21 Massimiliano Gnecchi (ed.), Mesenchymal Stem Cells: Methods and Protocols, Methods in Molecular Biology, vol. 1416, DOI 10.1007/978-1-4939-3584-0_2, © Springer Science+Business Media New York 2016 Chapter 2 Bone Tissue Engineering: Past–Present–Future Rodolfo Quarto and Paolo Giannoni Abstract Bone is one of the few tissues to display a true potential for regeneration. Fracture healing is an obvious example where regeneration occurs through tightly regulated sequences of molecular and cellular events which recapitulate tissue formation seen during embryogenesis. Still in some instances, bone regeneration does not occur properly (i.e. critical size lesions) and an appropriate therapeutic intervention is necessary. Successful replacement of bone by tissue engineering will likely depend on the recapitulation of this flow of events. In fact, bone regeneration requires cross-talk between microenvironmental factors and cells; for example, resident mesenchymal progenitors are recruited and properly guided by soluble and insoluble signaling molecules. Tissue engineering attempts to reproduce and to mimic this natural milieu by deliver- ing cells capable of differentiating into osteoblasts, inducing growth factors and biomaterials to support cellular attachment, proliferation, migration, and matrix deposition. In the last two decades, a significant effort has been made by the scientific community in the development of methods and protocols to repair and regenerate tissues such as bone, cartilage, tendons, and ligaments. In this same period, great advance- ments have been achieved in the biology of stem cells and on the mechanisms governing “stemness”. Unfortunately, after two decades, effective clinical translation does not exist, besides a few limited examples. Many years have passed since cell-based regenerative therapies were first described as “promising approaches”, but this definition still engulfs the present literature. Failure to envisage translational cell therapy applications in routine medical practice evidences the existence of unresolved scientific and technical struggles, some of which still puzzle researchers in the field and are presented in this chapter. Key words Bone, Mesenchymal stem cells, iPSC, Cell therapy, Biomaterials, Scaffolds 1 Past Cell Therapy The standard approach proposed in the past implied the delivery of in vitro expanded cells (stem cells, progenitors, etc.) combined with biomaterials of various chemical nature and architecture. Osteoprogenitor cells have been isolated from a variety of tissues, including periostium, bone marrow, spleen, thymus, skeletal muscle, and adipose tissue [1–8]. Osteoprogenitors have also been isolated from other tissues, such as amniotic fluid [9], chorionic villi [10], infrapatellar fat pad [11], synovium [12], and the umbilical cord [13], 1.1 Cells