Stem cells are undifferentiated biological cells that can differentiate into
specialized cells and can divide (through mitosis) to produce more stem cells. They are found in
multicellular organisms. In mammals, there are two broad types of stem cells: embryonic stem cells,
which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms,
stem cells and progenitor cells act
as a repair system for the body, replenishing adult tissues. In a developing
embryo, stem cells can differentiate into all the specialized cells—ectoderm,
endoderm and mesoderm (see induced pluripotent stem cells)—but
also maintain the normal turnover of regenerative organs, such as blood, skin,
or intestinal tissues.
There are three known accessible sources of autologous adult stem cells in humans:
1.
Bone marrow, which requires extraction by harvesting,
that is, drilling into bone (typically the femur or iliac crest).
2.
Adipose tissue (lipid cells), which requires
extraction by liposuction.
3.
Blood, which requires extraction through apheresis, wherein blood is drawn from the donor (similar to a
blood donation), and passed through a machine that extracts the stem cells and
returns other portions of the blood to the donor.
Stem cells can also be taken from umbilical cord blood just
after birth. Of all stem cell types, autologous harvesting involves the least
risk. By definition, autologous cells are obtained from one's own body, just as
one may bank his or her own blood for elective surgical procedures.
Adult stem cells are frequently used in
medical therapies, for example in bone marrow
transplantation. Stem cells can now be artificially grown and transformed (differentiated) into
specialized cell types with characteristics consistent with cells of various
tissues such as muscles or nerves. Embryonic cell lines and autologous embryonic stem cells generated through Somatic-cell nuclear transfer or dedifferentiation have
also been proposed as promising candidates for future therapies.[1] Research into stem cells grew out of findings
by Ernest A. McCulloch and James E. Till at theUniversity of Toronto in
the 1960s.
Types of stem cells
Pluripotent Stem Cells (PS
cells)
These possess the capacity to divide for long periods and
retain their ability to make all cell types within the organism. The best known
type of pluripotent stem cell is the one present in embryos that helps babies
grow within the womb. These are termedembryonic stem cells. These cells form at the blastocyst stage
of development. A blastocyst is a hollow ball of cells that is smaller than a
pinhead. The embryonic stem cells lie within this ball of cells. Recent
research has enabled scientists to derive pluripotent cells from adult human
skin cells. These are termed induced pluripotent stem cells or iPS cells.
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Fetal stem cells
These are obtained from tissues of a developing human
fetus. These cells have some characteristics of the tissues they are taken
from. For example, those taken from fetal muscles can make only muscle cells.
These are also called progenitor cells.
Adult stem cells
These are obtained from some tissues of the adult body.
The most commonly used example is the bone marrow. Bone marrow is a rich source
of stem cells that can be used to treat some blood diseases and cancers.
Discovery of stem cells
Scientists first studied the potential of stem cells in
mouse embryos over two decades ago. Over years of research they discovered the
properties of these stem cells in 1998. They found methods to isolate stem
cells from human embryos and grow the cells in the laboratory.
Early studies utilized embryos created for infertility
purposes through in vitro fertilization procedures and when they were no longer
needed for that purpose. The use required voluntary donation of the embryos by
the owners.
Potential for use
Stem cell research is improving by leaps and bounds.
These may soon become the basis for treating diseases such as Parkinson's
disease, diabetes, heart failure, cerebral palsy, heart disease and host of
other chronic ailments.
Stem cells may also be used for screening new drugs and
toxins and understanding birth defects without subjecting human volunteers to
the toxins and drugs.
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