Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 325

numerous known and as-yet unidentifi ed factors that can infl uence the
structure and function of these very temperamental and fragile cells. In
vivo , VSMC phenotype was found to be modifi ed by the cells being seeded
into three-dimensional collagen gels compared with two-dimensional struc-
tures (Stegemann and Nerem, 2003).

12.6 Stem cells

Stem cells are a feasible source of vascular endothelial and smooth muscle
progenitor cells which may have valuable applications in vascular tissue
engineering. Employment of these cells in TEBVs for clinical use remains
distant, as much remains to be investigated. Embryonic and adult stem cells,
endothelial progenitor cells and induced pluripotential stem cells (iPSCs)
are being considered and investigated to augment or possibly replace tra-
ditionally used mature cell sources.
The pluripotency of stem cells is both advantageous and disadvanta-
geous. A stem cell's degree of potential pluripotency depends on whether
it is embryonic or adult, which in turn infl uences its capability of differen-
tiating into the cells constituting the required tissue type. Embryonic stem
cells may differentiate into practically any cell type while adult stem cells
generate cells particular to their tissue of origin. The fact that stem cells
are a relatively easily accessible source of autologous young cells is another
obvious advantage. There is an almost unlimited source of embryonic stem
cells which are relatively easy to manage and study in an in vitro environ-
ment. In contrast, adult stem cells, which are infrequently found in their
host tissue, are diffi cult to isolate and culture in vitro . If one is to generate
suffi cient quantities of vascular cells for seeding on an engineered vascular
conduit, large numbers of stem cells would be required. Embryonic stem
cells and iPSCs may therefore be more suitable for vascular tissue engineer-
ing applications.
There are a number of critical benefi ts to employing adult stem cells in
tissue engineering applications. Sourcing adult stem cells from the patient
who actually requires them extricates scientists and clinicians from the
ethical dilemma associated with embryonic stem cell use. In addition, a
patient's own adult stem cells may be expanded in vitro , differentiate and
then be reintroduced after being seeded into an engineered conduit (NIH,
2009). Employing autologous cells would reduce the possibility of rejection
of the newly transplanted cells. It is unknown whether patients, in whom
human embryonic stem cells are transplanted, will require immunosuppres-
sants to prevent rejection. The world's fi rst human clinical trial investigating
the safety of human embryonic stem cells was recently approved by the
FDA. This trial will employ oligodendrocyte progenitor cells derived from
human embryonic stem cells (NIH, 2009).

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