Q. Why is embryonic stem cell research considered controversial by some?

A. Human embryonic stem cells emerge five to seven days into the development of the embryo, when it is a hollow sphere made up of approximately 100 to 150 cells. At this stage, the embryo is known as a blastocyst. One portion of the blastocyst contains the "inner cell mass." This is where the embryonic stem cells are.

To obtain the cells, scientists apply chemicals to the embryo, which dissolve its coating, exposing the stem cells. This process destroys the embryo. Some people oppose the use of embryos for research because they consider them living beings.

The embryos that are studied at US universities have been donated for research by patients who have undergone fertility treatments. The embryos were created through in vitro fertilization. The embryos have been left over following the fertility treatments and, depending on the patient's request, would otherwise have been stored indefinitely or discarded.

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