IN THE 1990s, at Harvard University and her colleagues made a peculiar discovery. They found that women who had given birth to boys up to 27 years earlier . “We were very surprised – it really changed our thinking about pregnancy,” says Bianchi, who is now director of the US National Institute of Child Health and Human Development in Maryland.
Other groups later , even when the children had become young adults. Together, these findings suggest that while we are in utero, a small proportion of our cells cross into our mothers and vice versa, then stick around for decades.
But this goes even further because it is thought that we also harbour cells from older siblings, uncles, aunts and grandmothers. One study of 154 Danish girls, aged 10 to 15, found that . This was more likely to be the case if they had an older brother. This could occur if a mother absorbed cells from her son while he was in utero, then passed those cells on to her daughter during a subsequent pregnancy. In theory, if the daughter later passed her brother’s cells on to a child of her own, that child would carry their uncle’s cells.
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Such effects can also be seen years after . And we are only now starting to realise the extraordinary impact these cells can have, not only on our health but our behaviour too.
For instance, studies have found , suggesting they may trigger reactions, even if a causal link is yet to be established. Meanwhile, increasing evidence suggests that these cells also play an important role in repairing tissue damage and combating disease.
Fetal-maternal microchimerism
Some from women with thyroid disease. Part of the thyroid from one woman was entirely male, says Bianchi. They believe the male cells came from the woman’s son in utero and were subsequently helping to repair her damaged thyroid.
Similarly, and Uzma Mahmood at University College Cork in Ireland discovered and found signs that these had assisted with the healing process. Cells from female fetuses possess the same healing properties, but it is easier to look for male fetal cells in mothers because they carry the distinctive Y chromosome.
Remarkably, at the Icahn School of Medicine at Mount Sinai in New York and her colleagues found that when the hearts of pregnant mice are damaged, cells from their fetuses travel to the injury. There, they transform into beating heart cells. This may explain why some people with heart failure spontaneously recover while pregnant, says Chaudhry. “My theory is that there’s an evolutionary mechanism whereby the fetus protects the mother because it has to protect its home.”
Protecting damaged organs
The transfer of cells the other way, from mother to fetus, also seems to be beneficial. For example, during an autopsy, at the Fred Hutchinson Cancer Center in Seattle and her colleagues in a pancreas from an 11-year-old boy whose body hadn’t been able to produce insulin on its own. The female cells appeared to have come from his mother and were attempting to help regenerate his malfunctioning pancreas.
These cellular souvenirs from pregnancy have now been . Some of the most intriguing discoveries are being made in the brain. Most recently, there have been and turn into neurons that form new connections with the mother’s own brain cells. What they do then isn’t clear, but researchers hypothesise that these structural changes may play a role in the mother’s ability to love and care for the child.
There is clearly much to discover, but it appears we should be happy to have this menagerie of cells from different generations inside us. It means that we always carry little pieces of them around with us, keepsakes that can change our behaviour and safeguard our health.
This article is part of a special series on the body, in which we explore:
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