Fracture healing is a complex process and blood vessel formation,
controlled by locally produced growth factors and cytokines, has to be
tightly regulated and has to be coincident with the activation, proliferation
and differentiation of skeletal progenitor cells. Under optimal conditions,
these processes occur synchronously resulting in full restoration of
the original morphology and biomechanical properties of the fractured
bones [106]. The vascular system plays a key role in this process, not
only by supplying oxygen and nutrients, but also by delivering
osteoprogenitors to the fracture site giving rise to bone-forming cells
[54,67].
However, these healing processes are likely too slow for the
reconstruction of large bone defects created by tumor resection, trauma
or infection, where accelerated bone regeneration may be required to
prevent progression to a non-union. In addition, the regenerative
process might be disturbed in pathological conditions like avascular
necrosis or in diseases associated with impaired vascular function
[107]. To circumvent these problems, strategies might focus on novel
techniques for synthesizing scaffolds that serve as a template for
vascularization including 3D fiber deposition. Still, one of the main
problems in creating a cell-seeded construct of clinically relevant size
is the lag-time in blood vessel ingrowth, seriously hampering the
survival of the implanted cells in the scaffold. Therefore, an appealing
strategy might be to precondition the osteogenic cells prior to in vivo
implantation by adapting their metabolism to the environment. This
way, the implanted cells may remain viable during the early critical
time frame before blood vessels arrive, hereby hopefully improving
bone formation.
Taken together, tremendous progress has been made in unraveling
the importance of angiogenesis during bone healing, but novel
therapies have to be developed that favor vascular ingrowth and at
the same time promote the survival of the implanted cells, particularly
in patients with an inadequate angiogenic host response.