Biomedical Engineering Reference
In-Depth Information
germline-like embryonic stem cells, suggesting that these iPS cells were completely repro-
grammed [61]. The number of reprogrammed colonies increased following initiation of drug
selection at day 20 rather than day 3 post-transduction, which suggests that reprogramming
is a slow, gradual process and may explain why previous attempts resulted in incomplete
reprogramming. Recently, Yamanaka's group generated human iPS cells that are similar to
hESCs in terms of morphology, proliferation, gene expression, surface markers, and tera-
toma formation; additionally, Thompson's group showed that retroviral transduction of the
stem-cell markers OCT4, SOX2, NANOG, and LIN28 could generate pluripotent stem cells.
In both studies, the human iPS cells were similar but not identical to hESCs [62,63]. The
potential of reprogramming is limited only by our current understanding of the mechanisms
controlling it, which must be understood before clinical applications can begin.
Amniotic Fluid and Placental Stem Cells
Stem cells have also been isolated from amniotic fluid and fetal placenta. Amniotic fluid
and the placenta are known to contain multiple partially differentiated cell types derived
from the developing fetus. Our group isolated stem-cell populations from these sources,
called amniotic fluid and fetal placental stem cells (AFPSCs), which express embryonic and
ASC markers [64]. The undifferentiated stem cells can be extensively expanded without
feeders and have a doubling time of 36 hours; moreover, they can be maintained for over
250 population doublings and retain long telomeres and a normal karyotype. AFPSC,
unlike hES cells, do not form teratoma
in vivo
[64]. These cells meet the standard accepted
criteria for pluripotent stem cells in that clonal human lines, which have been verified by
retroviral marking, can differentiate toward the three cell lineages composing the embryonic
germ layer to ultimately generate fat, bone, muscle, endothelial, nervous, and liver adult
cells [64]. Furthermore, adult cells differentiated from these stem cells display specialized
functions, including nerve cells secreting the neurotransmitter L-glutamate or expressing
G-protein-gated inwardly rectifying potassium (GIRK) channels, liver cells producing urea,
and bone cells forming tissue-engineered bone. These stem cells could be obtained either
from amniocentesis or chorionic villous sampling in the developing fetus, or from the pla-
centa at the time of birth, and the cells could be preserved for self-use and used without
rejection [64]. Additionally, a cell bank of 100,000 specimens may be established that could
potentially supply 99% of the US population with a genetically identical match for trans-
plantation. Such a bank may be easier to create than with other cell sources, because there
are approximately 4.5 million births per year in the USA [64].
Extracellular Matrix
In Vivo
: Components and Behavior
of Cells on Them
Biomaterials for Use in Regenerative Medicine
Cell seeding onto a scaffold composed of appropriate biomaterials is the basis of cell-based
tissue engineering. Biomaterials mimic the biological and mechanical function of the native
extracellular matrix (ECM) found in tissues of the body. Exogenous ECMs are designed to
bring the desired cell types into contact in an appropriate three-dimensional environment
(Figure 4.2); furthermore, they allow for the delivery of cells and requisite bioactive factors (cell
adhesion peptides and growth factors) to targeted sites in the body [65]. As the majority of
mammalian cell types are anchorage-dependent, biomaterials supply a cell-adhesion substrate
to maintain cell viability. In addition, biomaterials can provide mechanical support against
in vivo
forces, to maintain the three-dimensional structure during tissue development.
