Perspective on the evolution of cell-based bone tissue engineering strategies.

Despite the compelling clinical needs in enhancing bone regeneration and the potential offered by the field of tissue engineering, the adoption of cell-based bone graft substitutes in clinical practice is limited to date. In fact, no study has yet convincingly demonstrated reproducible clinical performance of tissue-engineered implants and at least equivalent cost-effectiveness compared to the current treatment standards. Here, we propose and discuss how tissue engineering strategies could be evolved towards more efficient solutions, depicting three different experimental paradigms: (i) bioreactor-based production; (ii) intraoperative manufacturing, and (iii) developmental engineering. The described approaches reflect the need to streamline graft manufacturing processes while maintaining the potency of osteoprogenitors and recapitulating the sequence of biological steps occurring during bone development, including vascularization. The need to combine the assessment of efficacy of the different strategies with the understanding of their mechanisms of action in the target regenerative processes is highlighted. This will be crucial to identify the necessary and sufficient set of signals that need to be delivered at the injury or defect site and should thus form the basis to define release criteria for reproducibly effective engineered bone graft substitutes.