Laboratory research on adult stem cells is generally uncontroversial. Research with human subjects becomes controversial because some experimental “therapies” could harm patients. Debate can be acrimonious between researchers who want to perform additional studies on animals to try to better understand risks to humans, and those who don't want to delay testing procedures that might help patients.

Most opponents to embryonic stem cell research think that it is wrong to destroy a 2- to 6-day-old embryo, even if it is not destined to start a pregnancy. Others argue that it is immoral not to do this research, if doing so could lead to treatments for disease. The groups disagree as to whether an early embryo deserves the same protection as a fetus or an adult human.

The acquisition of unfertilized human eggs is another area of controversy. The procedure to retrieve eggs from women requires a series of drugs and surgery. Women who donate eggs face a small but real risk of death and are certain to endure discomfort. In most countries, women can be paid to donate eggs to infertile couples, but many ethicists, lawyers and women's rights activists feel that women should not be compensated for donating eggs for research. Other ethicists, lawyers and women's rights activists feel that they should, at least to compensate for time lost from work and other costs to them.

To surmount the supply and ethical problems of acquiring human eggs, some researchers have proposed inserting human nuclei into animal eggs. Any embryonic stem cells derived would not be used for therapies directly but instead used to conduct research that could lead to therapies. Other researchers hope to make research-grade materials through cell fusion or genetically engineering other types of cells.

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Regulatory agencies in the UK have launched a public discussion as to whether human-animal hybrids should be created. Researchers must agree not to let the embryos grow past 2 weeks, and the researchers must argue convincingly that their experiments address important questions that could not be answered any other way. However, some people object that mixing human nuclei with animal eggs offends human dignity or that scientists might not follow the regulations set for these experiments.