Levi, B. et al. Proc. Natl. Acad. Sci. USA 109, 20379–20384 (2012).

In vivo engraftment of stem cell–derived cells into damaged or diseased tissue has many challenges. For one, if pluripotent stem cells are introduced in numbers large enough for tissue regeneration, there is a high risk of teratomas in an immunodeficient mouse (a standard research model for studying engraftment). Levi et al. show that teratoma formation can be minimized by transplanting the cells into an engineered niche that promotes efficient differentiation. Pluripotent cells are implanted in a hydroxyapatite-coated poly-L-lactic acid scaffold that releases bone morphogenic protein 2. The implants achieve rapid and efficient repair of a calvarial defect with a very low incidence of teratomas (they are seen in 2 of 42 mice) after injection of 1 million human pluripotent cells.