A PLoS Medicine review looks at the present state of regenerative medicine for bones: "To overcome the drawbacks of the current bone graft materials, bone tissue engineering (BTE) using bone marrow stem cells has been suggested as a promising technique for reconstructing bone defects. Indeed, various animal studies have shown the capacity of BTE to produce bone, both in a non-bone environment (ectopic bone formation) and in a bone environment (orthotopic bone formation) ... Surprisingly however, until recently, no convincing successes have been achieved in humans. In this article, we review the available clinical data in the area of bone tissue engineering together with our own clinical experience. We discuss possible new directions that need to be exploited to make bone tissue engineering a clinical success. ... Cell survival is the most important requirement for achieving clinical success in cell-based bone tissue engineering. ... What is indisputable is that [mesenchymal stem cells] are crucial for the healing of bone defects."