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Stem Cells: A Primer
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NATIONAL INSTITUTES OF HEALTH Office of the Director May 2000

http://www.nih.gov/news/stemcell/primer.htm

This primer presents background information on stem cells. It includes an
explanation of
- what stem cells are;
- what pluripotent stem cells are;
- how pluripotent stem cells are derived;
- why pluripotent stem cells are important to science;
- why they hold such great promise for advances in health care; and
- what adult stem cells are.

Recent published reports on the isolation and successful culturing of the
first human pluripotent stem cell lines have generated great excitement and
have brought biomedical research to the edge of a new frontier.

The development of these human pluripotent stem cell lines deserves close
scientific examination, evaluation of the promise for new therapies, and
prevention strategies, and open discussion of the ethical issues.

In order to understand the importance of this discovery as well as the
related scientific, medical, and ethical issues, it is absolutely essential
to first clarify the terms and definitions.

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Definitions
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DNA - abbreviation for deoxyribonucleic acid which makes up genes.

Gene - a functional unit of heredity which is a segment of DNA located in a
specific site on a chromosome. A gene directs the formation of an enzyme or
other protein.

Somatic cell - cell of the body other than egg or sperm.

Somatic cell nuclear transfer - the transfer of a cell nucleus from a
somatic cell into an egg from which the nucleus has been removed.

Stem cells - cells that have the ability to divide for indefinite periods
in culture and to give rise to specialized cells.

Pluripotent -capable of giving rise to most tissues of an organism.

Totipotent - having unlimited capability. Totipotent cells have the
capacity to specialize into extraembryonic membranes and tissues, the
embryo, and all postembryonic tissues and organs.

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What is a stem cell?
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Stem cells have the ability to divide for indefinite periods in culture and
to give rise to specialized cells.

They are best described in the context of normal human development.

Human development begins when a sperm fertilizes an egg and creates a
single cell that has the potential to form an entire organism.

This fertilized egg is totipotent, meaning that its potential is total.

In the first hours after fertilization, this cell divides into identical
totipotent cells.

This means that either one of these cells, if placed into a woman's uterus,
has the potential to develop into a fetus.

In fact, identical twins develop when two totipotent cells separate and
develop into two individual, genetically identical human beings.

Approximately four days after fertilization and after several cycles of
cell division, these totipotent cells begin to specialize, forming a hollow
sphere of cells, called a blastocyst.

The blastocyst has an outer layer of cells and inside the hollow sphere,
there is a cluster of cells called the inner cell mass.

The outer layer of cells will go on to form the placenta and other
supporting tissues needed for fetal development in the uterus.

The inner cell mass cells will go on to form virtually all of the tissues
of the human body.

Although the inner cell mass cells can form virtually every type of cell
found in the human body, they cannot form an organism because they are
unable to give rise to the placenta and supporting tissues necessary for
development in the human uterus.

These inner cell mass cells are pluripotent — they can give rise to many
types of cells but not all types of cells necessary for fetal development.

Because their potential is not total, they are not totipotent and they are
not embryos.

In fact, if an inner cell mass cell were placed into a woman's uterus, it
would not develop into a fetus.

The pluripotent stem cells undergo further specialization into stem cells
that are committed to give rise to cells that have a particular function.

Examples of this include blood stem cells which give rise to red blood
cells, white blood cells and platelets; and skin stem cells that give rise
to the various types of skin cells.

These more specialized stem cells are called multipotent.

While stem cells are extraordinarily important in early human development,
multipotent stem cells are also found in children and adults.

For example, consider one of the best understood stem cells, the blood stem
cell.

Blood stem cells reside in the bone marrow of every child and adult, and in
fact, they can be found in very small numbers circulating in the blood stream.

Blood stem cells perform the critical role of continually replenishing our
supply of blood cells — red blood cells, white blood cells, and platelets —
throughout life.

A person cannot survive without stem cells.

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janet paterson
53 now / 44 dx cd / 43 onset cd / 41 dx pd / 37 onset pd
TEL: 613 256 8340 URL: http://www.geocities.com/janet313/
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