ISSUES of safety aside, the very idea of cloning people—of taking, say, a cell from the skin of a man or a woman and growing it into a new human being with exactly the same genes as its progenitor—is anathema to many. But what about taking such a cell and creating from it an egg or a sperm that can be used for in vitro fertilisation? That would enable infertile men and women, and gay couples, who wanted to raise children genetically related to both parents to do so, rather than relying on the assistance of an unrelated egg or sperm donor to start a family.

In people, this is not yet possible. But Katsuhiko Hayashi and his colleagues at Kyushu University, in Fukuoka, Japan, have done the equivalent in mice. As they report this week in Nature, there are animals now scampering around cages in their laboratory whose maternal antecedents are egg cells derived not from the ovaries of their mothers, but from body cells (known as somatic cells), in this case from those mothers’ tails. Nor does it stop there. In the past, using a slightly different technique from the one that he describes this week, Dr Hayashi has bred mice using somatic-cell-derived sperm.

Both of these sorts of animals have gone on to breed successfully. So, not only are Dr Hayashi’s creations viable, they are fertile. Moreover, in principle—though he has not yet done so in practice—he could fertilise his somatic-cell-derived eggs with his somatic-cell-derived sperm to create an entirely somatic-cell-derived adult animal. He might even, if he so chose, be able to derive sperm and eggs from the same animal, for the processes do not require that the eggs be made from female cells and the sperm from those of males. That would create a mouse which had only one parent, yet was not a true clone of that parent because the sex cells which united to form it would both have undergone the internal genetic mixing that biologists call meiosis.

One step back, two steps forward

Dr Hayashi and his colleagues do not create their eggs and sperm directly from somatic cells. First, those cells have to undergo an alchemical transformation to rejuvenate them into an ancestral form known as a pluripotent stem cell. Mature body cells (eggs and sperm included) derive from progenitors, known as stem cells, that have the power to divide, proliferate and eventually to turn into particular cellular components of a particular tissue. Pluripotent stem cells are, in turn, the ancestors of these tissue-forming stem cells.

In nature, pluripotent cells are restricted to embryos. The first students of cloning and its related arts had therefore to “harvest” them for their experiments—which created ethical dilemmas when the embryos involved were human. Such harvesting is no longer necessary. Instead, for mice, men and many other species, pluripotent cells can be made to order by taking an ordinary body cell and adding to it active copies of the four genes which encode the genetic switches that cause pluripotency.

It was such “induced” pluripotent cells that formed Dr Hayashi’s starting-point. As he had discovered in his experiments creating sperm, judicious application of a molecule called bone morphogenetic protein 4 turns pluripotent cells into primordial germ cells—the type of stem cell ancestral to both sperm and eggs. Which of these a primordial germ cell goes on to become depends on the sex of the tissue it finds itself in. In those earlier experiments Dr Hayashi injected them into the testes of newly born mice, thus persuading them to become sperm when they underwent meiosis. This time he used ovarian tissue extracted from mouse fetuses to induce egg-forming meiosis.

To keep track of this process, and to avoid confusion, Dr Hayashi took the cells used to make the eggs from a dark-eyed mouse. He then fertilised the eggs he had created in vitro with sperm from a pink-eyed male, and also implanted the resulting embryos into pink-eyed females. To everyone’s delight the pups born of this arrangement had dark eyes (see picture)—caused by a gene that could have come only from the tail-derived eggs. Furthermore, as had happened before with the somatic-cell-derived sperm, these pups developed normally into adults and were themselves able to reproduce.

All this is a long way from enabling scientists to perform the same trick with people. First and foremost, using human embryonic tissue in any part of the process is out of the question for ethical reasons. That means someone needs to work out exactly which of the chemicals in testes and ovaries tell primordial germ cells whether to become eggs or sperm. Second, at the moment the process is extremely inefficient. Only 3.5% of Dr Hayashi’s tail-derived embryos grew into pups, compared with about 60% of embryos from normal eggs. Third, though mice have proved useful models for examining many questions of human medicine, mere models is all they are. A lot more research will be needed before anyone (or, at least, anyone with any ethical sensibility) tries something similar on a human being. If and when that day comes, though, the unwillingly childless around the world will be watching with great interest.