Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 328

etic abnormalities may develop in cells that undergo differentiation in vitro
(Heng et al. , 2004; Rufaihah et al. , 2007; Sinha et al. , 2006).
The recognised markers commonly used to identify ECs include plate-
let EC adhesion molecule (PECAM), vascular endothelial growth factor
(VEGF) receptor 2 (Flk1) and vWF. Haematopoietic stem cells and hae-
matopoietic progenitor cells may also be identifi ed using PECAM, Flk1
and vWF, hence further complicating the identifi cation of ECs from the
haemangioblast population (Kim and von Recum, 2008). The differentia-
tion of mouse embryonic stem cells is dependent on time and the environ-
ment (Nishikawa et al. , 1998; Yamashita et al. , 2000). The differentiation
of mouse embryonic stems cells was promoted when these cells were cul-
tured in a two-dimensional environment, on specifi c ECM components,
such as type IV collagen and in the presence of cytokines. The pluripo-
tency of haemangioblasts was clearly verifi ed in Flk1 expressing cells
which differentiated into ECs in the presence of VEGF. In the presence
of platelet-derived growth factor-BB (PDGF-BB), Flk1 expressing cells
differentiated into mural cells, i.e. VSMCs and pericytes (Yamashita et al. ,
2000).
The differentiation potential of human embryonic stem cells has also
been demonstrated in vitro (Ferreira et al. , 2007b; Schuldiner et al. , 2000).
Human embryonic stem cells have been demonstrated to differentiate
into both ECs (Levenberg et al. , 2002) and VSMCs in two and three-
dimensional systems (Gerecht-Nir et al. , 2003; Levenberg et al. , 2002).
Although the report by Gerecht-Nir et al. (2003), was intended to empha-
sise the potential in vitro use of human embryonic stem cells, this study also
demonstrated the exciting potential of these cells differentiating into ECs
and VSMCs. The investigation of human embryonic stem cells in vivo is
non-existent due to the absence of a knock-out model for the human
species. Progress in investigating human embryonic stem cells is therefore
dependent on in vitro models (Yamashita, 2004).
Techniques have been reported to enhance human embryonic stem cells'
differentiation into functional vascular cells in vitro . One approach involved
seeding the human embryonic stem cell aggregates into a three-dimensional
dextran-based hydrogel together with VEGF-impregnated microparticles.
Following incubation in this environment for ten days, the cells were
extracted and incubated in two-dimensional environment. Spatial arrange-
ment and enhancement, by bioactive molecules, of human embryonic stem
cells enabled derivation of large quantities of vascular cells. The enhanced
human embryonic stem cells had a reduced capability to differentiate into
other germinal layers (Ferreira et al. , 2007a). In further reports, human
embryonic stem cells differentiated into ECs in the presence of VEGF,
while PDGF-BB induced the derivation of VSMCs (Ferreira et al. , 2007b;
Yamashita et al. , 2000). When these differentiated cells were injected

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