This allows for scaffold colonization and for cell differentiation, before grafting of this processed composite material at the affected site, prior to implantation into the same patient [73]. For bone reconstruction purposes, human MSCs have been seeded and cultured on porous calcium VX-809 phosphate ceramics in osteogenic

media (dexamethasone, ascorbic acid, β-glycerophosphate). Early proposals lead to clinical studies with low numbers of patients using this approach, but the outcomes were inconsistent showing low efficacy in bone regeneration. From these, it is clear that the strategy requires significant tuning [74] and [75]. The reasons of the limited clinical success may be due to several bottlenecks in the multidisciplinary field of bone tissue engineering, particularly about biomaterials and cell limitations. Biomaterials used as bone void fillers are inspired by the bone extracellular matrix (hydroxyapatite, collagen I) but need to be colonized by cells and vascularized in order to promote bone tissue formation and healing. The regenerative capabilities of current biomaterials are still limited to small bone defects. Regarding cell limitations,

barriers are found in the autologous approach, the cell selection, the association DAPT cost of cells and materials, and the

osteogenic differentiation of implanted cells. The autologous approach for isolation and osteogenic differentiation of MSCs is highly Ribonucleotide reductase demanding in terms of logistics, production and safety of culture conditions leading to a costly therapeutic procedure. The selection of a restricted population of cells from different donors with age and genetic diversities remains a challenge for regenerative medicine at this early stage of research due to patient variability. The association of biomaterials and osteoprogenitor cells raises technical challenges (i.e. cell sources, types, doses, timing) and regulatory issues (devices with medicinal drugs) to implement clinical trials. Moreover, bone formation requires different cell populations that cooperate to set up complex 3D tissue under the guidance of biomechanical cues while vascularization plays a major role in tissue healing. Finally, osteogenic differentiation induced in vitro is not fully supported by the in vivo release of osteogenic factors from the graft itself. An alternative to the previous strategies is to implant the composite material (cell + scaffold) into a heterotopic site, e.g.