Biomedical Engineering Reference
In-Depth Information
cardiomyocytes or occur as a function of paracrine effects of transfected cells. Others
maintain that there is evidence for a transdifferentiation event. Cell fusion has been
demonstrated between cardiomyocytes and noncardiomyocytes in vivo and in vitro,
and the data in support of transdifferentiation (particularly with HSCs) have not
always been replicable. Further research is needed to clarify these issues and
reconcile the contradictory claims as well as provide additional information about
the extent of cell fusion and when it occurs. A limitation of the majority of the
clinical studies with adult noncardiac stem cell transplantation relates to the potential
stability of the differentiated phenotype because these studies have primarily
examined the short-term benefits.
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12.3.3 Resident Cardiac Progenitor Cells
Cardiac progenitor cell are capable of multilineage differentiation. Beltrami and
coworkers
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were the first to describe the existence of a resident cardiac progenitor
cell (CPC) capable of differentiating into myocytes as well as endothelial and
smooth muscle cells. It has been shown that when these resident cardiac progenitor
cells are injected into the postinfarct heart, there is some improvement of cardiac
function.
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12.3.4 Adult Stem Cells
Adult or somatic stem cells exist throughout the body after embryonic development
and are found inside of different types of tissue. These stem cells have been found in
tissues such as the brain, bone marrow, blood, blood vessels, skeletal muscles, skin,
and liver. They remain in a quiescent or nondividing state for years until activated by
disease or tissue injury. They are multi(pluri)potent.
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Adult stem cells are unique cells that can divide or self-renew indefinitely,
enabling them to generate a range of cell types from the originating organ or
even regenerate the entire original organ. They have the ability to differentiate
through a committed lineage. It is generally thought that adult stem cells are limited
in their ability to differentiate based on their tissue of origin, but some evidence
suggests that they can differentiate to become other cell types. These adult stem cells
comprise at least three different groups: bone marrow-derived stem cells, the
circulating pool of stem or progenitor cells, and tissue resident stem cells Adult
stem cells typically generate the cell types of the tissue in which they reside (e.g., a
blood-forming adult stem cell in the bone marrow), and they normally gives rise to
the many types of blood cells such as red blood cells, white blood cells, and platelets.
Stem cells from one type of tissue may be able to give rise to cell types of a
completely different tissue; that is, they exhibit plasticity (e.g., blood cells becoming
neurons and heart muscle, liver cells that can be made to produce insulin).
Stem cells, depending on their lineage commitment, possess the ability to
differentiate into cells of various tissues; this property of stem cell is called
differentiation. Embryonic cells are pluripotent and can generate tissues belonging
to all three germ layers. Adult stem cells are thought to be more committed and
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