Two methods may dramatically increase the availability of embryonic stem cells. Using biochemical techniques, three independent teams reprogrammed mouse skin cells into cells nearly indistinguishable from the embryonic kind. Using a virus as the messenger, the researchers inserted into adult mouse skin cells four key genes that are expressed in embryonic cells, effectively turning back the clock on the skin cells. When injected into a blastocyst (an embryo just a few days old), they contributed to the development of its layers—and mixed in with the mature animals' germ cells.

These findings, reported online June 6 by Nature and in the June 7 issue of Cell Stem Cell, build on past work of a team led by Shinya Yamanaka of Kyoto University, which had created more limited reprogrammed cells. Researchers may know in the next year whether the process works in human cells.

The June 7 issue of Nature also describes a method to derive stem cells from single-celled fertilized embryos, or zygotes; previously, many scientists believed that only unfertilized eggs would do. Instead of removing a zygote's nucleus, a separate Harvard University group swapped the chromosomes of an adult cell for those of the zygote before its nucleus re-formed during cell division. The approach yielded harvestable stem cells as well as mouse clones. Fertilized zygotes, which are routinely discarded at in vitro fertilization clinics, are more abundant than unfertilized eggs.