Cell-seeded type II collagen scaffolds for articular cartilage tissue engineering

Abstract

Defects in articular cartilage exhibit little spontaneous healing response, in part due to the limited number of chondrocytes available to infiltrate the defect and the absence of a provisional fibrin scaffold to accommodate cell migration into the lesion. One variable related to tissue engineering strategies employing cell-seeded scaffolds to treat such defects is the amount of cartilage formed in the construct prior to implantation. The objectives of this thesis were to evaluate effects of scaffold cross-link density and bioreactor culture environment on chondrogenesis in cell-seeded type II collagen scaffolds in vitro, and to begin to test effects of implant compositional maturity (viz. glycosaminoglycan, GAG, content) on chondral defect repair. Scaffold cross-link density, a determinant of cell-mediated scaffold contraction and degradation, affected chondrogenesis; scaffolds of low cross-link density that experienced contraction exhibited greater cartilaginous tissue formation compared to highly cross-linked scaffolds that resisted contraction. In addition to tissue-level effects on histogenesis, cross-link density was found to direct phenotypic differentiation at the cellular level. When employing marrow-derived stem cells as an alternative to chondrocytes, scaffolds with lower cross-link densities (and thus less resistance to contraction and degradation) favored chondrocytic differentiation.

(cont.) In comparison to these findings, bioreactor culture of chondrocyte-seeded scaffolds demonstrated little benefit over static culture with respect to histogenesis within the first 2 weeks of culture. To begin to investigate effects of implant maturity on in vivo repair outcome, chondrocyte-seeded type II collagen scaffolds achieving 30% of the GAG content in native cartilage were implanted in chondral defects in a caprine model. Repair tissue evaluated at 15 weeks consisted primarily of fibrocartilage and small amounts of hyaline tissue. Implantation of the construct reduced fibrous tissue formation compared to controls, but did not significantly affect other outcome variables. Future animal investigations will evaluate effects of implanting constructs with GAG contents 50% and 75% of that in normal cartilage. An additional study evaluated a construct comprised of a non-cell-seeded type II collagen scaffold and a bone graft substitute for treating osteochondral defects in a goat model. These implants qualitatively improved bone formation, but did not significantly improve repair of cartilage compared to controls.