PROPERTIES OF STEM CELLS All stem cells share three basic properties,

1. They can divide and renew themselves by mitotic cell division. Most somatic cells are limited to about 50 divisions due to the loss of DNA from the telomeres with each pass through the cell cycle. It is theorized that the enzyme telomerase is active in stem cells which allows the lost telomeres to be replenished.

2. They are unspecialized. They do not display any specific structures associated with carrying out specific functions in the body. 3. They can give rise to specialized cell types. Over 200 different types of specialized cells can be found in the human body. All arise from stem cells. The process in which a stem cell becomes specialized is called differentiation.

Link to the Stem Cell Guy : http://learn.genetics.utah.edu/content/tech/stemcells/sci ntro/

THE POTENTIAL OF STEM CELLS Click here to learn about the potential uses of stem cells.

THE STEM CELLS PREFERRED FOR RESEARCH AND THERAPY ARE PLURIPOTENT 

Totipotent stem cells are not typically used in research or therapy because they are not abundant. They can only be obtained from a zygote (offers one cell), and embryos in the 2, 4 and 8 cell stage (offer 2, 4 or 8 stem cells).

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The multipotent stems cells from an adult or the umbilical cord are not preferred for research and therapy because they are so restricted in the type of cell they can become. The multipotent stem cells of the umbilical cord are limited to differentiating into a type of blood cell. Adult stem cells are limited into differentiating into a cell type specific to a particular tissue or group of tissues. Pluripotent stem cells are easier to obtain and still retain the ability to differentiate into almost every cell type in the human body.

METHODS FOR CREATING PLURIPOTENT STEM CELL LINES 1. In Vitro Fertilization One source of pluripotent stem cells are from embryos formed in vitro (translates to "in glass") for couples with fertility problems. During the in vitro fertilization process, the female is subjected to hormones that will stimulate multiple secondary oocytes to be released from the ovary. The secondary oocytes are surgically removed from her body and fertilized in a test tube. (Typically between 10 and 30 secondary oocytes are harvested.) Once the embryos reach the blastocyst

(blastula) stage, a few are selected and implanted into the female’s uterus. The remaining embryos are discarded, frozen for future use, or donated for scientific research. Pluripotent stem cells can be removed from the donated embryos and used to start a stem cell line.

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Advantages: These stem cells are very abundant (as many infertility couples do not use all of their fertilized eggs) and easy to obtain. Disadvantage: Obtaining these stem cells involves the destruction of the embryo and this creates ethical issues. Some feel that the embryos will ultimately be discarded anyway so they should be used for research and therapy. Others feel that life begins at conception and the destruction of the embryo to obtain its stem cells is unacceptable.

2. Therapeutic Cloning Pluripotent stem cells can also be created by taking the nucleus out of an unfertilized egg and injecting the egg with DNA from a differentiated cell (such as a skin cell). The "donor" cell nucleus is diploid and already has the full complement of chromosomes. The egg is never fertilized by sperm in this process. The "virgin" egg can be stimulated to divide as a zygote would. When it reaches the blastocyst (blastula) stage, pluripotent stem cells can be removed and used to start a stem cell line. (The stem cells will be genetically identical to the individual that donated the nucleus.)

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Advantages: Therapeutic cloning allows for the creation of stem cells that are a genetic match to the recipient so there are no issues with immune rejection of the cells. Since the "embryo" not produced by a sperm fertilizing an egg, fewer individuals have ethical issues with the procedure. Disadvantages: Therapeutic cloning has had a very low success rate. The vast majority of the "fertilized eggs" don't develop. There are still ethical issues with this procedure because although the embryo is created though unconventional methods, it is still a human embryo and must be

destroyed to harvest the stem cells. There is also concern that this procedure may lead to the cloning of humans. If the embryo was allowed to complete development, it would be a clone of the individual that donated the skin cell. A final concern is the question of how the human eggs would be obtained for the procedure. A large number of unfertilized eggs would be needed. Could young, healthy females "sell" their eggs for use in therapeutic cloning? Is this ethical?

3. Nuclear Reprogramming This is a new technique for making pluripotent stem cells that is still under study. Genetically engineered viruses are used to transfer master regulatory genes into differentiated skin cells. The addition of these genes alters the expression of other genes within the cell reprogramming the cell back to an undifferentiated state. The "induced pluripotent stem cells" (IPSCs) can be cultured on petri dishes and should be able to differentiate into any of the 220 specialized human cell types.

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Advantages: Nuclear reprogramming would create genetically matched cells to the individual whose skin cell had been reprogrammed. It could be used to repair or replace damaged tissue without concern that the pluripotent stem cells would be rejected by the immune system of that individual. In addition, since the procedure does not involve the creating of an embryo, there are no ethical issues.

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Disadvantages: Creating IPSC's has some risks. There is no way to control exactly where the virus inserts its genes into the cell's chromosomes. The viral genes could insert themselves into genes that regulate the cell cycle and cause cancers. In addition, one of the genes within the genetically engineered virus is a cancer causing gene. Researchers may find the IPSC's have subtle yet significant differences from embryonic stem cells that prevent them from being able to differentiate in every cell type.

Link to Stem Cell Video http://www.pbs.org/wgbh/nova/body/stem-cellsresearch.html