Microenvironment Design for Stem Cell Fate Determination - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 248

screened immobilized small molecules for their ability to induce hMSCs to dif-
ferentiate down several pathways. The small molecules were chosen to incorporate
functionalities found in the extracellular environment of the target cell types.
Specifically, PEG hydrogels were modified with small amounts of carboxyl groups
to mimic glycosaminoglycans in cartilage, phosphate groups for their role in bone
mineralization, or t-butyl groups to mimic the lipid-rich environment in adipose
tissue. HMSCs were encapsulated in the tethered PEG hydrogels and cultured in
standard hMSC media, without the added cytokines or steroids typically used in
differentiation media. Most interestingly, these synthetic matrices were successful
in inducing the differentiation of hMSCs into the chondrogenic, osteogenic or
adipogenic pathways respectively [ 111 ]. This study was the first example where
synthetic matrices were shown to control induction of multiple hMSC lineages
purely through interactions with small-molecule chemical functional groups
tethered to the hydrogel material. Strategies using simple chemistry to control
complex biological processes would be particularly powerful as they could make
production of therapeutic materials simpler, cheaper and more easily controlled.
3.2.4 Mechanical Features of the Matrix
Stem cells sense and respond to the mechanical properties of the extracellular
matrix. These properties are determined primarily by the matrix chemical com-
position, water content and structure. Previous studies using differentiated cell
types have demonstrated that the mechanical properties of a material affect cell
behaviors such as growth and migration [ 176 - 182 ]. In particular, adhesion ligands,
which bind to integrins and other cell surface receptors, serve as mechanical
transducers between the external material and the internal cytoskeleton of the cell,
allowing cells to sense and respond to the stiffness of their substrates. Tensional
homeostasis with the microenvironment thereby induces cellular cytoskeletal
organization and alters gene regulatory pathways.
The extent of matrix mechanical effects on stem cell fate in 3D microenvi-
ronments and the underlying biophysical mechanisms are still under investigation.
Recent studies have demonstrated the integral role of mechanical cues in the
commitment of stem cell fate. For example, McBeath et al. [ 183 ] demonstrated
that hMSC lineage commitment could be largely regulated by cell shape and size
via related changes in the cytoskeleton tension. In their study, they used spatial
micro-patterning of adhesion molecules to control cell shape and degree of
spreading with single cell precision. HMSCs patterned on a small island tended to
undergo adipogenic differentiation, whereas cells patterned on a larger island were
able to spread and develop high cytoskeleton tension, and tended to undergo
osteogenic differentiation [ 183 ]. The researchers suggested an explanatory
mechanism for their results, in which the commitment signal for the osteogenic
differentiation required actin-myosin-generated tension.
In an additional study, the correlation between tension forces and cell differ-
entiation in monolayer cultures of hMSCs was nicely demonstrated [ 184 ]. Ruiz