Stem cells can be urged to grow into various tissues for transplantation
Stem cell research - the basic questions answered, courtesy of Tom Shakespeare, a contributor to Ouch! It's a Disability Thing - the BBC's dedicated disability lifestyle website.
What is stem cell therapy?
Stem cells are undifferentiated cells - lacking qualities that make them different or unique - which are capable of developing into any of the 200 different types of cell in the human body. They are derived from embryos, from the umbilical cord or, with greater difficulty, from the scarce stem cells in adults or children.
Stem cells can be used to grow tissues for transplantation - for example, heart muscle or brain cells or liver cells. They can also be used as models for disease, which can then be used in research - meaning better knowledge or less reliance on animal experimentation.
Who might be helped by these therapies?
People who have diseases or impairments which are caused by tissue damage or degeneration can potentially be helped by stem cell therapy. For example, people with diabetes, liver disease, Parkinson's and Alzheimer's and possibly people with spinal cord injuries.
A new trial is exploring whether stroke survivors could benefit too, whilst the latest news suggests that stem cells from patients' own bone marrow could help reverse the early signs of MS.
Most of these therapies are only at the stage of initial trials in humans - for example, studies on corneal blindness and spinal cord injury are just starting.
Does it work?
A form of stem cell gene therapy has been successful in treating Severe Combined Immune Deficiency (SCID) and a few other conditions in research settings. While stem cell therapy sounds good in theory, in practice it is very hard to grow specific cell types and control their growth safely. Some of the children cured of SCID in France went on to contract a form of leukaemia. The disease, which only affects boys, leaves them with little if any immune system and forces them to live in sterile environments. Research continues to understand why, and to improve safety and effectiveness.
Clinical trials - why so long?
Clinical trials can take up to 10 years, so even if a therapy is shown to be successful, scientists or pharmaceutical companies then have to prove that it is safe. Animal trials have shown that therapies for spinal cord injury, muscular dystrophy and other conditions have great potential, but effective treatments are still a long way off. Therapies may be beneficial in the early days after a spinal cord injury, but not benefit those who have been injured for a long time.
What are the ethical issues?
Embryonic stem cell therapy depends on destroying embryos - usually surplus embryos from IVF treatment. Those who believe that life starts at conception take issue with the use of destroyed embryos.
Former US President George W Bush opposed the use of destroyed embryos in research.
The arrival of President Barack Obama in the White House in January signalled a change in US government policy over federal funding for stem cell research.
Mr Bush restricted federal funding to around 60 stem cell lines created from embryos destroyed prior to August 2001, which posed both funding and logistical problems as researchers were forced to separate their work - in some cases creating new labs - in order to comply with the rules.
Tom Shakespeare is a research fellow at Newcastle University, he is the author of Disability, Genetics and Global Justice.