Biologists come close to cloning primates
Cloned monkey embryos transferred into mothers.
US biologists have created cloned monkey embryos, and successfully transferred them into monkey mothers. Although none of the resulting pregnancies lasted more than a month, this is by far the closest scientists have come to cloning a primate.
The study was unveiled yesterday by reproductive biologist Gerald Schatten of the University of Pittsburgh, Pennsylvania, at the annual meeting of the American Society for Reproductive Medicine in Philadelphia. Schatten's group copied a technique used earlier this year to clone a human embryo and extract embryonic stem cells.
If researchers are able to repeat this process in monkeys, it might help them to refine the tricky technique without experimenting on human eggs and embryos, which are very difficult to obtain and raise a host of ethical objections. This in turn might help to resolve whether human embryonic stem cells, which can grow into a variety of tissues, will prove useful in medicine.
Previously, Schatten and his colleagues had struggled to create healthy monkey embryos by cloning, which involves removing the DNA-containing nucleus from an adult cell and inserting it into an egg stripped of its own nucleus.
In a 2003 study published in Science, Schatten suggested that extracting the nucleus from a monkey egg also robs it of two proteins essential for survival1. He found that all the resulting embryos had fatal chromosomal defects, and speculated that cloning any primate, including humans, might be impossible.
That view proved false in February this year, when scientists from South Korea announced in Science that they had successfully cloned human embryos, and used them to grow embryonic stem cells capable of morphing into numerous different tissue types2. One aim of such research is to grow replacement tissues to fight human disease.
In the Korean work, the cloned human embryos were allowed to divide in culture for just five or six days before being terminated. But, by adopting the Koreans' technique, Schatten's team made 135 cloned monkey embryos and transferred them into 25 mothers. The team experimented with transferring the nuclei from skin cells and from cumulus cells, which are found in the ovary.
Why does it work?
Schatten says he is not yet sure why the Korean method is so successful. Rather than sucking the nucleus out of the recipient eggs, the technique involves gently squeezing it out. This may remove less of the cell's cytoplasm and leave more of the essential molecules needed by the egg to direct embryo development; or it may simply cause less damage to the eggs.
None of the cloned monkey embryos resulted in a pregnancy that lasted more than a month. But Schatten says it is too early to say whether cloned monkeys will ever be born; it may just take more attempts. It is also impossible, he says, to use these results to predict whether a cloned human baby could survive long in development.
Schatten adds that his preliminary attempts to make embryonic stem cells from cloned monkey embryos failed. But at least his study confirms that the Korean cloning method works, something that has been difficult to prove because very few research groups work in such areas. "It shows that at least part of the technique is reproducible," he says.
- Simerly C., et al. Science, 300. 297 (2003).
- Hwang W. S., et al. Science, 303. 1669 - 1674 (2004).