Biomedical Engineering Reference
In-Depth Information
provide a unique system for studying the events in
human embryonic development. The hES cells have the
potential to generate all embryonic cell lineages when
they undergo differentiation. Differentiation of hES can
be induced in monolayer culture or by removing the cells
from their feeder layer and growing them in suspension.
This differentiation in suspension results in aggregation
of the cells and formation of embryoid bodies (EBs),
where successive differentiation steps occur.
From human ES cells we might be able to develop new
transplantation therapies to replace diseased or aged cells
or tissues. To this end, researchers need to develop
methods with which they can derive from human ES
cells their required cell types, such as cardiomyocytes or
hematopoietic cells. Chemical cues provided directly by
growth factors or indirectly by feeder cells can induce ES
cell differentiation toward specific lineages. While ES
cells show great promise for treating many diseases, such
as heart disease, diabetes, and Parkinson's disease, non-
matching ES cells would be rejected by patients' immune
systems unless they take immunosuppressant drugs. The
HLA (human histocompatibility leukocyte antigen)
system has a central role in the initiation and devel-
opment of immune rejection. However, hES cells and
their differentiated progeny express highly polymorphic
MHC (major histocompatibility complex) molecules
that serve as major graft rejection antigens to the immune
system of allogeneic hosts. To achieve sustained en-
graftment of donor cells, strategies must be developed to
overcome graft rejection without broadly suppressing
host immunity. One approach entails induction of donor-
specific immune tolerance by establishing chimeric en-
graftment in hosts with hematopoietic cells derived from
an existing hES cell line. To achieve best possible MHC
matching we could establish large banks of HLA-defined
and highly diversified hES cell lines, but this strategy
might not be sufficient since minor rejection antigens are
still present and difficult to define. Immunosuppressive
drugs such as cyclosporine are administered to transplant
recipients to prevent acute and chronic immune-mediated
rejection of allogeneic bone marrow and organ transplants
even with best possible MHC matching. Polymorphisms
in many non-HLA histo-compatibility antigens, includ-
ing highly polymorphic mitochondrial and H-Y gene
products, result in rejection even in HLA-matched
individuals.
heterodimer and activation of the JAK/Stat3 signaling
passway. Although the mechanism by which gelatin aids
in the maintenance of ES cells in an undifferentiated
state is not known, it is clear that surface properties of
specific substrates can exert powerful effects upon cell
growth and behavior.
By controlling the culture conditions under which ES
cells are allowed to differentiate, it is possible to generate
cultures that are enriched for lineage-specific precursors.
To this end, stem cells require an additional ability to
control growth and differentiation into useful cell types.
The effects of biomaterials on the behavior of stem cells
have not been studied in great detail. This is due in part
to the potential diversity of biomaterials and the diffi-
culty of large-scale hES cell production.
7.2.8.4.2 Somatic cell nuclear transfer
The isolation of pluripotent hES cells and breakthroughs
in somatic cell nuclear transfer (SCNT) in mammals
have raised the possibility of performing human SCNT
and generated potentially unlimited sources of un-
differentiated cells for use in research, with potential
applications in tissue repair and transplantation medi-
cine. The SCNT concept, known as ''therapeutic clon-
ing'', refers to the transfer of the nucleus of a somatic cell
into an enucleated donor oocyte. In theory, the oocyte's
cytoplasm would reprogram the transferred nucleus by
silencing all the somatic cell genes and activating the
embryonic ones. The ES cells would be isolated from the
inner cell mass of the cloned preimplantation embryo.
When applied in a therapeutic setting, these cells would
carry the nuclear genome of the patient; therefore, after
directed cell differentiation, the cells could be trans-
planted without immune rejection to treat degenerative
disorders such as diabetes, osteoarthritis (OA), and
Parkinson's disease among others.
A team led by veterinary cloning expert Woo Suk
Hwang and gynecologist Shin Yong Moon of Seoul Na-
tional University in South Korea showed that the cloning
technique can work in humans
[40]
. The researchers
described how they created a human ES cell line by
inserting the nucleus of a human cumulus cell into
a human egg from which the nucleus had been removed.
(Cumulus cells surround the developing eggs in an ovary,
and in mice and cattle they are particularly efficient nu-
cleus donors for cloning.) After using chemicals to
prompt the reconstructed egg to start dividing, the team
allowed it to develop for a week to the blastocyst stage,
when the embryo forms a hollow ball of cells. They then
removed the cells that in a normal embryo are destined
to become the placenta, leaving the so-called ''inner-cell
mass'' that would develop into the fetus. When these
cells are grown in culture, they can become ES cells,
which reproduce indefinitely and retain the ability to
form all the cell types in the body. The ES cell line the
7.2.8.4.1 Cell expansion and differentiation
Routine propagation of mouse ES cells in an un-
differentiated state can be achieved by culture upon
mitotically inactivated mouse embryonic fibroblasts
(MEFs) or upon gelatin-coated dishes in the presence of
the interleukin-6 family member cytokine leukemia in-
hibitory factor (LIF). The LIF stimulates ES cell self-
renewal following binding of the LIF receptor b/gp l30
