Clone rangers

Clone rangers
Jul 23rd 2009
From The Economist print edition
The technology of cloning is improving step by step





THIS mouse is one of a batch that represent the latest breakthrough in cloning technology. It was created by Zhao Xiaoyang and Li Wei, of the Chinese Academy of Sciences in Beijing, and their colleagues, and was reported this week in Nature. Cloning mice is nothing new, but this one and 26 like it are descended from what are known as induced pluripotent stem cells. These are created from the laboratory-cultured descendants of normal body cells by activating four usually quiescent genes. The result is something similar to an embryonic stem cell, from which it is already known that new, adult mice can be created. The question is, how similar? Dr Zhao and Dr Li have now shown that the answer is, “very similar indeed”.
To make their clones, Dr Zhao and Dr Li injected induced pluripotent cells into early-stage embryos called blastocysts. Normally, the result of doing this is a chimera—an animal that consists of a mixture of cells derived from the injected cell and the blastocyst. That proves the cells are, indeed, pluripotent; in other words, they can turn into a variety of tissues. But to make a true clone a cell needs to be not just pluripotent, but totipotent and thus able to turn into an entire animal.
To show that this, too, is possible, the researchers created a special sort of blastocyst whose cells have double the number of chromosomes found in normal mouse cells. These so-called tetraploid cells can go on to form placental tissue but cannot thrive in the embryo proper. So, if a mouse is born from a stem cell injected into a tetraploid blastocyst, that stem cell must have been totipotent.
Altogether, Dr Zhao and Dr Li created several lines of induced pluripotent stem cells that could produce successful chimeras. Of these, three could also pull off the trick of making a mouse when injected into a tetraploid blastocyst. And one of the 27 mice produced, a male, has also gone on to have a family of its own.
The importance of this work is its demonstration of just how similar to a real embryonic stem cell an induced pluripotent cell is—at least, in mice. If the same technology works in people, it may make the row about using human embryonic stem cells (which involves the destruction of human embryos) redundant. It might also bring closer the day when spare body parts can be grown from, say, skin cells of the person who needs them, eliminating the risk of tissue rejection.
At the moment, such thoughts are science fiction—as are thoughts of turning out human clones from skin cells. But Dr Zhao and Dr Li have taken their field a small step closer to making them fact.