Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 334

As our knowledge of stem cells becomes more sophisticated, the suit-
ability of employing these cells within engineered vascular grafts will
become clearer. Although stem cells present exciting opportunities in pro-
gressing towards the holy grail of 'off-the-shelf' vascular grafts, a number
of issues require clarifi cation. The in vitro expansion of human stem cells,
which may potentially differentiate into vascular cells, remains a challeng-
ing undertaking. In addition, the selection and phenotyping of these stem
cells subtype remains unrefi ned. In vivo , stem cells interact with other cells,
growth factors and other cofactors in their environment which stimulates
their natural differentiation into tissue-specifi c cell type. Crude selection
methods, while the stem cells are still suboptimally differentiated, may
lead to the selection of inappropriate stem cells which may induce contami-
nation, hence causing more harm than good to the engineered vascular
conduits. Recently, a technique for tracking the development and fate
of vascular progenitor injected into a murine model was reported
(Schmeckpeper et al. , 2009). Lentiviral vectors containing human EC-
specifi c, i.e. VE-Cadherin, and human SMC-specifi c, i.e. Smoothelin-B,
markers were used to drive the expression of green fl uorescent protein. Bone
marrow progenitor cells transduced with these vectors fl uoresced within 14
days in vitro and the cells injected subcutaneously into a murine model also
fl uoresced. Developing these techniques, to ensure their safe deployment in
tissue engineering, will require considerable fi nancing in a period of ever-
decreasing budgets. A revisiting of ethical concerns will also be necessary.

12.6.4 Induced pluripotent stem cells

iPSCs are an exciting potential new source of ECs and VSMCs which avoids
the ethical implications of employing embryonic stem cells (Taura et al. ,
2009). In a landmark discovery in 2006, Takahashi and Yamanaka repro-
grammed mouse and human skin fi broblasts, i.e. somatic cells, to a pluripo-
tent state using a defi ned set of transcription factors (Takahashi and
Yamanaka, 2006; Takahashi et al. , 2007). The transcription factors Oct 3/4,
Sox2, Klf4 and c-Myc were introduced into the cells using retroviral-
mediated vectors. The Oct3/4, Sox 2 and Nanog transcription factors are
considered essential to embryonic stem cells maintaining their pluripotency
(Boyer et al. , 2005; Loh et al. , 2006; Nichols et al. , 1998). The transcription
factors that were obligatory for the generation of iPSCs were Oct3/4 and
Sox 2 (Takahashi and Yamanaka, 2006). The development of a novel stem
cell population was partly motivated by reports that unfertilised eggs and
embryonic stem cells contain factors that confer pluripotency to somatic
cells (Cowan et al. , 2005; Tada et al. , 2001; Wilmut et al. , 1997). The iPSCs
are a potential source of patient-specifi c cells which may be used in tissue
engineering applications.

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