Researchers at the University of Wisconsin, Madison reported Sunday that they've whipped up an exciting — but intricate — new recipe that could someday treat spinal cord injuries or provide a cure for amyotrophic lateral sclerosis (search), better known as Lou Gehrig's disease.

Step one: Take human embryonic stem cells (search), the microscopic dots that have brought condemnation from the pope, opposition from the president and people generally opposed to abortion. Add pinches of chemicals, dashes of other biological ingredients implicated in brain growth at just the right moment and voila: brain cells called motor neurons (search) that control every body movement.

The conclusion, reported online in science journal Nature Biotechnology, is important for two reasons. First, stem cell scientists have struggled to accomplish what researcher Su-Chun Zhang and his colleagues have just accomplished. It took Zhang's team two years of tedious trial-and-error experiments to direct stem cells to turn into motor neurons.

Perhaps more important, Zhang's recipe shows researchers that timing is everything when adding their chemical cocktails to stem cell stews. Stem cells are vulnerable to successful human manipulation for only the briefest of moments — and at different intervals depending on the results each researcher craves.

"This shows that you can't dump whatever growth factors you want in there," Zhang said. "It's not that simple. It's very specific. You have to have the right cocktail in the right amount at the right time."

Other scientists said Zhang's work also will help researchers better translate data gleaned from decades of animal experiments into human terms. Scientists were losing faith that 25 years of work with the embryonic stem cells of mice had little direct correlation to humans, said Harvard University neuroscientist Ole Isacson.

But with Zhang, and others, showing that the biological clock ticks differently in different animals and in each type of cell, it appears translating animal data to human terms is more about timing than biology.

"That is also somewhat reassuring," said Isacson, who has created dopamine-producing brain cells from stem cells. Parkinson disease patients lose dopamine cells, which help regulate body movement.

Embryonic stem cells are created in the first days after conception and ultimately turn into the 220 or so types of cells that make up the human body. Scientists believe they can someday control what stem cells become and when, using that power to replace damaged and dead cells that cause a wide range of suffering, from diabetes to Parkinson's.

But harnessing that power has proved elusive in all but a few cell types such as heart and two other types of brain cells.

"This is an important contribution because stem cell biology is difficult," Isacson said. "It helps decode the locks."

There are political hurdles as well. Pope John Paul II and others believe the work is immoral because days-old embryos are destroyed during research. President Bush has limited federal funding of the science.

More maddening, though, are the scientific bugaboos. Scientists have had trouble controlling what the stem cells turn into. That's a problem if brain cells are the goal and heart cells the results.