In the early stages of human development, stem cells in the embryo 'differentiate' into all the cell types in the body - brain, bone, heart, muscle, skin, etc.

Scientists are excited about the possibility of harnessing the spectacular natural power of these embryonic stem cells to cure many different diseases. For example, Parkinson's and Alzheimer's diseases result from damage to particular groups of cells in the brain. By transplanting stem cells from an embryo into the damaged part of the brain, scientists hope to replace brain tissue that has been lost.

In the near future, stem cell research may revolutionise the way doctors treat many other 'killer diseases' such as stroke, diabetes, heart disease and even paralysis.

Different countries have different attitudes to the use of human embryonic stem cells for research and medical treatment. In Germany, for example, taking stem cells from a human embryo is illegal.

In Britain, by contrast, it is legal but strictly regulated: British scientists can use human embryos for research up to 14 days after fertilisation. At this point the embryo is a hollow ball of cells about a quarter the size of a pinhead (0.2 mm).

Many countries do not yet have explicit laws regulating human stem cell research.

Because using embryos is ethically controversial, scientists worldwide are hunting for other sources of stem cells. One possibility is a type of stem cell found in the bone marrow of adults. These stem cells already have the potential to 'differentiate' into a variety of different blood cells throughout life.

In the future, scientists hope to manipulate these adult stem cells so that instead of only making blood cells, they can make brain, liver, heart and nerves.

Picture courtesy of www.repromed.co.uk

In the meantime though, stem cells from embryos are likely to provide the most immediate prospects for new treatments and cures.

• What are stem cells?
• Where do the embryos come from?
• Other sources of human stem cells
• Human cloning
• Human cloning - the risks