The effect of moving point of contact stimulation on chondrocyte gene expression and localization in tissue...

Tissue engineering is a promising approach for articular cartilage repair. However, using current technologies, the developed engineered constructs generally do not possess an organized superficial layer, which contributes to the tissue's durability and unique mechanical properties. In this study, we investigated the efficacy of applying a moving point of contract-type stimulation (MPS) to stimulate the production of a superficial-like layer in the engineered constructs. MPS was applied to chondrocyte-agarose hydrogels at a frequency of 0.5, 1 or 2 Hz, under a constant compressive load of 10 mN for durations between 5 and 60 min over 3 consecutive days. Expression and localization of superficial zone constituents was conducted by qRT-PCR and in situ hybridization. Finite element modeling was also constructed to gain insight into the relationship between the applied stimulus and superficial zone constituent expression. Gene expression of superficial zone markers were affected in a frequency dependent manner with a physiologic frequency of 1 Hz producing maximal expression of PRG4, biglycan, decorin and collagen II. In situ hybridization revealed that localization of these markers predominantly occurred at 500-1000 μm below the construct surface which correlated to sub-surface strains between 10 and 25% as determined by finite element modeling. These results indicate that while mechanical stimuli can be used to enhance the expression of superficial zone constituents in engineered cartilage constructs, the resultant subsurface loading is a critical factor for localizing expression. Future studies will investigate altering the applied stimulus to further localize superficial zone constituent expression at the construct surface.