Exactly What are Stem Cells?

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The ethical and moral debate over the use of stem cells has taken center stage over the past decade. Stem cells are of great medical interest, since they have the potential to develop into almost any type of cell in the body. Regenerative medicine focuses on the potential uses of stem cells in medicine and how they can provide effective treatment for a range of diseases.

Stem cells have the capacity to divide indefinitely to replenish other cells in the body. When a stem cell divides, each daughter cell can remain a stem cell or become a more specialized cell, such as a red blood cell, a muscle cell or a nerve cell. An increasing body of evidence also suggests that molecular pathways and properties associated with normal stem cells is relevant to cancer development [1].

There are two types of mammalian stem cells: embryonic stem cells, which are found in blastocysts (the mammalian embryo at the stage at which it is implanted into the wall of the uterus), and adult stem cells, which are found in adult tissues.

Stem cells are undifferentiated, meaning that they aren’t committed to becoming a specific kind of cell in the body. They are characterized by the ability to self-renew (remaining undifferentiated) and the capacity to differentiate into specialized cell types (termed potency). Potency specifies a stem cell’s differentiation potential.

There are four classes of stem cell potency:

• Totipotent: A totipotent stem cell is produced from the fusion of an egg and sperm cell and has the ability to become any kind of cell in the body. For a brief period, each cell division creates identical totipotent cells — any one of these cells could develop into a fetus if placed in a woman’s uterus. By the fourth day, the totipotent cells begin to specialize, forming a blastocyte, the type of cell that forms the outer layer of a blastocyst. These cells will go on to form the placenta and other supporting tissues of the uterus necessary for fetus development.

• Pluripotent: The inner cluster of cells in a blastocyst, called inner mass cells, are pluripotent. Pluripotent stem cells have the ability of to become any kind of cell in the body other than cells of the placenta or other supporting tissues of the uterus. Because of this, pluripotent cells cannot form a fetus if placed in a woman’s uterus. Embryonic stem cells are generally considered pluripotent.

• Multipotent: As pluripotent cells continue to specialize, they become cells that only lead to the development of specific tissues. Multipotent stem cells have the ability to form several cell types of a closely related family of cells and are generally referred to by their tissue origin, e.g. cardiac stem cells, neural stem cells, bone marrow stem cells. Multipotent cells function as a repair system for damaged tissue. Adult stem cells are multipotent stem cells.

• Unipotent: Unipotent stem cells have the ability to produce only one specialized cell type. Unipotent cells maintain the property of self-renewal, thus distinguishing them from non-stem cells.

Pluripotent stem cells can be derived from a number of sources, all of which are associated with moral issues:

1. Surplus embryos that are the by-product of in vitro fertilization
2. Embryos created in the lab from donated sperm and eggs
3. Embryos created through a cloning technique called somatic cell nuclear transfer
4. Embryos from aborted fetuses

Pluripotent stem cells can also be obtained from umbilical cord blood, placenta and amniotic fluid.

Although pluripotent stem cells have greater potential, multipotent adult stem cells are currently being used therapeutically. Adult stem cells can be found in many organs and tissues, including bone marrow, peripheral blood, brain, muscle, liver, skin and heart. Nevertheless, a number of limitations exist with adult stem cells [2]. Adult stem cells are found in small quantities in adult tissues and umbilical cord blood, raising doubt that they could be grown in clinically significant quantities. Additionally, adult stem cells have not been found for all tissues of the body, necessitating the use of pluripotent stem cells for the generation of certain tissue types. Since adult stem cells are multipotent, they cannot be induced to develop into any cell type, i.e. manipulated to differentiate into a specialized cell type different than their final tissue type.

For more information on stem cell research and current federal policy on embryonic stem cell research, see Research!America.

References

1. Lobo et al. The biology of cancer stem cells. Annu Rev Cell Dev Biol. 2007;23:675-99.
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2. Vats et al. Stem cells. Lancet. 2005 Aug 13-19;366(9485):592-602.
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