DNA from male cells, most likely from a fetus or sibling, are often found in the brains of women, according to a study released yesterday (September 26) in PLOS ONE. The findings are the first demonstration of microchimerism—in which cells that originated in one individual integrate into the tissues of another—in the human brain, and could have implications for disease.

“Knowing cells are in the brain brings home the idea that we’re a little more diverse than we thought we were,” said Nelson. “So conceptually, it may be more appropriate to think of ourselves as an ecosystem rather than a single genetic template.”

Researchers have suspected that the human brain may harbor microchimeric cells, which are present in other human organs, and previous studies in mice have shown that such foreign cells can break through the blood-brain barrier. But the study, led by Lee Nelson of the Fred Hutchinson Cancer Research Center in Seattle, revealed that microchimeric cells could not only migrate to the brain, but do so frequently: more than 60 percent of autopsied brains contained DNA from another individual.

Microchimerism most commonly arises during pregnancy when cells from a fetus pass through the placenta and into the mother’s body—and vice versa. The foreign cells can then migrate to various tissues and set up chimeric cell lines, which has raised many unanswered question about immune disorders and other links to disease risks. Other studies have found that fetuses can also acquire microchimeric cells from a twin or even from an older sibling, as some fetal cells linger in the uterus. In rare cases, microchimerism can occur from blood transfusions in immunocompromised patients.