UC San Diego

Cartilage develops specific forms, sizes, and functional properties under the guidance of naturally present biophysical and biochemical stimuli during in vivo growth and maturation. Analogously, controlling the physical and biochemical environment of cartilage during culture may facilitate the in vitro manipulation of tissue properties. This dissertation explores flexural deformation as a physical stimulus capable of inducing changes in the free- swelling shape of articular cartilage explants and specific biochemical agents in modulating the shape plasticity, size, and maturity of cartilage through altered matrix metabolism and remodeling. The bulk of the work described herein was conducted with a model system of immature articular cartilage explants from bovine calves, with consideration of the potential translation of the experimental approaches to other native and engineered chondral tissues. A system for analyzing mechanically- induced reshaping of cartilage was developed through the creation of a novel bioreactor to apply flexure to cartilage explants and a means of sensitively quantifying specimen shape. Using these tools, static flexure was shown to produce significant changes in the free-swelling shape of cartilage in a duration-dependent manner. The flexural stimulus was characterized with micromechanical strain analysis techniques, and the observed asymmetrical strain distributions within the tissue were found to be a result of the pronounced tension-compression nonlinearity of cartilage mechanical properties. Inhibitors of chondrocyte and cartilage extracellular matrix metabolism were used to probe potential mechanisms of cartilage reshaping, and findings indicated that the process of reshaping was largely independent of chondrocyte-mediated matrix synthesis or remodeling. However, a strong temperature-dependent response suggested a possible biophysical mechanism of matrix remodeling. Altering matrix metabolism and remodeling by supplementing cultures with specific biochemical agents or growth factors resulted in changes in tissue composition with respect to the predominant collagen and glycosaminoglycan matrix components. These treatments differentially modulated cartilage volume and functional properties, including shape plasticity and mechanical properties in compression. These studies further elucidate the regulation of the shape, size, and maturity of cartilage by biophysical and biochemical stimuli. The tools and techniques developed here may be translatable to creating chondral grafts with desired properties for joint repair or craniofacial reconstruction