Figure 4.

The tissue engineering paradigm. Cells, signals and scaffolds are the major elements of tissue engineering approaches to cartilage repair. For this purpose, many different cell sources (autologous cells, allogeneic cells or stem cells) have been tested in vitro. Neotissue has been cultured ex vivo with various stimuli and chemicals to enhance synthesis and chondrogenic potential. To further improve the integrity of neotissues, scaffolds have been used to create a 3D environment to maintain the phenotype of, and carry, integrated cells in vivo, and to recruit cells from the host environment. Combinations of these factors drive the three major cartilage engineering strategies that exist: cell-free, scaffold-based implants that promote cell recruitment with chemoattractants; cell-seeded scaffolds that mimic the structure of native tissues; and scaffold-free, cell-based biomimetic techniques. Abbreviations: AMIC, autologous matrix-induced chondrogenesis; ECM, extracellular matrix; GAG, glycosaminoglycans; MACI, matrix-assisted autologous chondrocyte implantation.