A debate on the ethical and moral implications of using embryonic stem cells in research and curative health care has raged for more than a decade.
|An embryo dies once stem cells are extracted from it [EPA]
Embryos are effectively destroyed once stem cells are extracted for research.
In 1995, the US Congress said that governments funds would no longer be made available for research in which human embryos were destroyed.
In 2001, the administration of George Bush, the former US president, restricted funding to stem cell research on already existing stem cell lines, prohibiting the extraction of new stem cells from embryos.
Barack Obama, the US president, said he would introduce measures to end eight years of limitations on embryonic stem cell research. It is expected that federal funds will be released for continued research.
What are stem cells?
Stem cells are the master cells of the human body from which all other cells - such as brain, blood, heart muscle or bone cells - are created.
Stem cells are considered to be unique because they are the only type of cells which have the ability to divide into daughter cells.
Daughter cells can then be used to create more stem cells or specialised cells, those tasked with a particular function within the human body.
Embryonic stem cells are those that have been grown in a laboratory by way of in vitro fertilisation of an egg.
In this process, a human embryo that is four to five days old contains about 150 cells.
These cells can then be divided into daughter cells or given specialised tasks to repair diseased tissue and organs in the human body.
What are stem cell lines?
Stem cell lines are a group of cells derived from a singular original stem cell. As they have no genetic defects, they are harvested to create more stem cells, bypassing the need for the extraction of original stem cells, which consequently destroys the embryo.
Is the embryo removed from a fertilised egg in a woman's body?
No. Embryos are derived from fertilisation of a man's sperm cells and a woman's egg at in vitro clinics. Once the fertilised egg is not wanted or discarded as excess, it is not implanted in a woman's uterus but is frozen for later research.
Why is stem cell research controversial?
Stem cell research is controversial because the best source of stem cells is human foetal tissue.
Harvesting the stem cells destroys the embryo, which many see as morally problematic.
In other words, to get stem cells, scientists either have to use an embryo that has already been conceived or else clone an embryo using a cell from a patient's body and a donated egg. In either case, the embryo is destroyed.
Though that embryo may only contain four or five cells, some religious leaders say that destroying it is equivalent to taking a human life.
Why is there so much scientific interest in them?
Scientists believe stem cell research can be a revolutionary approach to combating human diseases.
Stem cells have the potential to be used to repair other diseased, dysfunctional or injured cells and replace specific tissues or grow organs.
Stem cells have already been used in bone marrow transplants.
A better understanding of normal cell development will allow scientists to understand and perhaps correct the errors that cause these medical conditions.
In theory, scientists say stem cell research could one day lead to curing such degenerative diseases as Alzheimer's or Parkinson's disease.
Another potential application of stem cells is making cells and tissues for medical therapies.
Stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases.
While the potential of stem cell therapy has yet to be realised, adult stem cells are already being used to treat medical conditions such as heart failure.