Chondrocyte micro-aggregates enhances neo-cartilage formation

Purpose: We recently developed a novel injectable in situ crosslinkable Dextran-Tyramine 14kDa-DS=15 (Dex-TA) hydrogel, which has shown high potential for cartilage regeneration. However, cartilage repair strategies based on autologous chondrocyte implantation still rely on in vitro expansion to obtain sufficient cells with all inherent drawbacks such as dedifferentiation. Cartilage repair using mixtures of a limited number of chondrocytes with Mesenchymal Stem Cells (MSCs) can potentially overcome this hurdle. It remains unclear whether the hydrogels should be seeded with single-cell suspension or with preformed microaggregates of defined size. To test, this we have seeded hydrogels with microaggregates of chondrocytes, MSCs or a mixture of both and compared their performance with single-cell seeded hydrogels.

Methods and Materials: High throughput formation of microaggregates of 50, 100 and 200 cells was achieved in micromolds. Microaggregates were prepared of chondrocytes, MSCs or a mixture of both. Morphology, stability and chondrogenic capacity was evaluated. Aggregates with the optimal cell density of 100 cells were incorporated into Dex-TA hydrogels, cultured in vitro and in vivo and compared to single-cell seeded hydrogels.

Results: Micro-aggregates were formed in a very controlled manner and successfully incorporated into Dex-TA hydrogels. Aggregates formed by 100 cells showed a superior balance between stability and gene expression profile, with higher collagen type2 and Aggrecan expression. After incorporation of micro-aggregates into Dex-TA, long term stability and survival was observed, as well as enhanced matrix production, when compared to single-cell seeded hydrogels. More cartilage was formed in micro-aggregates consisting of a 50%/50% mixture of chondrocytes and MSCs, compared to both micro-aggregates of pure cell populations and to single-cell mixtures.

Conclusions: We conclude that neocartilage formation is greatly improved by seeding hydrogels with micro-aggregates instead of single-cell suspensions. In addition, this system provided preliminary information about the effect of micro-cocultures, showing enhancement of neocartilage formation when using 50% chondrocytes mixed with 50% MSCs.