快猫短视频

WHILE therapeutic cloning is storming ahead, hopes that adult stem cells could be as fertile a source of replacement tissue as embryonic stem cells have taken a blow. Studies apparently showing that stem cells taken from adults can develop into a variety of tissues may have been flawed.

This warning comes from two groups, one led by Austin Smith of the University of Edinburgh and the other by Edward Scott of the University of Florida in Gainesville. They found that rather than forming a range of tissue types by themselves, adult stem cells may be forming abnormal hybrids with embryonic stem cells that could be mistaken for pristine new tissues.

鈥淲e are not saying that those findings are wrong,鈥� says Naohiro Terada of the Florida team. But researchers shouldn鈥檛 conclude that their stem cells can form different tissue types without checking whether fusion is the cause.

This is just the latest of a series of setbacks to adult stem cell research. In February, one group largely retracted a previous claim that muscle stem cells could give rise to blood cells. Another failed to reproduce earlier studies claiming neural cells could turn into blood cells.

These results come at a politically important time, as countries such as the US and Australia are currently drawing up new laws on the use of embryonic stem cells (ESCs). Many people are opposed to ESC research because it involves destroying human embryos, and argue that adult stem cells show so much promise there is no need to mess around with embryos.

Now, however, it will be much harder for critics and scientists to argue that this promise is real. 鈥淚t鈥檚 incumbent upon us to prove whether or not fusion is responsible for [what] we have called 鈥榩lasticity鈥�,鈥� says Diane Krause of Yale University.

The Edinburgh and Florida groups grew neural and blood stem cells together with ESCs, mimicking earlier experiments in which neural stem cells grown this way were later shown to turn into various tissue types when injected into embryos. Both the neural and blood stem cells did indeed seem to revert to a blank-slate state.

But closer examination revealed that the cells behaved that way because they had fused with the ESCs. And when one of the groups injected these abnormal fused cells into mouse embryos, a few of their descendants were found to have contributed to different tissues throughout the bodies of the adult mice. That鈥檚 a cause for concern, because this is seen as the ultimate test of a stem cells鈥� plasticity.

Experts disagree on how significant the results are, however. Krause contends that the way both groups did their experiments encouraged the formation of fused cells. And the authors themselves admit that fusion is a very rare event.

Catherine Verfaillie of the University of Minnesota argues that the adult stem cells she鈥檚 discovered form too high a proportion of tissues when injected into mice for this to be due to rare fusion events. Verfaille鈥檚 work, revealed by 快猫短视频 (26 January, p 4), is considered some of the most promising to date. And Terada agrees that the way she carried out her experiments suggests fusion wasn鈥檛 involved.