The role of hydrogel structure and dynamic loading on chondrocyte gene expression and matrix formation

Crosslinked poly(ethylene glycol) (PEG) hydrogels are attractive scaffolds for cartilage tissue engineering because of their ability to mimic the aqueous environment and mechanical properties of native cartilage. In this study, hydrogel crosslinking density was varied to study the influence of gel structure and the application of dynamic loading (continuous, 1 Hz, 15% amplitude strain) on chondrocyte gene expression over approximately 1 week culture. Gene expression was quantified using real-time RT-PCR for collagen II and aggrecan, the major cartilage extracellular matrix (ECM) components, and collagen I, an indicator of chondrocyte de-differentiation. When chondrocytes were encapsulated in PEG gels with low or high crosslinking, a high collagen II expression compared to collagen I expression (1000 or 100,000:1, respectively) indicated the native chondrocyte phenotype was retained. In the absence of loading, relative gene expression for collagen II and aggrecan was significantly higher (e.g., 2-fold and 4-fold, respectively, day 7) in the low crosslinked gels compared to gels with higher crosslinking. Dynamic loading, however, showed little effect on ECM gene expression in both crosslinked systems. To better understand the cellular environment, ECM production was qualitatively assessed using an in situ immunofluorescent technique and standard histology. A pericellular matrix (PCM) was observed as early as day 3 post-encapsulation and the degree of formation was dependent on gel crosslinking. These results suggest the PCM may protect the cells from sensing the applied loads. This study demonstrates that gel structure has a profound effect on chondrocyte gene expression, while dynamic loading has much less of an effect at early culture times.