Microenvironment Design for Stem Cell Fate Determination - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 234

clearly offer some distinct advantages over genetic manipulation. For example, in
contrast to genetic manipulation, the effects of small molecules are typically fast
and reversible, providing more precise temporal regulation of protein function.
Synthetic molecules have been playing increasingly important roles in both
elucidating the fundamental biology of stem cells and facilitating the development
of therapeutic approaches to regenerative medicine. Such approaches will involve
therapies using homogenous functional cells produced under chemically-defined
conditions in vitro. The almost unlimited structural and functional diversity
endowed by synthetic chemistry provides small molecules with unbounded
potential for precisely controlling molecular interactions and/or recognition, a
feature that can be extensively explored by design and screening.
In most cases, the signaling molecule, either natural or synthetic, is simply
added to the growth medium of the cells. However, current stem cell research is
increasingly oriented towards spatio-temporal presentation of signaling molecules,
in similar fashion to that of the native ECM.
3 Synthetic Extracellular Matrix
The insoluble matrix, i.e., the ECM, is an important constituent of the microen-
vironment, affecting stem cell fate. Much attention has been given in previous
years to the design of matrices able to promote the appropriate spatial cell
arrangement. In addition to the obvious advantage of a high surface area per
volume ratio compared to monolayer systems, culturing in 3D matrices also
enhances cell-cell and cell-matrix interactions and allows better cell distribution
[ 48 ]. Specifically, the cultivation of hESC in scaffolds was shown to result in an
appropriate cell differentiation and neo-tissue formation, including blood vessels,
and an integration with the host upon implantation [ 49 ], in contrast to the
incomplete differentiation observed in two-dimensional (2D) studies.
Choosing the optimal scaffold requires studying stem cell interactions on both
the molecular and cellular levels, methods of scaffold fabrication and optimization
of scaffold properties to the specific needs of the stem cell population. The scaffold
is required to be biocompatible, biodegradable over an appropriate time scale, and
highly porous with large interconnected pores to provide efficient mass transport,
cell permeation and interstitial fluid flow [ 20 ]. The scaffold temporarily provides
the physical support for the seeded cells in culture until they produce their
own ECM. Therefore, the matrix should be mechanically stable and suitable for
cultivation either in a bioreactor or at the implant site [ 50 ]. However, the matrix
should be also flexible enough to allow cell reorganization into a 3D tissue and its
subsequent integration with the host tissues [ 51 ].
The ideal scaffold for stem cell applications is also required to introduce the
stem cell to the right biochemical cues in a spatio-temporal fashion, similar to that
of the native ECM. Preferably, the scaffold should dynamically interact with the
cells and be adaptable to various cellular changes in culture.