COMBINING two or three cloned embryos to make a single one greatly increases the chances of a live animal being born.
Cloning has always been very inefficient, with hundreds of embryos having to be created to get a few live births. The cloned embryos of many species also have fewer cells than normal and often do not express genes that are key to development, such as Oct-4. So K. John McLaughlin at the University of Pennsylvania, in Philadelphia, wondered if adding more cells to cloned embryos might improve their chances.
His team created genetically identical cloned mouse embryos using cells from one mouse, let them divide to the four-cell stage and then merged two or three. Eight pregnancies resulted from the implantation of 98 double-clone embryos, compared with just four from 391 single-clone embryos. Oct-4 expression was also shown to be normal in triple-clone embryos (The EMBO Journal, vol 22, p 5304).
Advertisement
But the results suggest it is not increasing the number of cells in the embryo per se that boosts survival. The problem with cloned cells is that they are not always correctly 鈥減rogrammed鈥� to become embryos. In other words, the genes needed for an embryo to develop, such as Oct-4, may be switched off, or the wrong ones may be turned on. In the combined embryos, the cells from one embryo may compensate for defects in those from another, and vice versa, the team thinks.
McLaughlin says the technique should work in other animals. Several companies hope to use cloning to produce multiple copies of the best farm animals or of genetically engineered animals.