Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 323

fi nite proliferation capability of these cells which contribute considerably
to maintaining vessel homeostasis.
In an interesting study using a rabbit model, Yu et al. reported that
seeding ECs together with VSMCs on to the lumen of a PTFE graft
improved the retention of ECs. This occurred despite the PTFE conduit
being grafted into, and hence subjected to, high velocity and pressure fl ow
in the rabbits' aorta. Cell retention was determined after only one hour of
blood fl ow through the PTFE conduit. Despite this limitation, the
improvement in cell retention with dual seeding of ECs and SMCs, com-
pared with ECs being seeded alone, was signifi cant (Yu et al. , 2003). The
aforementioned study provides more evidence that TEBVs should
perhaps copy the naturally occurring vessels in structure in order to mimic
their function.
ECs have been seeded onto synthetic grafts and decellularised scaffolds
with varying degrees of success. Various techniques have been employed
to seed ECs onto the luminal surface of the conduit. In addition, the luminal
surface has been coated with various substances to encourage initial adhe-
sion and subsequent sustained retention of ECs.
An interesting application of ECs and their progenitor cells is in the
promotion of angiogenesis. One of the critical factors restricting progress
in the development of tissue engineered organs in three dimensions is a
reliable blood supply.
Autologous, mature ECs harvested from various sources, including sub-
cutaneous fat, may not function in a similar fashion, as their phenotype
varies remarkably from one tissue to another (Aird, 2007; Tiwari et al. ,
2003a). It may thus be preferable to employ ECs harvested from the vas-
culature to line tissue engineered vessels, although there is probably some
phenotypic variation between ECs harvested from veins and arteries.
Whether one should contemplate preferentially employing ECs, harvested
from an arterial source, for use in a tissue engineered conduit, which is to
be grafted into the arterial circulation, is unclear. Phenotypic variation
may even exist between ECs harvested from larger diameter arteries and
those harvested from smaller diameter arteries. Restricting the source of
ECs for vascular tissue engineering applications to arteries with a diame-
ter which the tissue engineered conduits are being manufactured to
replace or augment would be extremely limiting. This restriction would
compound the diffi culties already being reported with the use of ECs.
ECs harvested from patients' blood vessels are restricted in their prolifer-
ation potential and expand very slowly. Large numbers of ECs would
therefore be harvested for therapeutic use (Alsberg et al. , 2006). Alter-
nate sources of ECs are now being sought which will be able to be used
without concerns about limited resources and phenotypic variability.
Although employing stem cells in engineered vascular grafts is being

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