Scaffold properties influence the behavior of the implanted cells and
it is obvious that the porosity is important, not only for osteoblast
proliferation and matrix production but also for blood vessel ingrowth.
However, conflicting datamake it difficult to extrapolate in vitro findings
to the in vivo situation. In vitro, endothelial cell proliferation was
enhanced in scaffolds with smaller pore sizes [99], whereas osteoblast
proliferation was diminished [100]. In contrast, higher porosity and
pore size resulted in improved vascularization aswell as bone formation
in vivo [101]. A recently developed and interesting technique is the
fabrication of a vascular network within a polymer-based scaffold, for
example by photolithography [102] or 3D fiber deposition [103]. Here,
2D and 3D structures are created, serving as template to seed EC. An
alternative way to realize in vitro prevascularization is the co-culture
of EC and osteoblast cells. Several reports indicate that these co-cultures
were able to form microcapillary-like structures within the scaffold,
which remained stable during culture [104]. However, only limited
evidence exists whether these in vitro pre-made networks can connect
to the host vasculature when implanted in vivo [105].