What are stem cells?

Stem cell research offers potential cures for diseases

By Julian Siddle of BBC Science

Stem cells are young cells which, in embryos, have the ability to develop into any of the body's 130 different tissue types.

President Bush's decision has important implications for scientific research aimed at providing new drugs and alternatives to organ transplants.

Scientist hope that by growing such cells in the laboratory they can programme them to form specific tissue such as kidney, heart or even brain tissue

Scientists hope that by growing such cells in the laboratory they can programme them to form specific tissue such as kidney, heart or even brain tissue.

This tissue could then be used to repair damaged organs, and so provide an alternative to organ transplants.

It is possible to extract stem cells from fresh human embryos obtained from fertility treatments or abortions.

A more reliable supply, however, would be obtained by copying or cloning embryos specifically for their stem cells.

One embryo could, therefore, be a source for many thousands of cells.

Heart breakthrough

Recently scientists in Israel made the first heart tissue from stem cells.

They took one, modified it to act like a heart cell, and then allowed it to divide into other cells and form a tissue.

It's not clear whether this process would be allowed in the US.

It's likely that scientists involved in US-based stem cell research will take their expertise abroad, closely followed by their sponsors, the US biotechnology industry.

Most countries do not have any laws on embryo research or cloning, but the most likely destination is Britain, where limited cloning is allowed.