived cells in the defect region was observed with stratified constructs maintaining certain features of stratification after 4 weeks in vivo which were distinct from mixed constructs suggesting a role of cell organization and phenotype in cartilage repair. Clarifying cell source (implanted vs. host) and fates contributes to a mechanistic understanding of the repair of articular cartilage defects and possible tissue engineering repair strategies for the growing population of patients with impaired joint function