Making an Egg In Vitro.

As reported in New Scientist, embryonic stem cells have been turned into eggs. The feat was achieved with embryonic stem (ES) cells from mice, but the same technique should work in humans. The work was published May 1 in Science online by Hans Schler and his colleagues from the University of Pennsylvania (3451 Walnut St., Philadelphia, PA 19104; Tel: 215/898-5000; Website: www.upenn.edu).

Previously it had been thought ES cells could produce everything but the germline. This work proves that ES cells are "totipotent"-able to give rise to every tissue in the body, including those used for reproduction.

"The work is exceptional," adds researcher Gerald Schatten from the University of Pittsburgh, Pennsylvania. "The results convincingly demonstrate that ESCs from mice have universal, not just restricted, developmental capabilities."

Several groups of researchers were trying to make eggs from ESCs, but with little success. In the end, the key was straightforward- letting nature take its course.

Instead of searching for special chemical factors to coax ESCs into becoming eggs, Schler's group let ESCs grow in basic conditions but at a high density. "The procedure is just too simple," he says. "There's no complicated cocktail of growth factors."

Growing at high density, some of the cells detach from the lab dish and form floating aggregates. The clumps look like the type of debris most scientists would probably throw away. But researcher Karin Hbner took those clumps and placed them in new dishes. "She found that in four days they proliferated like crazy." As it turned out, those aggregates were like miniature ovarian structures, each with small cells nurturing a bigger cell that was to become the egg.

Schler's team not only followed the aggregates morphologically, but monitored the expression ...