Biomedical Engineering Reference
In-Depth Information
liver. Both embryonic and fetal stem cells generate the developing tissues and
organs. At this stage such stem cells are designed as “multipotent,” and they are
more tissue-specific rather than generating all of the body's 200 different cell types
[
2
]. Such stem cells are generally designated as “multipotent.” However, some
research suggests that at least some multipotent stem cells may be more plastic than
first thought and may, under the right circumstances, become pluripotent.
Up until week 12, fetal stem cells (as well as the embryonic stem cells which
preceded them) can be transplanted into an individual without being rejected. This
is because they have little to none of immune-triggering proteins—HLA antigens on
their surface.
After the 12th week, fetal stem cells acquire these proteins, and they remain
present on stem cells from this point on, including on adult stem cells. Thus, while
some advocate therapeutic use of stem cells derived from cord blood, adult bone
marrow or the blood stream, these sources pose the problem of possible rejection
reactions. Therefore, stem cells derived from these sources may have therapeutic
potential only when given to the individual from whom they were derived (“autolo-
gous” transplantation) or from an immunologically matched donor (“allogenic”
transplantation).
Adult stem cells
are at a more advanced stage of development. For a long time
adult stem cells were considered not capable of differentiating into the endoderm,
ectoderm, or mesoderm, because they are already at a developed stage as one of the
three types of tissues and cannot be rejuvenated back to an early developmental
stage. They can be found in the blood, cornea, bone marrow, dental pulp of the
tooth, brain, skeletal muscle, skin, liver, pancreas, and gastrointestinal tract [
3
] .
These cells are capable of making identical copies of themselves, and usually divide
to make “progenitor” or “precursor” cells capable to develop into specific cell lines.
Adult stem cells have been identified in many organs and tissues, but in a very small
number in each tissue. They are thought to reside in a specific area of each tissue
(niche) where they may remain quiescent (non-dividing) for many years until they
are activated by disease or tissue injury. The adult tissues reported to contain stem
cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle,
olfactory mucosa, skin, and liver [
4
] .
Adult stem cells typically generate the cell types of the tissue in which they
reside. A blood-forming adult stem cell in the bone marrow, for example, normally
gives rise to the many types of blood cells such as erythrocytes, granulocytes, lym-
phocytes, and platelets. Until recently, it had been thought that a blood-forming cell
in the bone marrow—which is called a hematopoetic stem cell—could not give rise
to the cells of a very different tissue, such as nerve cells in the brain. However, a
number of experiments over the last several years have raised the possibility that
stem cells from one tissue may be able to give rise to cell types of a completely dif-
ferent tissue—a phenomenon known as plasticity [
3
]. Examples of such plasticity
include blood cells becoming neurons, liver cells that can be made to produce insu-
lin and hematopoietic stem cells that can develop into heart muscle. Furthermore,
the concept of plasticity has been revised by the Ratajczak's group which has devel-
oped recently and proved the concept of very small embryonic like cell (VSEL),
