Combining Cloned Mice Embryos Improves Survival

Scientists have found that when two genetically identical cloned mouse embryos are combined, the aggregate embryo is considerably more likely to survive to birth. The results were reported in the October 1, 2003, issue of the Journal of the European Molecular Biology Organization (EMBO).

Despite the successful cloning of many mammals, the process remains very inefficient. Out of every 100 cloned mice, roughly one survives to birth. The researchers found that when they transferred the cloned hybrids back into the uteri of recipient mice, the survival rate jumped to 8%. The researchers were able to produce a litter of four cloned mouse pups, in contrast to the usual single pup. They are not quite certain as to why aggregation boosts survival, although they speculate that the combination of two embryos helps to compensate for genetic deficiencies in either.

"The genetic reprogramming of a cloned embryo never seems to occur with 100% accuracy,” said corresponding author K. John McLaughlin, assistant professor of animal biology at the University of Pennsylvania (Philadelpha, USA). "However, the group of genes that fails to reset properly differs in each individual embryo, meaning that each embryo that contributes to an aggregate can help mask the shortcomings of the other. By combining cloned embryos, you might end up with an embryo that's 99% reprogrammed rather than just 90%.”

When the researchers cut wild-type embryos in half, they found that the expression of key developmental genes was not affected, which suggests that the developmental deficiencies of cloned embryos are not due to low cell counts alone. They speculate that cells in a blastocyst may communicate in a way that is compromised in a smaller cloned embryo.




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