| Cartilage cells (chondrocytes) are sparsely populated in an extracellular matrix (ECM) that provides cells with biochemical and biomechanical cues. The exact mechanisms, however, are largely unknown. This dissertation employed three-dimensional photopolymerized synthetic hydrogels to mimic and isolate aspects of cartilage in a controllable manner. By employing poly(ethylene glycol) (PEG) hydrogels, this work elucidated that cell maturity and loading parameters (e.g., frequency, strain) were significant factors in mediating chondrocyte response. Adult cells exhibited diminished capabilities to produce matrix and higher catabolic activities compared to adolescent cells, which was further enhanced by loading. Chondrocyte response was further mediated by the hydrogel structure, which acted to mimic changes in the cartilage crosslinked structure. Under compressive strains, the gel structure regulated cellular deformation and how cells sensed and responded to static and dynamic loading. Dynamic loading largely resulted in matrix inhibition where additional studies suggested that fluid flow alone was sufficient to inhibit matrix production. To assess the role of ECM in mediating chondrocyte response, matrix molecules were introduced into the PEG hydrogels either as tethers of Arg-Gly-Asp (RGD) oligopeptide to promote cell attachment or negatively charged chondroitin sulfate. Chondrocytes dedifferentiated with increasing RGD concentrations, while mechanical stimulation was able to counteract this negative response and promote matrix biosynthesis. The addition of chondroitin sulfate inhibited matrix synthesis; however, when mechanical stimulation was applied matrix synthesis was significantly up-regulated. These studies suggest that the addition of matrix molecules is an important cue for enhancing matrix production, but only when combined with mechanical stimulation. Findings from this dissertation also pinpointed to the importance of physiological osmotic environments for promoting cell survival and matrix deposition when low cell densities are employed, while exogeneosly delivering proinflammatory cytokine, interleukin-1a, led to cell death, increased NO production, and decreased matrix synthesis. In summary, this work contributes to our current knowledge of chondrocyte (mechano)biology and the role of structure and chemistry in mediating chondrocyte response to a range of parameters (e.g., cell deformation, fluid flow, osmolarity, cell-matrix interactions, streaming potentials). This knowledge will also aid in developing superior strategies for cartilage repair. |
