Tuesday, March 25, 2008

What are stem cells - embryonic and adult stem cells

Stem cells are relatively primitive cells that have the ability to divide rapidly to produce more specialized cells. Stem cells in the embryo are capable of huge variation in the kinds of tissues they make, reproduce rapidly and have attracted interest of researchers for decades. However embryonic stem cells are hard to get hold of in humans - you need a supply of human embryos, which requires either breaking the law in some countries or applying for complex licenses in others.

Embryonic stem cells are also hard to control, and hard to grow in a reliable way. They have "minds" of their own, and embryonic stem cells are often unstable, producing unexpected results as they divide, or even cancerous growths. Human embryonic stem cells usually cause an immune reaction when transplanted into people, which means cells used in treatment may be rapidly destroyed unless they are protected, perhaps by giving medication to suppress the immune system (which carries risks).

One reason for intense interest in human cloning technology is so-called therapeutic cloning. This involves combining an adult human cell with a human egg from which the nucleus has been removed. The result is a human embryo which is dividing rapidly to try and become an identical twin of the cloned adult. If implanted in the womb, such cloned embryos have the potential to be born normally as cloned babies, although there are many problems to overcome, including catastrophic malformations and premature ageing as seen in animals such as Dolly the sheep.

In theory, therapeutic cloning could allow scientists to take embryonic stem cells from the cloned embryo, throw the rest of the embryo away and use the stem cells to generate new tissue which is genetically identical to the person cloned. In practice this is a very expensive approach fraught with technical challenges as well as ethical questions and legal challenges.

An alternative is to try to create a vast tissue bank of tens of thousands of embryonic cells lines, by extracting stem cells from so many different human embryos that whoever needs treatment can be closely matched with the tissue type of an existing cell line. But even if this is achieved, problems of control and cancer remain. And again there are many ethical considerations with any science that uses human embryos, each of which is an early developing but complete potential human being, which is why so many countries have banned this work.


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