Biomedical Engineering Reference
In-Depth Information
a degree of pluripotency when removed from their stem-cell niche and when appropriate
growth factors and reagents are added to the ESC culture medium. They obtained pluripo-
tent ES-like cells from germ cells without any selection procedure and claimed that this simple
method may be more suitable if the procedure is refined for clinical application to obtain
pluripotent cells to cure disease.
Although SSCs are unipotent stem cells that express the transcription factors Oct4,
Sox-2, lin-28, Myc, and Klf4 (Kruppel-like factor 4), the expression of Nanog is not detected
in SSC culture [24].
Regulation of PSCs
Pluripotent cells such as ESCs, iPSCs, and ES-like cells from mouse testis show unlimited
proliferation in the presence of feeder, growth factors, and high concentration of fetal bovine
serum (FBS), while maintaining their pluripotency state during
in vitro
culture. Growth
factors such as BMP4 (bone morphogenetic protein 4) and LIF (leukemia inhibitory factor)
can be substituted with serum and feeder cells in mouse ESC (mESC) culture, while keeping
their self-renewal and pluripotency [25]. The LIF and BMP4 signaling induce activation of
signal transducer and activator of transcription 3 (STAT3) and SMAD1 (mothers against
decapentaplegic homolog 1) respectively [25, 26]. In human ESC (hESC) culture, BMP4 pro-
motes differentiation to trophoblast [27], while Activin/Nodal and fibroblast growth factor
(FGF) signaling pathways maintain self-renewal and pluripotency of hESCs during
in vitro
culture [28]. In addition, of these signaling pathways, activation or inhibition of STAT3 [29],
mitogen-activated protein kinase (MAPK), and glycogen synthase kinase 3 (GSK3) [30] are
very significant for maintenance of the pluripotency state.
Nichols and Smith proposed two phases of a naive and prime pluripotency state [31].
Naive cells similar to ESCs or early epiblast cells can form teratoma and chimera whereas
prime cells similar to late epiblast stem cells or post-implantation epiblast stem cells can
form teratoma but they rarely form chimera [31].
It is generally accepted that the expression of the core transcriptional pluripotency
genes, which include Sox2, Nanog, and Oct4, in addition to other genes, play important
roles throughout the development and regulation of pluripotent stem cells
in vitro
. These
transcription factors regulate the expression of lineage-specific gene families and keep
maintenance of pluripotent cells from differentiation [32]. It has been well documented
that microRNAs also can regulate expression of core pluripotency transcription factors in
hESCs [33]. Expression of the Oct4 gene inhibits neural ectodermal differentiation
and supports mesendodermal differentiation, while Sox2 hampers mesendodermal
differentiation and enhances neural ectodermal differentiation [34]. In ESCs, a critical
amount of Oct4 mRNA expression seems to be necessary to maintain an undifferentiated
state of pluripotency [35]. For example, it has been shown that overexpression of Oct4
mRNA up to about twofold causes loss of pluripotency and differentiation into primitive
endoderm and mesoderm cells, while 50% repression of Oct4 promotes dedifferentiation
to trophectoderm cells [36]. Sox2-deficient blastocysts were shown not to be successful in
shaping a pluripotent inner cell mass (ICM), and ESCs with depletion of Sox2 lost pluri-
potency and differentiated to terophectoderm cells [37].
Nanog could sustain mESCs self-renewal independently of the LIF/STAT3 signaling
pathway and Nanog null ICM fail to generate epiblast cells [38]. Nanog null mESCs lead to
loss of pluripotency and differentiation into extraembryonic endoderm lineage [38]. Nanog
inhibits gastrulation and cell migration in the embryo [39] and is required specifically for
germline development [40]. Expression of the c-myc gene functions in cell growth,
