TAKE the fertilised egg of a pig. From each cell in the resulting embryo cut out a gene or genes that promote the development of the animal’s heart. Inject human stem cells from a patient who needs a new heart into the embryo and then place it into the womb of a sow. Wait nine months. The result is an adult pig with a heart made of human cells. The pig can be slaughtered and the heart transplanted into the patient who provided the stem cells, for whom the organ will be a genetic match.

That, at least, is the hope of a panel of researchers who presented their work to the the annual meeting of the American Association for the Advancement of Science, in Austin, Texas. For, though this kind of biological melding may trip the disgust circuits, the value of such a procedure, were it possible, is clear.

First, transplantable organs are scarce and demand for them is increasing. As life in general, and cars in particular, become safer, the supply of bodies with healthy organs in them is shrinking. Meanwhile, people are living longer. As a result, some 75,000 individuals in America alone are waiting for organs. Every day around 20 of them die. Besides increasing the supply of organs, growing them in animals might also increase their utility. The genetic match between organ and patient would mean those receiving transplanted organs would no longer need to take immunosuppressive drugs to stop rejection.

All this makes growing organs in livestock a tantalising alternative to harvesting them from the dead. Thousands of years of breeding have yielded beasts which grow fast, so a patient need not wait long while his future heart develops inside a young animal. Sheep, cows and pigs are all roughly the right size to host human organs. And, since such animals are already raised for their flesh and skin, their use to grow more valuable things should meet with no objection beyond squeamishness.

Creating human organs in this way would rely on the union of two recent developments in biotechnology. CRISPR/Cas9, a genetic-engineering tool discovered in 2012, would snip away portions of the animal-host’s genome that control the development of the organ being grown. This would create a “genetic vacuum” which could be filled by induced pluripotent stem cells, the second breakthrough, made in 2006. Human pluripotent stem cells can grow into many different kinds of tissues, filling the void left in the developing animal with an organ made from the patient’s own cells.

That this combination works in principle was first shown last year, when a group at the Salk Institute in California reported making mice with eyes, pancreases, hearts and other organs composed of rat cells. Such mixed-species creatures are known as chimeras, after a monster in Greek mythology. Many of these mouse-rat chimeras lived to adulthood, and one reached its second birthday which, for a small rodent, is old age. A second group, led by Hiromitsu Nakauchi of Stanford University showed that a mouse pancreas grown this way in a rat, which then had parts of it transplanted into a mouse genetically identical to the one that supplied the stem cells employed, could control diabetes in that mouse. Dr Nakauchi had thus created a working, transplantable organ.

The creation of chimeras that include human organs is more challenging, because people are less closely related to sheep and pigs than mice are to rats. Nevertheless, Dr Nakauchi and his group have followed up their mouse work by growing human pancreas cells in pig fetuses. Another panellist, Pablo Ross of the University of California, Davis, said he had managed a similar feat in sheep. In both cases the chimeric animals were not brought to term. Were that to happen, their human pancreas cells might hypothetically be extracted and transplanted into people suffering from diabetes, in order to revive a patient’s ability to produce insulin.

Dr Nakauchi and Dr Ross both performed their tricks by using CRISPR/Cas9 to snip a gene called Pdx1 from the embryos of their pigs and sheep. This gene encodes a protein crucial to pancreatic development, thus creating the genetic vacuum that the human pluripotent stem cells go on to fill. But Pdx1 is not the only gene that can be silenced in this way. Daniel Garry of the University of Minnesota uses the technique to shut down Etv2 in pigs. Etv2 controls the development of the vascular system, including the heart. Dr Garry, too, is then able to persuade human stem cells to grow into organ cells—in his case heart cells—though at the moment they form only a small proportion of the cells in the resultant embryonic hearts.

In their quests for pancreatic cells, Dr Nakauchi and Dr Ross have encountered similar problems of rarity. But these are early days. Dr Garry, whose laboratory is now producing two or three pig-human fetuses a week, is studying those fetuses to try to understand why it is that in some only one heart cell in 100,000 is human while in other fetuses the number is one in 100. If he can discover the underlying principle, then the aim of replacing pig cardiac cells entirely with human ones will have come closer.