You May Have Other People’s Brain Cells in Your Head

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In 1979, Stanford University researchers first discovered that women have some male Y-chromosomes in their blood. Women have only X-chromosomes, so the Y-chromosomes had to come from someone else. In this case, the women were pregnant, so it was inferred that the chromosomes came from the male babies in their wombs. It was interesting enough that not only does the mother’s genetic material enter the baby, but the genetic material from the baby (and thus from the baby’s father) enters the mother.

This cell-swapping phenomenon is known as microchimerism. Since those early studies, we have realized that women who have never had sons may also have these foreign Y-chromosomes in their bodies. These cells also may affect a woman’s immune system, and a study in 2012 showed that the foreign cells are found in the brain.

Women Who’ve Never Had Sons

A 2005 study by researchers at the Fred Hutchinson Cancer Research Center in Seattle focused on microchimerism in women who have never had sons. They found the Y-chromosome cells present in 21 percent of their test subjects. Results ranged from the equivalent of 0 to 20.7 male cells per 100,000 female cells.

"Male microchimerism was not infrequent in women without sons."

— study, Fred Hutchinson Cancer Research Center

The study, published in the American Journal of Medicine in August 2005, concluded: “Male microchimerism was not infrequent in women without sons. Besides known pregnancies, other possible sources of male microchimerism include unrecognized spontaneous abortion, vanished male twin, an older brother transferred by the maternal circulation, or sexual intercourse.”

Foreign Cells in the Brain, Correlation With Alzheimer’s

A 2012 study also conducted at the Fred Hutchinson Cancer Research Center looked for male microchimerism in the brains of deceased women. They found it in the brains of more than 60 percent of the women.

Women with Alzheimer's disease were found to have fewer microchimeric cells in their brains.

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Women with Alzheimer’s disease were found to have fewer microchimeric cells in their brains. This finding was counterintuitive, because the disease is more common in women who’ve had a high number of pregnancies.

Correlation With Other Illnesses

The microchimeric cells can be found in a woman’s body up to 38 years after pregnancy, according to a 2003 article by Michael Verneris at the University of Minnesota. They may have long-term effects on a woman’s health, but little is known with certainty about these effects.

Some microchimeric cells are similar to stem cells, which are able to transform into various kinds of tissue.

Some microchimeric cells are similar to stem cells, which are able to transform into various kinds of tissue. These microchimeric cells have been known to gather around damaged tissues on a repair mission, morphing into whatever kind of cells are needed.

But, they have also been found in cancerous tumors and it’s not clear what they were doing there.

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The cells are perceived by the body’s immune system as being partly foreign, since half the fetus’ genetic material is from the mother and half from the father. This may prime the immune system for detecting cancer cells, which arise due to genetic mutations, according to Scientific American.

On the other hand, the cells are more common in people with Multiple Sclerosis, an autoimmune disease. This suggests they may set off an autoimmune attack in the body.

We’re not really sure whether they harm, help, or have relatively little effect. Diana Bianchi, a geneticist at Tufts Medical Center, told Scientific American: “There isn’t enough evidence to accuse or acquit microchimeric fetal cells.”