Why, of all animals, have scientists used cloned pigs to use for organ transplant for humans

A.� The five piglets born on Christmas day are clones produced with the intention of providing suitable organs for transplanting to humans, officially known as xenotransplantation.

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There are several reasons why pigs, rather than other animals, are thought to be well suited to providing organs for humans.

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Firstly, pig hearts are similar in size to human ones. In fact valves from pig hearts have been used in human heart surgery for over a decade.

Cloning is still a relatively new field, but pigs have already been cloned and genetically engineered and so scientists have a better understanding of them.

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One of the biggest problems in finding suitable donor organs for a human is that once transplanted the recipient's body rejects them. Scientists have been able to reduce this risk of rejection when using pig organs by switching off the relevant gene.

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Q.� They sound perfect, what's the hitch
A.� There are several major hurdles still to be overcome, including the problem of human recipients becoming infected with a pig virus after transplantation.

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Q.� Why not use an animal much closer to us genetically, like apes or monkeys
A.� Although pigs are mammals like us, they're by no means our closest living relatives, which begs the question, why not use an animal with a much closer genetic makeup to our own, such as monkeys or apes

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First of all, unlike pigs and other animals not as much cloning work has been done on our fellow primates, making them much more of an unknown quantity.

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Experiments so far suggest that producing a cloned monkey is quite a tricky matter.

Cloned monkey eggs look fine but are actually full of genetic mistakes, which suggest that there is something unique about primate eggs that will make cloning monkeys far more difficult than cloning other animals. At the very least, the experiments show that there's a lot to learn before primates can be cloned.

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Another huge hurdle lies in the moral issue of performing experiments on and cloning apes and monkeys. Although their close relationship to us theoretically makes them perfect candidates for donating organs it also raises many ethical concerns about whether we should be using our close relatives for such purely selfish means.

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by Lisa Cardy