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
Embryonic stem cells are the primitive cells that grow into the roughly 200 types of cell that comprise the body's tissues.
Scientists aim to coax these cells into becoming lab-dish replacements for heart, liver, skin, eye, brain, nerve and other cells destroyed by disease, accident, war or normal wear-and-tear.
Parkinson's disease, Alzheimer's, Type 1 diabetes, cancer and cardiac degeneration are among the many disorders that, in theory, could be healed by this wonder cure.
Of the two categories of stem cells, the biggest interest by far has focused on embryonic stem cells. These are so-called pluripotent cells, meaning that they have ability to differentiate, or diversify, into many different tissues.
But embryonic stem cell research has been controversial. These master cells are extracted from early embryos that are allowed to grow for five to six days in culture.
The harvested stem cells are kept in self-replicating 'lines' for study, but the embryos themselves – usually surplus embryos from in-vitro fertilisation (IVF) – are destroyed by the process.
American Christian conservatives have long opposed this research, saying that a human embryo equates to a human life.
In August 2001, former president George W. Bush banned all U.S. federal funding for research that entailed new lines of human embryonic stem cells. That move caused an outcry among U.S. researchers, who warned investment and talent in their field would shift to other countries.
In July 2006, an attempt in the senate to lift some restrictions was barred by Bush, wielding his presidential veto for the first time.
President Barack Obama has vowed to scrap the ban, and a White House official said the president would sign executive order Monday reversing Bush administration restrictions on federal funding for embryonic stem cell research.
The official would not divulge the exact wording of the order, but confirmed, on condition of anonymity, that it would be in line with Obama's campaign vow to restore funding to embryonic stem cell research.
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.
