Microenvironment Design for Stem Cell Fate Determination - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 243

EBs and induced the formation of cystic spheroids uniquely resembling the
phenotype and structure of early mouse embryos [ 154 ].

Covalent Binding of Signaling Molecules onto the Matrix

Covalent binding of signaling molecules onto the matrix facilitates their prolonged
presentation and biological activity, in case long-term persistence of the signal is
needed [ 155 , 156 ]. The covalent binding of the factor to the scaffold overcomes the
natural internalization process of the factor-receptor complex, which includes its
endocytosis and degradation inside the cell. Thus, tethering the signaling molecule
to the matrix may prevent ligand depletion from the environment. Moreover,
tethered ligands represent a mimicry of ligands that are naturally insoluble, matrix-
''tethered'' i.e., a ligand which acts from the matrix, presented continuously to cell
surface receptors [ 157 ]. However, as opposed to the temporal presentation of
factors

by natural ECM, covalent binding presents an unnatural permanent

presentation.
Using this strategy, Fan et al. [ 156 ] showed that biomaterial surfaces covalently
modified with epidermal growth factor (EGF) promoted both MSC spreading and
survival, more strongly than saturating concentrations of soluble EGF added to the
medium. By sustaining mitogen-activated protein kinase kinase-extracellular-
regulated kinase signaling, tethered EGF increased the contact of MSCs and
conferred resistance to cell death induced by the pro-inflammatory cytokine, Fas
ligand. Thus, the authors suggested that tethered EGF may offer a protective
advantage to MSCs in vivo during acute inflammatory reaction to tissue engi-
neering scaffolds [ 156 ].
Immobilization of growth factors to biomaterial surfaces in specific spatially-
defined patterns was carried out by bio-printing technologies [ 158 , 159 ]. For
example, inkjet bio-printing technology was used for the creation of an immobi-
lized BMP2 printed pattern on fibrin substrate. The results indicated that
the differentiation of primary muscle-derived stem cells towards the osteogenic
lineage was confined to the printed patterns of BMP2, whereas cells off-pattern
differentiated towards the myogenic lineage [ 158 ]. This approach may also be
useful for understanding cell behavior towards immobilized biological patterns.
For instance, recent work has focused on the effect of spatially-controlled gradi-
ents on MSC migration [ 160 ].

Affinity Binding and Presentation of Signaling Molecules

Despite the fact that growth factors and other signaling molecules were
originally described as soluble molecules, evidence shows that the binding of
signaling factors to ECM is a major mechanism regulating their activity.
A bio-inspired approach for presentation and release of these factors exploits
their natural interactions with native ECM. For instance, in the native cellular