Purpose: Articular cartilage is a physiologically hypoxic tissue with a gradient of oxygen tension ranging from about 10% oxygen at the cartilage surface to less than 1% in the deepest layers. The overall goal of the study was to determine whether an injectable allogeneic/autologous fibrin scaffolds in combination with mesenchymal stem cells (MSCs) is suitable for articular cartilage tissue engineering, and to determine the effect of hypoxic culture conditions on the stability of cell-fibrin scaffolds. The secondary goal was to enhance the accumulation of extracellular matrix (ECM) inside the fibrin scaffold under these conditions.

Method: Chondroprogenitor clonal cell line RCJ3.1C5.18 (C5.18) and human mesenchymal stem cells (hMSCs) were encapsulated in fibrin hydrogel and fibrin glue scaffolds. The stabilization of fibrin scaffolds and development of ECM components were evaluated using zymography, SDS-polyacrylamide electrophoresis (SDS-PAGE), immunochemistry, spectrophotometry, RT-PCR including real time and histology (Ahmed TA., et al. Tissue Engineering2007;13(7): 1469–77).

Results: After encapsulation of C5.18 and hMSCs, fibrin gels quickly degraded under normoxic conditions (21 % oxygen) due to upregulation of plasminogen and matrix metalloproteinases (MMPs) genes especially MMP-2, -3, and -9. Protease inhibitors such as aprotinin and galardin (GM6001), in combination or separately, prevented the fibrin-C5.18 hydrogels breakdown for up to 5 weeks. Only a combination of aprotinin and galardin resulted in accumulation of ECM components such as collagen II and aggrecan. In contrast, fibrin-hMSCs hydrogels were found to be stable under hypoxic conditions (5% O2) for up to 4 weeks in the absence of inhibitors, suggesting that hypoxic conditions may downregulate the expression of the enzymes responsible for fibrin-hydrogel breakdown.

Conclusion: These results suggest that in C5.18 and MSCs cell lines, expression of matrix metalloproteinases (MMPs) and plasmin is upregulated under normoxic conditions and is responsible for fibrin-hydrogel breakdown. Moreover, inhibition of both proteases is required to enhance the accumulation of ECM. However, fibrin hydrogel scaffolds were stabilized under low oxygen tension, which is more physiological than normoxia and therefore these constructs may be stable after implantation in the absence of protease inhibitors.