Three years ago, Macchiarini made headlines by implanting an artificial trachea created from donor tissue combined with stem cells from the recipient, Claudia Castillo, whose windpipe had been damaged by tuberculosis.
Since the protein needed to transform 5-fluorocytosine could only be churned out by souped-up stem cells, only the gliomas were exposed to the poison, killing them but sparing normal tissue, which the stem cells ignored.
The issue of cloning and embryonic stem cell research intersect when it comes to possible uses of cloned tissue as a source of stem cells.
They broke these up, in order to extract cardiac stem cells from them (these cells can be identified by the presence on their surfaces of a particular protein), and then bred the stem cells in tissue cultures until they numbered millions.
ECONOMIST: Stem cells and medicine: Repairing broken hearts | The
However, he said that embryonic stem cells remained the most obvious source of cells for such transplants, as foetal tissue - used in some experiments - was scarce, and so-called "adult stem cells" had never been converted into the right type of brain cells, despite 15 years of research.
Other companies have developed stem cells from adult tissue, sidestepping the controversy.
They strongly advocate the use of stem cells from adult tissue.
The evidence suggests that umbilical cord stem cells may be even better than embryonic stem cells when it comes to replacing damaged tissue.
Another U.S. company, Reneuron, plans to conduct trials involving stem cells taken from fetal tissue in Britain this year.
It was previously assumed that the cells damaged by cancer therapies were the stem cells in the gut tissue.
When such a cell divides, at least one of its daughters is also a stem cell (the other may set off on the route to specialisation that allows stem cells to generate new tissue).
After years of work his team, in early 2003, unearthed a rare group of stem cells present in human breast cancer tissue.
There is real hope that stem cells will soon be available to regenerate tissue for any part of the body.
In addition to the extremely expensive process of cloning, for each patient you have to culture stem cells and reliably turn them into the tissue you want with 100% efficiency, so you don't get a single left over stem cell that will cause tumors.
CNN: Human stem cell cloning: 'Holy Grail' or techno-fantasy?
At the moment colonies of human stem cells have to be grown on layers of tissue derived from mice.
It is hoped that work of this kind will eventually enable scientists to turn mature cells back into stem cells which have the ability to become any type tissue in the body.
Dalton is a leader in uncovering exactly what it is about embryonic stem cells that allows them to turn into any kind of tissue in the body.
Researchers highly value embryonic stem cells because of their potential to turn into any organ or tissue cell in the body.
For the tissue-growing experiment, the researchers focused on the clump of cells containing embryonic stem cells.
Dr Frisen reckons that they may offer several advantages over human embryonic stem cells which, although easier to manipulate, are a controversial source of new tissue because they are still mainly obtained from aborted fetuses.
The long-term hope is to treat patients with some sort of stem cell that could turn into beating heart muscle and replace dead cardiac tissue with fully functioning new cells.
Osiris Therapeutics of Baltimore has several trials under way using adult stem cells to treat the intestinal inflammation known as Crohn's disease, repair damaged tissue following a heart attack and prevent the progression of arthritis.
This chemical cascade, called the Wnt pathway, is turned on when organisms are injured, and it could in theory be used to make stem cells that are already present in the heart, brain and other organs regenerate damaged tissue.
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