AMERICAN researchers say they have found a way for male animals - including humans - to achieve virtual immortality by freezing the cells that generate sperm and transplanting them into another species.
Dr Ralph Brinster and colleagues at the University of Pennsylvania's veterinary school said they froze the stem cells that produce sperm in rats and transplanted them into mice. The mice, which had been made sterile, went on to produce healthy rat sperm.
"This fairly simple but stunning experiment is, in a very real way, the immortalisation of an individual", the science journal Nature Medicine, which published part of Dr Brinster's research, said. "This work opens up new areas of research for reproductive biologists, offers possible help for sterile men and raises profound questions for medical ethicists."
Dr Brinster, whose findings were also published in the sister journal Nature, said they would greatly improve on current methods used in animal breeding, treating male infertility and genetic research, all of which use frozen sperm.
"However many sperm you have you only have the end product. You do not have the generator", he said in a telephone interview.
In addition, the sperm of many species is difficult to freeze. Human and bull sperm, for example, freeze well and have produced generations of healthy offspring. But Dr Brinster said mouse sperm still had not been successfully frozen.
"It will be important for preserving livestock animals for generations", he said. "It will be valuable for endangered species. Even if an animal dies before it can make sperm, you can freeze the stem cells and some day we can re culture them and introduce them into another species to produce sperm.
Dr Brinster said he had shown that the technique could cross wide species barriers. Mice and rats may look alike but they diverged on the evolutionary tree 11 million years ago and are genetically more different from one another than humans are from chimpanzees.
He said mice could be used as living factories to produce the sperm of other species.
Fellow researchers said the findings were significant but Dr Brinster had more work to do.
"The authors need to demonstrate that the rat sperm are capable of fertilising rat eggs with the production of viable offspring", Dr Robin Lovell Badge of Britain's Medical Research Council said.
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