Biomedical Engineering Reference
In-Depth Information
Nanoengineering Approaches to Study Stem-Cell Fate
Self-renewal or differentiation of stem cells into other cell types depends on the biochemical
cues of the stem-cell microenvironment as well as the stiffness of the substratum and cell
density. Different nanoengineered tools, like SAMs or microcontact printing, have aided to
regulation and understanding of the fate of stem cells.
It has been found that SAMs when conjugated to various ligands, such as peptides or
proteins, influence the proliferation and differentiation of the stem cells. For instance,
RGD peptide conjugated SAMs induce osteogenic or adipogenic differentiation [23, 24].
In addition different polymer coatings also induce stem-cell differentiation. In this regard,
Joy
et al.
have coated the matrix surfaces with different polymer compositions and studied
the effect on the differentiating ability of the stem cells, and the results suggested that the
surface chemistry had a major influence on the osteogenic and adipogenic differentiation
of hMSCs [25]. High-throughput biomaterial platforms also can aid in the study of deter-
mining stem-cell fate. Derda
et al.
have investigated the growth of hESCs with arrays hav-
ing 18 different laminin-derived peptides [26]. It was found that RNIAEIIKDI laminin
peptide could not promote differentiation, whereas LGTIPG peptide resulted in
differentiation of hESCs. Thus, they demonstrated that high-throughput platforms will be
useful for quick identification of peptide sequences that play a role in regulating stem-cell
fate. Furthermore, in another study, Anderson
et al.
have examined the growth and
differentiation of hESCs into cytokeratin positive cells on a microarray containing 1728
polymer mixtures [27].
A major breakthrough with microcontact printing and understanding of stem-cell
fate was achieved by McBeath
et al.
, who showed how cell-ECM interaction and cell
shape regulated stem-cell behavior [28]. They exposed the hMSCs to bipotential
differentiation medium that contained inducers for both adipogenic and osteogenic dif-
ferentiations and studied the behavior of the hMSCs. It was found that the hMSCs that
were confined to small ECM areas selectively underwent adipogenesis, whereas the
hMSCs present on large ECM islands were differentiated into osteogenic lineage. This
osteogenic-adipogenic switch between large-spread versus small spread areas necessi-
tates generation of cytoskeletal tension by a RhoA/ROCK-dependent actomyosin con-
tractility mechanism. In this regard, Connelly
et al
. have shown that confinement of the
cell-spread area to small ECM islands would result in terminal differentiation of human
epidermal stem cells [29].
Different soluble factors found in the stem-cell niche have been demonstrated to play a
major role in regulating stem-cell growth and differentiation. For instance, cytokine leu-
kaemia inhibitory factor (LIF) helps to retain the self-renewal ability of embryonic stem cells
in the presence of serum but absence of embryonic fibroblast feeder cells [30]. Furthermore,
combination with other factors can vary the effect. For example, bone morphogenetic protein
4 (BMP4) and LIF in combination with serum-free media promote embryonic stem cells for
self-renewal, whereas BMP4 alone promotes ventral mesodermal differentiation [31]. In this
regard, several approaches have been utilized to obtain a gradient concentration of soluble
factors found in the stem-cell microenvironment. Among these, microfluidic technology
along with laminar flow can achieve molecular gradients in soluble form within the microflu-
idic channel. Micro- or nanofluidics allow regulating tiny volumes of solutions containing
multiple soluble factors. For example, microfluidic alginate hydrogel is able to control the
release of different soluble factors in three-dimensional microenvironments. Therefore, micro-
or nanoengineered tools can control the chemical environment on a nanometer scale within
a macroscopic scaffold, which in turn is useful for engineering complex tissue models.
