A family affair Swapping cells isn’t just limited to a mother and her child (see main
at the University of Washington, Seattle, but studies suggest it does
story). Connections may reach much deeper into your family tree. Take, for example, a woman pregnant with a baby girl, her second child. We know that microchimeric cells can stick around for decades, so it’s easy to imagine cells from her eldest still running around her body. They could get transferred to her new baby. If the eldest was a boy, the daughter now has cells from her brother. These could conceivably be passed on when the daughter has a child of her own – who would
happen. Rather than just one other human, we could have a whole family album’s worth of cells inside us, all exerting an effect on us (see diagram, right). “Another thing to consider,” she says, “is that microchimerism can also occur after a miscarriage.” The same is true for abortions. In both cases, a woman would carry the cells of her unborn child, and any subsequent pregnancies conceivably could pass those cells on to the next generation. It doesn’t stop there. Twins can share
therefore have cells from their uncle. David Haig at Harvard University points to this scenario as a hiccup in studies that assume Y chromosomes found in a woman’s bloodstream must have come from her son. “One thing that I think is crying out to be done is to identify exactly who the cells are from,” he says. The odds of carrying your sibling’s
cells in the womb, which they then carry out into the world with them. Finally, consider the fact that more pregnancies begin with twins than end with them. In rare cases, one fetus simply vanishes, absorbed by the mother, the placenta or the other twin. These “vanishing twins” may in fact leave traces of themselves, in their mothers and
cells are probably low, says Lee Nelson
brothers and sisters.
neurons in their mothers’ brains. So could the same thing be happening in humans, helping to form the cells that carry information about your senses, your movement, your thoughts? Nelson and her team are now looking to see if this is the case and are expecting results in the next few months. They are also looking in the opposite direction, to see whether maternal cells reach the brains of their children. “I wouldn’t be surprised if there were maternal cells in the brain,” says Nelson, “and I wouldn’t be surprised if they were an important part of normal development.” How microchimeric cells interact with our immune system is also a key point of interest. After all, the immune system is there to defend our bodies from invaders, yet microchimeric cells seem impervious to it. That’s promising for things like organ transplants, but how do the cells dip below the immune system’s radar? Nelson’s colleague Hillary Gammill points out that microchimeric cells can turn into a type of immune cell: they literally embed themselves into our body’s defences. Chaudhry is currently looking at the molecules on the surface of microchimeric cells to try to get to the bottom of this. The health benefits and downsides may not
Family ties You could have cells from various relatives coursing through your blood A woman can pass cells on to her children and have cells from her children and mother in her body (solid lines). She could theoretically even have cells from older siblings (dotted lines)
DECEASED The cells of a dead child could have transferred in the womb to the mother, so can be passed on to her subsequent children – though this is likely to be rare
be limited to mother and child. Gammill has looked at how a woman’s cells could give a helping hand to the next generation when she becomes a grandmother. Pre-eclampsia is a complication seen in 6 per cent of pregnancies. In a study of women who developed the condition, Gammill found that none of them carried cells from their own mothers. By contrast nearly a third of the women in the study who didn’t get pre-eclampsia did. Intriguingly, in these women, the number of their mother’s cells in the blood got a boost during the pregnancy’s third trimester, when pre-eclampsia is most common.
Protective hand The results raise the intriguing possibility of some kind of protective hand being extended to the fetus from their grandmother, says Gammill. Jen Kotler at Harvard University is using the same green fluorescent tags as Chaudhry to trace cells across generations, and see whether cells from grandparents can end up in the brains of their grandchildren. Why does microchimerism happen at all? Kotler’s colleague David Haig points out that evolutionary pressures may be at play (see “You
Men get cells from their mother and possibly older siblings
are multiple, but why?”). “You might expect that [fetal cells] will be enhancing bonding of the mother to the child,” says Haig – thus increasing the child’s chance of survival. “We know there are changes in the brains of mice after pregnancy that are involved with the delivery of maternal care,” says Haig (see “The real baby brain”, page 36). “We are raising the possibility that offspring cells could be having a say in the matter as well.” For Nelson, the weird world of microchimerism turns our understanding of the “biological self” on its head. “To me, the best working paradigm is that we are an ecosystem,” she says, one made up of a patchwork of humans and which can have both positive and negative effects on our health. “Microchimeric cells are present in fairly low numbers, so that will tend to limit their influence,” says Haig. “But small numbers of cells can have big influences. ” In future, we may be able to invite new helpful humans to join our body’s ecosystem, and encourage less helpful ones to leave. Being human is about to get a whole lot more complicated. n Andy Ridgway is a freelance writer based in Bristol, UK 9 January 2016 | NewScientist | 29
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