ADULT stem cells may be able to perform many of the same tricks as embryonic stem cells. If so, the ethical debates about the use of human embryos could be avoided.
Adult mammals have about 20 types of stem cell. It was thought these cells gave rise to only specific cell lines-for example, that blood stem cells could only turn into blood cells-but scientists are now discovering how versatile these cells are.
Angelo Vescovi of Italy鈥檚 National Neurological Institute in Milan showed in 1999 that mouse brain stem cells could produce blood cells when injected into mice whose bone marrow-the normal blood-making tissue-had been largely destroyed.
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Since Vescovi made this discovery, many similar observations have been made. 鈥淭he concept that these cells could differentiate into different tissues was very surprising,鈥� says Margaret Goodell of Baylor College of Medicine in Texas, who recently discovered that cells from muscles could repopulate the blood system of mice.
But just how versatile are adult cells? 鈥淎lthough the cells show potential, it鈥檚 premature to say they can totally substitute embryonic stem cells,鈥� Goodell cautions.
Jonas Fris茅n鈥檚 work at the Karolinska Institute in Stockholm, however, suggests that brain stem cells can perform many of the same feats as embryonic stem cells. His group injected early embryos with the adult stem cells and found descendants of these cells in various organs, including the heart, liver, intestine and nervous system.
But was a rare kind of brain stem cell responsible, or is there something in the embryo that can reprogram an adult stem cell? No one knows, but researchers are hoping it鈥檚 a rare cell that can be cultured.
Taking brain stem cells from people, though, is not very practical. Fortunately, other adult stem cells seem just as versatile. Malcolm Alison of the Imperial College School of Medicine recently showed that liver cells can be derived from blood stem cells, for example.
Such cells could be taken from patients themselves, but they are scarce, and Alison has yet to discover how to isolate them. A method developed by David Scadden at Massachusetts General Hospital, however, could help Alison and others. Scadden uses electrical pulses to kill large cells, leaving behind more of the smaller stem cells.
Even when they can be isolated, adult stem cells lose their ability to divide after a time, whereas embryonic stem cells divide indefinitely. 鈥淔inding a way to turn blood stem cells into liver cells, or into other cell types, and keep them dividing in a culture dish, is the huge challenge of the future,鈥� says Alison.
