Cloning and Stem Cells

In embryonic stem cell therapy, one problem that must be overcome is the chance of rejection by the host. Embryonic stem cells are heterologous, meaning they are taken from an individual other than the one who receives them. Since the stem cells have a different genetic make-up than the host's cells, there is a potential for rejection by the immune system.

Some researchers believe that a solution to this problem may be found with human cloning. A clone would be genetically identical to the donor, so the donor's immune system would recognize cells from a cloned embryo as self.

Since the goal of the cloning is not to create a baby, but rather to treat a disease or injury, it is called therapeutic cloning. The actual procedure is called somatic cell nuclear transfer.

Somatic cell nuclear transfer begins with two cells: an unfertilized human egg, and a body cell (somatic cell) from the donor. The nucleus is removed from both cells; then the nucleus of the somatic cell is injected into the egg. Now the egg has a complete set of diploid DNA, just like a naturally fertilized egg, and it can develop into an embryo. When the embryo has reached the blastula stage (an early stage of development in which the first structures begin to form), the inner cell mass is removed, killing the embryo. These cells are embryonic stem cells with the same DNA as the donor.

Because cloning for stem cells requires the destruction of human embryos, the procedure has all the same ethical drawbacks of conventional embryonic stem cells. Some bioethicists believe that killing an embryo is a form of murder. In addition, therapeutic cloning itself opens a number of new ethical problems.

In somatic cell nuclear transfer, an embryo is created for the sole purpose of destroying it. Conventional embryonic stem cell research often uses stem cells taken from embryos that were created via in vitro fertilization for reproduction. Some bioethicists believe that the embryo does not have the full rights of a person, but that it nevertheless deserves respect. They believe that using "leftover" embryos is ethically acceptable, but creating embryos just for experimentation is not.

The development of cloning technology for therapeutic purposes may also make reproductive cloning possible. Many ethicists are concerned with reproductive cloning, arguing that it would violate the human rights of the clone. They are concerned that therapeutic cloning will inevitably lead to reproductive cloning.

In 2001, the United States government outlawed all forms of human cloning, including both reproductive and therapeutic cloning. In 2004, researchers in South Korea announced they had successfully obtained stem cells from a cloned human embryo, but their claims were later shown to be based on fabricated data. To date, no cases of successful human cloning are known.

• Goodstein, Laurie, and Denise Grady. "Cloning and Stem Cells: The Debate ." New York Times, February 13, 2004.

• Hanna, Kathi E. "Cloning/Embryonic Stem Cells ." National Human Genome Institute, April 2006.