Less Promise for Marrow Cells

In a finding that could escalate the scientific and ethical debate over human embryo cell research, blood cells from bone marrow seem unable to transform directly into other kinds of cells and tissues -- as some scientists had recently come to believe. Nonetheless, researchers said, the cells may have significant potential for medical therapy.

The findings are the latest in the politically explosive new field of research that is examining the potential use of human "stem cells" to repair ailing tissues and organs.

Human stem cells come in two forms: those derived from human embryos and those retrieved from parts of the adult body, including bone marrow. Embryonic stem cells can morph into all kinds of tissues and so may be useful for repairing damaged organs, but they are controversial because embryos are destroyed to obtain them.

Some recent experiments had offered unexpected evidence that stem cells from bone marrow may have similar potential, in some cases apparently turning into liver cells, for example. The findings bolstered opponents of human embryo research, who have argued that adult cells are adequate. But the evidence for these seeming transformations has been incomplete.

Now a pair of studies published in yesterday's online edition of the journal Nature offer the first detailed analyses of how bone marrow stem cells turn into liver cells -- at least in mice. The cells, it turns out, do not simply change their identities by rewriting their genetic programs, as embryonic stem cells do. Rather, when they come in contact with liver cells, they fuse with them, forming hybrid cells with double the standard number of chromosomes. These cells are abnormal and would not be therapeutically useful, but some of them spontaneously "spit out" their extra DNA and then take on the characteristics of new and functioning liver cells.

Further studies will be necessary, scientists said, to see if human versions of such hybrid cells are normal enough to justify their experimental use in people.

-- Rick Weiss

More Afraid Than We Should Be

Americans wildly overestimated the risk of terrorism after the Sept. 11, 2001, attacks, according to a nationwide "emotion experiment."

In surveys conducted after the attacks, people estimated there was a 20 percent chance that they would be directly affected by a terrorist attack in the next year and said the "average American" faces a 48 percent risk, said researchers examining the links between emotions, policy and reality.

"There was an overwhelming overestimation of risk," said Jennifer Lerner, an assistant professor of social and decision sciences at Carnegie Mellon University. For even the 20 percent estimate to be accurate, we would have had to have Sept. 11 "every day and then some," she said.

Lerner and her colleagues also found that people who watched media reports that made them fearful -- such as a report on bioterrorism -- were likely to make higher risk assessments, while people who got news that made them angry -- like a report of some Arabs celebrating the attacks -- perceived relatively less risk. Both groups, however, greatly overestimated the actual risk of future attacks.

Fear and anger prompted support for different public policies: Fearful people tended to support policies of "conciliation," such as improved ties with the Muslim world, while angry people supported more "vengeful" policies, such as deporting foreigners without valid visas, the researchers found.

Lerner's paper, published this month in the journal Psychological Science, also found that women tended to respond with more fear, while men tended to respond with more anger. She said the findings suggested that the government and the media can unwittingly alter risk perception by making people either fearful or angry. Used responsibly, that connection could also be used to better communicate the real degree of risk, she said.

-- Shankar Vedantam

European Population Shrinking

If Europeans don't watch out -- actually, do more than just watch out -- there will be a lot fewer of them by the end of this century.

A team of Vienna-based researchers reported in the journal Science last week that the continent reached a demographic watershed in 2000. After decades of delayed childbearing and smaller families, Europe is now in "negative population momentum."

If the current fertility rate persists until 2020, there will be 88 million fewer people in the 15 countries of the European Union at the end of the century than there are today.

"Low fertility leads to smaller numbers of children than parents, locking in future decreases in the number of parents and a tendency toward population decline," wrote Wolfgang Lutz of the Vienna Institute of Demography of the Austrian Academy of Sciences.

If the average age of childbearing stops rising, fertility would actually rise from 1.5 to 1.8 births per woman, the researchers found. That, in turn, would lead to a smaller population drop of 49 million by century's end.

Even if that happens, however, the "support ratio" -- the number of working-age people per each person older than 65 -- will fall to somewhat below 3 to 1. If the average age of childbearing continues to rise, the ratio will be close to 2 to 1. Either number would mean a severe economic burden on the young, the researchers said.

"Over the coming decades," they added, the trend "will challenge social security and health systems, may hinder productivity gains, and could affect global competitiveness and economic growth."

Immigration may increase these ratios, but increases in longevity would lower them, the researchers noted.

-- David Brown