Cartilage tissue engineering: Directed differentiation of embryonic stem cells in three-dimensional hydrogel culture

The clinical goal of tissue engineering is to restore, repair, or replace damaged tissues in the body. Significant advances have been made in recent years using stem cells as a cell source for cartilage tissue engineering and reconstructive surgery applications. Embryonic stem cells have demonstrated the potential to self-renew and differentiate into a wide range of tissues including the chondrogenic lineage, depending on culture conditions. Three-dimensional scaffolds play an important role in tissue regeneration by providing attachment sites as well as bioactive signals for cells to grow and differentiate into specific lineages. The precise microenvironments required for optimal expansion or differentiation of stem cells are only beginning to emerge now, and the controlled differentiation of embryonic stem cells in tissue engineering remains a relatively unexplored field. Hydrogels are a class of polymer-based biomaterials that have been extensively utilized in tissue engineering as scaffolds. We have demonstrated that embryonic stem cells encapsulated within poly(ethylene glycol)-based (PEGDA) photopolymerizing hydrogels and cultured in an appropriate growth factor and medium conditions undergo chondrogenic differentiation with extracellular matrix deposition characteristic of neocartilage (Hwang et al., Stem Cells 24, 284-291). Another hydrogel that has been widely used for encapsulating chondrocytes in cartilage tissue engineering is alginate. This hydrogel also has potential for tissue engineering applications using stem cells. Here, we describe the three-dimensional culture of embryonic stem cell-derived embryoid bodies in hydrogels and their differentiation toward chondrogenic lineage in chemically defined chondrogenic medium in the presence of TGF-beta1 (chondrogenic inducing conditions). We also discuss various tools and assays used for characterizing the tissue-engineered cartilage.