Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 329

subcutaneously into a murine model, human microvessels were observed
after 28 days (Ferreira et al. , 2007b).
In the fi rst report of its kind, the derivation of ECs from human
embryonic stem cells through genetic modulation with recombinant DNA
was examined. Human embryonic stem cells were transduced with an
adenoviral vector expressing the human VEGF 165 gene which facilitated
the over-expression of the VEGF transgene. The increased presence of
VEGF in differentiating human embryonic stem cells resulted in a fi ve-
fold increase in CD133, an EC marker (Rufaihah et al. , 2007). The afore-
mentioned technique was employed owing to the ineffi ciency of adding
growth factors and cytokines to the culture medium in which the human
embryonic cells are bathed (Gerecht-Nir et al. , 2003; Heng and Cao,
2006; Levenberg et al. , 2002). The use of genetic modulation of human
embryonic stem cells in vivo raises critical safety concerns which must be
rigorously examined before this technique is employed (Rufaihah et al. ,
2007).
When compared with other sources of ECs, embryonic stem cells are
capable of higher rates of proliferation, pluripotency and low immunoge-
nicity (Levenberg et al. , 2007; Reubinoff et al. , 2000; Thomson et al. , 1998).
Before human embryonic stem cells may be employed in clinical applica-
tions, methods need to be developed that will ensure consistent isolation
and focused differentiation into potent vascular cells. The techniques that
are required have to be further developed before they be regarded as akin
to those used to study murine embryonic stem cells (Kim and von Recum,
2008).
In another approach, human embryonic stem cells were cultured in a
two-dimensional environment as a monolayer. After the addition of reti-
noic acid to the culture milieu, human embryonic stem cells differentiated
into SMCs, in a dose-dependent manner. SMC markers such as α SMA,

SMMHC and SM-22 were employed to detect human embryonic stem

cells which had successfully differentiated into SMCs. The results of this
report are intriguing as cells, with SMC-like characteristics were derived
which were capable of contracting (Huang et al. , 2006). Retinoic acid was
also employed to derive cells, with SMC-like characteristics that responded
to vasoactive agents, from mouse embryonic cells (Drab et al. , 1997).
Using an alternative technique, human embryonic stem cells obtained
from embryoid bodies differentiated onto SMC-like cells in vitro in a two-
dimensional model. By altering the culture surface coating from matrigel
to gelatine and the constituents of the culture milieu, cells adopted physical
and functional characteristics resembling SMC-like cells (Xie et al. , 2007).
The derived cells expressed SMC markers in a time-dependent manner
which was quantifi ed using real-time reverse transcription polymerase
chain reaction (RT-PCR).

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