EXCITEMENT about the potential of stem cells for curing all manner of ills is being tempered by two new studies that highlight the potential dangers. They show that even stem cells taken from adults can turn cancerous if they are allowed to multiply for too long outside the body.Researchers have long known that there is a cancer risk with stem cells extracted from very early embryos. Until they change into more specialised tissue, they can form aggressive cancers called teratomas when injected into animals.Until now, it has been widely assumed that adult stem cells, such as those taken from bone marrow, do not form cancers. But the latest studies suggest that adult stem cells are safe only if the number of times they are allowed to divide outside the body is limited.A team at the Autonomous University of Madrid in Spain grew human mesenchymal stem cells extracted from fat tissue for up to eight months. During this time the cells divided between 90 and 140 times. When transplanted into animals, the oldest cells formed cancers (Cancer Research, vol 65, p 3035).
