Hospitals and laboratories throughout the world use human cells in experiments. Usually, these are samples taken from tu- mors, organs, or other tissues with the permission of the pa- tient. Another use foreseen for human cells is called therapeutic cloning. The idea is to harvest cells from an adult, not to make a copy of a person, but to create new cells that could be useful in fi ghting disease. For example, clones could be transformed into the precursors of cells that the body has a hard time replacing, such as muscle and neurons. If scientists learn enough about the genes that guide tissue development, they may even be able to grow complete organs in the laboratory.
Such cells would be useful in treating many diseases caused by the death of particular kinds of cells. Cell death is a natural process—red blood cells, for example, have a lifespan of about 120 days—and the body can replace many of them. It does so by drawing on adult stem cells stored in various places in the body. In humans, these are capable of replacing some types of cells and repairing some tissues, but not all. Mammals have lost the ability to rebuild major tissues and organs, but species such as salamanders can regrow entire limbs and organs that have been lost or damaged. The most powerful regenerators may be worms called planaria, which can be cut up into hundreds of pieces, each of which will develop into a full worm.
One way to fi ght degenerative brain conditions and other wasting diseases might be to implant fresh stem cells that fi nd
the damage and transform themselves into new neurons or oth- er types of cells. This has been done successfully in animals, and it is what happens with bone marrow transplants. The marrow is a factory and storage facility for many types of stem cells. People with certain genetic diseases or cancers produce defective blood cells; marrow from a healthy person can some- times provide a source of healthy ones. A similar strategy has been tried in Parkinson’s and Huntington’s diseases, which are caused by the death of neurons in specifi c regions of the brain. Stem cells might grow into healthy neurons that can at least delay the course of the disease. The main problem is obtaining and keeping certain types of adult stem cells. They are rare, and when extracted from the body, they almost immediately develop into specialized types.
In organ transplants, it is often diffi cult to fi nd a donor who matches a patient; the same is true of cell transplants. Injecting one person’s cells into someone else would probably trigger an immune reaction and possibly disease or death. This problem could be overcome by using a person’s own cells. However, it is extremely diffi cult to fi nd and remove adult stem cells from a human being, and even when possible, if the problem is a ge- netic disease, these cells are likely to be defective, too.
Samuel Wood’s success in creating clones from nuclei taken from skin cells suggests that it may be possible to create embry- os from adult cells and use them to make new stem cells. Oth- erwise, a current practice is for hospitals to collect and freeze tissue taken from the umbilical cord of newborns, which is rich in embryonic stem cells. The idea is to keep this tissue until a person might need it, then to use it to produce various types of more specialized stem cells.
In either case, researchers may need to work with cells that might develop into a full human being if they were implanted into a woman’s womb. Thus, some people consider experi- ments with embryonic stem cells to be the moral equivalent of conducting experiments on human beings. This issue was introduced above in the section on reproductive cloning, but it bears reconsidering, because surveys show that people have different attitudes about the two types of cloning. They may be
unsure about the meaning of the terms. For instance, a Gallup poll from May 2002 showed that 59 percent of the Americans who were surveyed approved of the “cloning of human organs or body parts that can then be used in medical transplants,” but only 34 percent approved of the “cloning of human embryos for use in medical research.” And 51 percent approved of the “clon- ing of human cells from adults for use in medical research.” It is unclear whether people are aware that it may be necessary to make “embryos” to obtain such cells.
Many researchers argue that cells that will never be put into a womb to develop will never become human, so they are no different than blood samples or other types of tissue grown in cultures in hospitals and laboratories throughout the world. If the potential to become human were enough to give cells the same rights as fully developed human beings, then there would have to be new rules regarding the handling of single sperm and egg cells, which also—under the right circumstanc- es—might become human beings. Obviously, the question of abortions would need to be handled differently as well. Under the current laws of the United States and many other countries, a pregnancy may be terminated during the fi rst three months of an embryo’s development. After that, a fetus has a special status under the law and may not be aborted except in care- fully defi ned circumstances. This is a practical solution to a very complex ethical issue involving the defi nition of human life, the rights of mothers to make decisions regarding their bodies and their health, and the protection of the unborn, but it has left many people unhappy, including religious leaders.
In the United States, experiments involving human cells or tissues have to be approved by ethics committees. A scientist has to demonstrate that there is a need for the experiment, that it cannot be done using other methods, and that ethical con- siderations have been taken into account. The most common source of embryonic stem cells are frozen embryos that have been created and maintained by fertility clinics for in vitro fertil- ization (IVF). In trying to help families that are otherwise unable to have children, such clinics produce far more embryos than can ever be implanted or used. According to statistics from the
Human Fertilisation and Embryology Authority (HFEA) in the United Kingdom, a country in which stem cell experimentation is permitted, a total of 925,747 embryos were created between 1991 and 2000 by clinics for use in IVF. Only 3.5 percent of these were successfully implanted in mothers and survived to birth. All the rest, except for 118 embryos that were donated to research, were simply destroyed. Advocates of stem cell re- search argue that it would have been better to put the cells to use in research that might one day benefi t other people.
It is possible to create embryonic stem cells engineered in such a way that they could never become human beings. For example, scientists could delete an essential protein, such as a molecule needed for the embryo to attach itself to the lining of the mother’s womb. The stem cell would still be useful in therapies, but it could never become human; some feel that this eliminates the moral issue.
Most scientists agree that it would be unethical to manipu- late the human germline—the reproductive cells that go on to make humans. So the focus of molecular medicine, described in the last chapter, has been to develop therapies that can be used to cure genetic defects in adults or fully developed embryos. Some of the most promising methods involve siRNAs, or gene therapies involving viruses (discussed in the previous chapter). Most people see these methods as similar to normal medicines and do not have ethical problems with their use.