The knee meniscus plays an integral role in providing lubrication, load distribution, and shock absorption, yet is frequently compromised through traumatic injury or disease. Unfortunately, many of the injuries sustained by the meniscus are unable to heal, and current clinical therapies lack the ability to restore full tissue functionality. Tissue engineering efforts provide a possible solution to this problem. To engineer functional meniscal cartilage, however, researchers need specific design criteria from native tissue as well as an abundant cell source for tissue generation. Tissue engineering efforts must also take into account the complex geometry of the meniscus, as well as regional variations in biochemical and biomechanical properties. In this thesis, meniscus cells and tissue are characterized regionally to identify key parameters for tissue engineering, and an alternate cell source is evaluated for in vitro engineering of fibrocartilage.
Towards understanding regional meniscus characteristics important for tissue engineering efforts, meniscus cells were characterized biomechanically and an effective method for isolating these cells for tissue engineering was determined. It was found that the meniscus contains cells that are biomechanically distinct, with outer meniscus cells showing higher stiffness than inner cells. It was also determined that meniscus cells as a whole were more biomechanically similar to ligament cells than to articular chondrocytes, indicating that tissue properties may correlate with cellular mechanics. In addition to showing regionally distinct biomechanical properties, enzymatic isolation of meniscus cells was found to cause varying phenotypic changes in cells from the inner, middle, and outer regions. A comparison of isolation techniques also indicated that sequential digestion of meniscus tissue with pronase and collagenase was able to yield more cells with higher viability than other techniques tested, and those isolated cells created stiffer and more glycosaminoglycan (GAG) rich constructs when used in a tissue engineering modality than cells isolated using only collagenase. The identification of an effective mode of isolating meniscus cells is of great use to tissue engineering efforts, as they often require a large cell numbers. These findings illustrate that known regional variations in meniscus cell phenotype and biochemical composition are also evident in cellular mechanics, and phenotypic responses of these cells to isolation are varied and distinct.
To be successful tissue replacements, tissue engineered meniscus constructs must not elicit an immune response and must have sufficient mechanical properties to survive when implanted. To determine if allogeneic or xenogeneic implantation of scaffold-free meniscus constructs could be feasible, the immunogenicity of bovine and leporine meniscus cells and articular chondrocytes were determined in an in vitro model system. It was found that neither bovine nor leporine meniscus cells or articular chondrocytes caused activation of leporine immune cells, suggesting that they may serve as allogeneic or xenogeneic cell sources for meniscus engineering. Additional analysis of the mechanical role of meniscus GAGs indicated that they are mechanically important in all regions of the meniscus, but especially in the inner region where the relatively high GAG content affects both compressive and tensile properties. Therefore, tissue engineering efforts should try to recapitulate GAG content and distribution to enhance the functionality of meniscus replacements.
As a major obstacle for meniscus engineering is the identification of an abundant cell source, this thesis also investigated the use of skin cells as an alternative to primary cells for tissue engineering. Previously identified chondroinducible dermis cells were found to have multilineage differentiation capacity, and were subsequently termed dermis isolated adult stem cells (DIAS). DIAS cells were also able to be expanded in monolayer without losing chondroinductive capacity, and were able to create constructs with cartilaginous properties which could be varied with growth factor application. Given the ease of expansion and ability of DIAS cells to form fibrocartilaginous tissue, these cells present an abundant cell source for meniscus tissue engineering.
Together, the studies performed in this thesis 1) offer valuable design parameters for meniscus cells and tissue from different regions, 2) provide indications for the immunogenicity of possible cell sources for meniscus engineering, and 3) enhance the understanding and utility of DIAS cells for engineering the cartilage spectrum.
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