Cartilage tissue engineering with silk scaffolds and human articular chondrocytes

Adult cartilage tissue has poor capability of self-repair, especially in case of severe cartilage damage due to trauma or age-related degeneration. Autologous cell-based tissue engineering using three-dimensional (3-D) porous scaffolds has provided an option for the repair of full thickness defects in adult cartilage tissue. Mesenchymal stem cells (MSCs) and chondrocytes are the two major cell sources for cartilage tissue engineering. Silk fibroin as a naturally occurring degradable fibrous protein with unique mechanical properties, excellent biocompatibility and processability has demonstrated strong potential for skeletal tissue engineering [Wong Po Foo C, Kaplan DL. Genetic engineering of fibrous proteins: spider dragline silk and collagen. Adv Drug Deliv Rev 2002; 54: 1131–43; Altman GH, Diaz F, Jakuba C, Calabro T, Horan RL, Chen J, et al. Silk-based biomaterials. Biomaterials 2003; 24: 401–16; Altman GH, Horan RL, Lu HH, Moreau J, Martin I, Richmond JC, et al. Silk matrix for tissue engineered anterior cruciate ligaments. Biomaterials 2002; 23: 4131–41; Jin HJ, Kaplan DL. Mechanism of silk processing in insects and spiders. Nature 2003; 424: 1057–61; Jin HJ, Fridrikh SV, Rutledge GC, Kaplan DL. Electrospinning Bombyx mori silk with poly(ethylene oxide). Biomacromolecules 2002; 3: 1233–9]. The present study combined adult human chondrocytes (hCHs) with aqueous-derived porous silk fibroin scaffolds for in vitro cartilage tissue engineering. The results were compared with a previous study using the same scaffolds but using MSCs to generate the cartilage tissue outcomes. Culture-expanded hCHs attached to, proliferated and redifferentiated in the scaffolds in a serum-free, chemically defined medium containing TGF-β1, based on cell morphology, levels of cartilage-related gene transcripts, and the presence of a cartilage-specific ECM. Cell density was critical for the redifferentiation of culture-expanded hCHs in the 3-D aqueous-derived silk fibroin scaffolds. The level of cartilage-related transcripts (AGC, Col-II, Sox 9 and Col-II/Col-I ratio) and the deposition of cartilage-specific ECM were significantly upregulated in constructs initiated with higher seeding density. The hCH-based constructs were significantly different than those formed from MSC-based constructs with respect to cell morphology, zonal structure and initial seeding density needed to successfully generate engineered cartilage-like tissue. These results suggest fundamental differences between stem cell-based (MSC) and primary cell-based (hCH) tissue engineering, as well as the importance of suitable scaffold features, in the optimization of cartilage-related outcomes in vitro. The present work diversifies cell sources in combination with silk fibroin-based tissue engineering applications. Together with our previous studies, the present results show great promise for engineered 3-D silk fibroin scaffolds in autologous cell-based skeletal tissue engineering.