After a woman is pregnant, fetal cells linger in her body long after her baby is brought out into the world. They cross the placenta and congregate in her thyroid, breasts, brain, scars... and elsewhere. The phenomenon is called "fetal microchimerism," a reference to the hybrid monster of Greek mythology that strikes me as both whimsical and menacing.
But what do these cells do? An entertaining and informative National Geographic blog post highlights a recent review study published in BioEssays that seeks to answer this question. The evidence we have so far is contradictory and messy, not yielding much in the way of patterns: Sometimes cells collect more in diseased tissues, other times in healthy ones. But when viewed through an evolutionary lens, things start to make sense, argue the paper's authors. These cells allow a baby to inadvertently influence her mother's body in her own interest, which is sometimes - but not always - in the mother's interest, too.
Writer Ed Yong explains:
Some of those changes, like faster healing, benefit the mother too. Others may not. For example, foetal cells could stimulate the breast to make more milk, either by releasing certain chemical signals or by transforming into glandular cells themselves. That’s good for the baby but perhaps not for the mother, given that milk takes a lot of energy to make—mothers literally dissolve their own bodies to create it. And if the foetal cells start dividing too rapidly in the breast, they might increase the risk of cancer.
Similarly, the thyroid gland produces hormones that control body temperature. If foetal cells integrate there and start dividing, they could ramp up a mother’s body heat, to a degree that benefits her baby but also drains valuable energy. And again, if they divide uncontrollably, they might increase the risk of cancer. Indeed, thyroid cancer is one of the only types that’s more common in women than men, but is not a reproductive organ like the ovaries or breasts.
Such influences would have developed gradually over hundreds of millions of years in a subtle evolutionary contest between mother and fetus - it is in the mother's interest for the fetus to do well, but not to monopolize all her resources, so it's not unlikely that mothers evolved counter-measures. The paper authors don't have any conclusions yet, but their point is that within this evolutionary framework, it makes sense that fetal cells both help and harm the mother.
Previous research on microchimerism has only asked about such cells' presence, not their function. The paper's authors hope to organize a workshop to test some of the hypotheses they proposed, which means gathering microchimeric fetal cells and sequencing their genes, then working out which of the mother's genes they are activating and whether these correlate with any traits like milk production or temperature. The possibilities for further research are immense:
And then, there’s the matter of cells that travel in the other direction—from the mother to the foetus. What do they do in their new homes? These paths can get even more complicated. It’s possible that the cells from one foetus can travel into its mother, hide out, and then into a sibling during a later pregnancy. “At one point, we started trying to draw family trees, and trying to work out where all the microchimerc cells could be going,” says [co-author Athena Aktipis, PhD]. “It got really messy.”
Previously: How a child's cells may affect a mother's long term health
Related: The yin-yang factor
Photo by Simone Tagliaferri