Tuesday, May 10, 2005

The Word of the Day is Chimerical


In mythology, a chimera was a beast assembled from parts of other, more common animals, such as the eponymous Greek Himera (above), the Egyptian Sphinx, or the lawyer's favorite Chinese dragon-goat Hsieh-Chai. In biology, a chimera is an animal that contains not only its own genetic material, but that from some other zygote as well. The mythological chimera doesn't really exist. The biological one does.

If you've ever received an organ transplant, you are a chimera. If--and this is not as uncommon as it sounds--you were conceived alongside a twin who was then reabsorbed into you and your mother, you are a chimera. If you are the mother in such a scenario, you are a chimera. If you were ever gestated in a living person, there's a chance that you're a chimera.

If television is to be believed, which of course it is, such a state would enable you (CSI) or your supposedly dead twin (The X-Files) to commit crimes undetected.

Now the New York Times has a story on American cyclist Tyler Hamilton, who is about to be banned from racing because small amounts of another person's blood were detected in his blood. The conclusion the arbitration panel jumped to: he's been blood doping to gain a competitive edge. But Hamilton is sticking by his chimera defense.

One route to this odd state, called chimerism, is the vanishing twin. Dr. Helain Landy of Georgetown University, who has no involvement in the Hamilton case, has found that 20 to 30 percent of pregnancies that start out as twins end up as single babies, with one twin being absorbed by the mother during the first trimester.

Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.

Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.

Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it. It seemed not to exist in the past, she said, because no one was explicitly looking for small amounts of foreign cells in people's bodies.

Link. The word of the day today is, as it was for over a year in my friend's AIM profile, "chimerical," which, according to Webster's, means "existing only as the product of unchecked imagination; fantastically visionary or improbable; [and] given to fantastic schemes."

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