**5. Summary**

formation in tissue engineered bones as it emerges as the most crucial factor in ensuring graft

Having a network of blood vessels within a tissue-engineered graft is important for maintain‐ ing cellular survival particularly within the core of large bone grafts [67]. It has been shown that after implantation, neo-bone tissues were found only at where a vascular network was present [68]. Poor angiogenesis has been identified as the main reason for implant failure and

Currently, there are various strategies that are under investigation for improving vasculari‐ zation in tissue engineered grafts. These include the induction of vascularization in vivo, the design of scaffolds specific to improving vascularization, and prevascularization techniques using coculture systems [72]. The prospect of functional vascularized bone graft for defect

Angiogenesis and vasculogenesis are natural vascularization process that occurs in tissue development and wound healing. The endothelial cells function as the main mediator of neovascularization through forming new blood vessels by angiogenesis and they can be expanded by vasculogenesis. In some studies, endothelial cells were used to generate capillary-like structure and connect vessels in vitro [73]. However, it is unclear as to whether these vascular generation approaches are effective in inducing vascularization in engineered bone graft in

In addition to directly using endothelial cells to form vessel network, some growth factors related to angiogenesis are used as another method to improve vascularization in engineered tissue. These factors include vascular endothelium growth factor (VEGF), platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) [74]. A major advantage of utilizing growth factors instead of living cells is that the risk of host rejection can be excluded. However, it seems inefficient using growth factors alone, because of the random formation of new vessels within implanted bone [68]. More importantly, the inappropriate delivery of growth factors may induce excessive angiogenesis which could cause severe pathogenic process such as

Aside from the selection of molecular and cellular mediator for effective vascularization, the choice of material used is also closely related to the vascular formation ability of the engineered bone graft. The scaffolds have been designed to allow vascular in-growth through a macro‐ porous structure and incorporating vascular cues such as with the use of growth factors and/

Over the past years, the selection of material candidates for bone tissue engineering scaffolds has been focused on the compatibility of bone cell attachment and growth. But now, much attention has also been switched to homing vascular formation [76]. The materials can impact

tumour development, atheroscleosis, and proliferative retinopathies [68, 71].

is currently acknowledged as a major challenge in tissue engineering [69-71].

healing brings a bright future for clinical application.

**4.2. Induction of angiogenesis and vasculogenesis**

**4.3. Scaffold design to promote vascular formation**

or cells, rather than just serving as osteoconductive surfaces [75].

survival and hence bone repair.

606 Regenerative Medicine and Tissue Engineering

vivo.

A large number of bone grafts are required annually for clinical treatment of severe bone fracture. The limitations in autograft and allograft restricted their clinical application. Alter‐ natively, tissue engineering approach may offer a new solution to produce bone grafts for clinical use. Over the last twenty years, tissue engineering of the bone has made remarkable progress, although the problems of translating into clinical application still remain.

Various types of stem cells have been used to form mineralized bone in vitro. In contrast, there were much fewer studies focused on the healing efficacy and its potential side effects. One main barrier is the complicated in vivo environment, which has profound interactions with the implanted cell types. This is especially so for allogeneic cells, where the host immune reaction is likely to play a very important role, with the macrophage system currently being under intense investigations [90, 91].

The use of biomaterials and development of scaffolds are especially important for engineering bone grafts, because they need to provide mechanical support during bone repair and bioactive aspect for bone formation. In order to obtain optimal mechanical properties, and high biocompatibility, numerous composite materials were designed to acquire integrated proper‐ ties from the individual components. Recently, more stringent requirements brought forth in the scaffold design for complete bone healing efficacy, such as inducing neo-vascularization during the formation of bone.

The achievements in engineering bone tissue so far are encouraging, while new challenges and opportunities are bringing the perspective of bone tissue engineering to a new height in clinical application. In the near future, tissue engineering approaches will achieve full tissue trans‐ plantation and engineered bone graft will be mature enough for bone fracture repair treatment.
