Even more ethical issues are raised when cloning enters the discussion. Some scientists have obtained embryonic stem cells using a process known as therapeutic cloning. During this process, scientists replace the nucleus of an egg cell with the nucleus from another cell in the body to grow an embryo from which to harvest stem cells with specific properties, such as a new gene. This process differs from reproductive cloning, during which an egg is given the new nucleus and implanted into a woman's uterus so it will grow into a full-term fetus.

Legal limitations
Laws in the United States also place limitations on the use of embryonic stem cells for research. In 2001, an executive order limited federal funding for research on human embryonic stem cells.

This order means that if funded by federal money — as many research studies are — a scientist can only work with a limited number of already existing human stem cell lines that meet certain criteria. A stem cell line is a group of cells that can self-replicate outside the body for an extended period of time. Stem cell lines are appealing to scientists because the lines can be grown and shared indefinitely, eliminating the need to go through the difficult process of isolating and obtaining the stem cells again.

The allowable stem cell lines originated from embryos that already existed for reproductive purposes and were no longer needed for those purposes. Those who donated these embryos weren't paid for them and consented to their use in research.

Working exclusively with these stem cell lines presents some challenges for researchers. The number of available lines is limited. Some researchers question the quality and genetic diversity of the lines. Many of these cell lines have been exposed to animal-derived substances, which may increase the risk of certain infections. And, because private parties developed the lines, they don't have to share them and can charge researchers any amount they choose.

In March 2004, a group of scientists announced the existence of 17 new stem cell lines developed with the use of private funds. They plan to share these lines with other researchers. But research funded by government monies can't use these lines because they aren't derived from embryos that existed prior to the 2001 executive order.

The great unknown

Much remains to be learned about stem cells, including potential hazards. Real applications, for the most part, are still years away. But if progress with blood and bone marrow transplants is any indication, stem cell research may someday help many people. These tiny cells may significantly advance disease treatment and expand human knowledge of the body's basic processes.