Biomedical Engineering Reference
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retention of 'stemness' in mouse embryonic stem cells (mESCs) and the
degree of cell attachment.
65
Increased cell spreading and the absence of
three-dimensional (3D) colony organisation were observed to coincide
with loss of stem cell marker expression. However, others have suggested
that a comparatively larger colony size is optimal for maintaining cell
pluripotency.
66
Preliminary studies have demonstrated that tailoring a surface's mech-
anical properties can direct stem cell differentiation to a particular cell
type.
2,67-70
For example, mesenchymal stem cells (MSCs) were shown to be
very sensitive to substrate elasticity, allowing for elasticity-directed cell dif-
ferentiation (Figure 10.2A).
2
Polyacrylamide-based gradient gels were pre-
pared where the matrix ranged from soft to relatively rigid depending on the
extent of cross-linking. The soft substrates, mimicking the elasticity of brain
tissue, resulted in the stem cells displaying a neuronal phenotype. On the
rigid surfaces whereby the surface had comparable elasticity to bone,
osteoblast-like characteristics were observed from the cells. Similarly, on the
intermediate elasticity surfaces, muscle cells were evident. Expression of
transcriptional markers associated with neuronal, muscle and osteogenic
cell lines were consistent with morphology observations.
Curran et al.
62,71
investigated the effect of different surface functional
groups on the differentiation of MSCs (Figure 10.2B). A variety of silanes with
different terminal groups including -NH
2
, -CH
3
, -SH, -OH and -COOH were
deposited onto glass slides. Real time polymerase chain reactions were used
to observe the expression of several differentiation markers including:
b-actin (increase in expression), orinithine decarboxylase (indication of
proliferation), collagen II (chondrocytes), CBFA1 (bone transcription factor),
collagen I (MSC marker), TGF-b3 (ECM production)). MSCs cultures main-
tained their stem-cell phenotype on -CH
3
modified surfaces but -COOH,
-OH, -NH
2
and -SH surfaces promote osteogenesis. McBeath et al.
72
dem-
onstrated that the cell shape, namely the spreading area, influences hMSC
differentiation through growing cells on 'spots' of Fn proteins of different
sizes (Figure 10.2C). hMSCs that were allowed to spread out (i.e., larger
spots) were shown to differentiate into osteoblasts, whilst hMSCs with re-
stricted spreading underwent adipogenesis.
Through the use of microarrays displaying 576 combinations of 25 poly-
mers (derivatives of acrylate, diacrylate, dimethacrylate and triacrylate
monomers), Anderson et al.
56
observed that the majority of such combin-
ations were able to influence the differentiation of human embryonic stem
cells into epithelial-like cells. This technique is promising for tissue engin-
eering applications to create specific tissue constructs based on the com-
position of the underlying polymer surface. Similarly, Derda et al.
73
have
utilised microarrays of peptides immobilised onto self-assembled mono-
layers (SAMs) to determine peptidic surfaces that support embryonic stem
cell growth and self-renewal. They investigated 18 different laminin-
containing peptides to identify which properties supported proliferation and
self-renewal. It was found that five of the 18 peptides supported proliferation
d
n
3
r
4
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