Stem cells could help medicine in three general ways: cell-based therapies, drug discovery and basic knowledge. Cell therapies would use stem cells, or cells grown from stem cells, to replace or rejuvenate damaged tissue. Scientists also want to use stem cells to understand disease and find drugs that might treat it.
Embryonic stem cells could be used to make more specialized tissues that have been lost to disease and injury. For tissues that are constantly replaced, like blood and skin, stem cells would probably be replaced directly. Researchers are also exploring ways to use stem cells to treat diabetes, Parkinson's disease, spinal cord injury, heart disease and vision and hearing loss, among others.
As of April 2007, however, no therapies using cells derived from embryonic stem cells have been tested in humans. The efficacy of stem cell therapies depends on the introduced cells arriving where they are needed and either replacing or rejuvenating damaged cells. They should not contain undifferentiated embryonic stem cells, and either the cells, the patient or both should be treated so that the patient's immune system will not attack the transplants.
As an alternative to cell therapies, some researchers are looking for traditional drugs that would prompt adult stem cells to come out of hiding and replace damaged tissues. In one early study, rats with a strokelike injury had more control over their movement after being treated with a compound that stimulates stem cells in the brain.
Embryonic stem cells could be grown into more specialized cells for screening potential drugs. Cultures of cancer cells are already used for screening cancer drugs, and growing embryonic stem cells into heart, liver or nerve cells could be useful for testing drugs that affect those organs. Ideally, the human cells could be custom-made to represent the genetic diversity and traits typical of people who suffer from the disease being studied. Right now, potential drug molecules are tested first in mice and rats, but results of these animal tests do not always correlate with what happens in humans. Drugs that poison a human liver, for example, might do no harm to a rat's.
Many scientists think that testing pollutants and potential drugs on cells grown from human embryonic stem cells could be more accurate than current tests. This could mean that fewer animals would be killed for research and also make research faster and cheaper. However, if such experiments are to work, scientists will have to develop techniques to make sure that the cells and culture conditions remain constant; otherwise, differences between experiments could be due to factors other than the drug candidates being tested.