Eggs and more grown from mouse stem cells. (Biology).

In a series of recent experiments, scientists have transformed cells from mouse embryos into skin, heart muscle, and even eggs. In addition to providing insight into how such tissues develop, these feats have renewed the political and ethical debate over whether similar experiments should be conducted with cells derived from human embryos.

Known as embryonic stem cells, the lab-grown mouse cells are unspecialized and can grow in lab dishes seemingly forever. Scientists at the University of Pennsylvania created apparently normal mouse eggs from such stem cells, according to an upcoming report in Science. While observing batches of stem cells, the biologists detected a small number of cells that began to display a genetic marker of egg cells.

The stems cells that appear to have become eggs cells also ended up surrounded by structures similar to the so-called follicles that envelop typical eggs. These appear to be the first mammalian eggs formed completely in a lab dish, but researchers haven't yet shown that these eggs can generate normal mice if fertilized with sperm.

"It is particularly surprising that [the researchers do] not appear to have done anything really unusual ... to get the cells to make follicles," says John Eppig of Jackson Laboratory in Bar Harbor, Maine.

In another experiment, reported in the April 15 Circulation, researchers from Brigham and Women's Hospital in Boston screened 880 compounds for their capability to induce mouse stem cells to take on the attributes of heart muscle cells. They found that only ascorbic acid, better known as vitamin C, did the trick.

And in the May 13 Current Biology, a group from the French Institute of Health and Medical Research in Nice, known as INSERM, reports that mouse embryonic stem cells become skin cells called keratinocytes if the stem cells are grown on a matrix of proteins and other molecules in the presence of vitamin C or a substance called bone morphogenic protein-4. The resulting skin cells form layers reminiscent of normal embryonic skin.--J.T.