Incredible potential: Image shows IPS embryonic stem cells (L, top), which can develop, into cells of many tissues, such as muscle (L, bottom), nerves (R, bottom) and cartilage (R, top). Harnessing the power of stem cells would revolutionise medicine.
Credit: AFP/Kyoto University/Shinya Yamanaka
Access to embryonic stem cells in other countries has also been restricted by laws or regulations governing the source of the embryos.
In contrast to embryonic stem cells are so-called adult stem cells, which are are genetically programmed to differentiate into a much more limited number of specific cell types.
Adult stem cells were initially thought to be very small in number, but the tally has been found in many more tissues in recent years. They have now been discovered in brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.
There are already several types of therapy involving adult stem cells, the best known of which, dating from the 1960s, is the bone marrow transplant. Most, though, are at still at experimental or laboratory level.
Compared with embryonic stem cells, adult stem cells are less versatile and are harder to culture in the lab.
In 2007, researchers said they had found a way to make pluripotent (and therefore more versatile) adult stem cells by 'reprogramming' adult stem cells taken from skin, though this has yet to be proven.
And in January 2008, a team led by Robert Lanza at Advanced Cell Technology (ACT), a Massachusetts biotech company, announced they had devised a method to create the first human embryonic stem cells without destroying the embryo.
Biomedical researchers caution that several big questions remain to be answered before stem cell research fully delivers on its great promise.
One of the big challenges is understanding exactly how a stem cell differentiates into specialised cells. Another is how to ensure that transplanted stem cells are not attacked as alien by the immune system.
One area of work is to clone stem cells from a patients own cells, so that they carry the DNA of the patient and thus are not treated as foreign. Another method would be to induce a patients own adult stem cells to become pluripotent and therefore able to turn into as many cell types as embryonic stem cells, but this has yet to be achieved.
