The differentiation of mesenchymal stem cells (MSCs) into chondrocytes (native cartilage cells), or chondrogenesis, is a key step in the tissue engineering of articular cartilage. Chondrogenesis is regulated by transforming growth factor-beta (TGF-β), a short-lived cytokine whose effect is prolonged by storage in the extracellular matrix (ECM). Tissue engineering applications require the complete differentiation of an initial population of MSCs, and two common strategies used to achieve this in vitro are (1) co-culture the MSCs with chondrocytes, which constitutively produce TGF; or (2) add exogenous TGF-β. We are motivated by recent experiments where a tissue engineering construct is seeded with cells in spatially separated layers, with a layer of MSCs lying below a layer of chondrocytes, and is then stimulated with exogenous TGF-β from above.
To investigate the efficacy of this seeding strategy we develop a reaction-diffusion model for the interactions between the TGF-β, MSCs and chondrocytes. A feature of this model is that it describes the different forms TGF-β takes throughout its lifecycle, some of which are bound to the ECM (and so not diffusible), and the process by which the TGF-β cytokine is cleaved from the molecular latency complex it is secreted with to become available to cell receptors. Using this model we demonstrate that the concentration of TGF-β required to induce chondrogenesis of the MSCs in the lower layer is not physically realistic for layers of equal depth (as used experimentally). We then suggest improvements to the current experimental protocols, as well as compare the use of the layered cell seeding strategy with a strategy where the initial cell populations are well-mixed (which we have previously investigated with an ordinary differential equation model).