Microenvironment Design for Stem Cell Fate Determination - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 250

suggested that cells interpret changes in the physical properties of adhesion
substrates as changes in adhesion-ligand presentation [ 187 ].
Whereas all the above-mentioned studies utilize static polymer systems, their
native counterparts reside in a dynamic environment in which elasticity may
change spatially and/or temporally. In a recent study, Tse et al. [ 188 ] tried to
explore the potential signal of physiological stiffness gradients to MSC differen-
tiation. To that end, they cultured MSCs on a photo-polymerized PA hydrogel of
varying stiffness and provided the first evidence that MSCs indeed appear to
undergo directed migration, or durotaxis, up stiffness gradients rather than remain
stationary. Temporal assessment of morphology and differentiation markers indi-
cated that MSCs migrated to stiffer matrix and then differentiated into a more
contractile myogenic phenotype. This study may indicate that stiffness variation,
not just stiffness alone, can be an important regulator of MSC behavior [ 188 ].

3.2.5 Complex Hierarchical Matrices

New generations of interactive biomaterial scaffolds are now being developed for
the construction of hierarchically complex tissues, such as vascular networks,
interfaces, structural hierarchy, and other complex functional features. These
scaffolds need to establish compositional gradients or sub-compartments, temporal
changes, and control over cell-driven tissue and organ morphogenesis. For
example, in an attempt to design scaffolds for the appropriate layered structure of
various native tissues (as skin and cartilage-bone interface), layered systems have
been developed, to allow the creation of optimized, tissue-specific biological
environments in each respective layer via variations in mechanical, structural and
chemical properties. The bi-layered system designs were shown to be of great
potential for organized tissue growth, when implanted in vivo, either seeded with
mature cells [ 189 ], or acellular [ 190 ]. Furthermore, recent studies have pointed
out that the use of bi-layered system may be a powerful tool for the spatially-
controlled simultaneous induction of stem cells into distinct lineages.
One example is the design of a composite bi-layered hydrogel of PEGylated
fibrinogen and type I collagen, aimed at approximating the layered structure of
skin. The bi-layered matrix was able to control the bidirectional differentiation of
adipose-derived stem cells into endothelial cells and pericytes. Specifically,
matrix-driven phenotypic changes into a fibroblast-like morphology were observed
in the collagen layer, whereas a tube-like morphology was simultaneously detected
in the PEGylated fibrin layer. The matrix composition dictated the lineage spec-
ification and was not driven by soluble factors [ 191 ].
In another study, injectable, biodegradable hydrogel composites of cross-linked
oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs)
were utilized to fabricate a bi-layered osteochondral construct, consisting of a
chondrogenic layer and an osteogenic layer. Rabbit MSCs were encapsulated in
both layers, and were able to undergo chondrogenic differentiation, in the presence
of TGFb1-loaded MPs. Although simultaneous differentiation of MSCs into