A Placenta With an Immigration Policy
The mother’s bloodstream and the fetus’s bloodstream run right next to each other inside the placenta — the large, tangled bundle of blood vessels that links mother and child through the umbilical cord — but until the 1970s they were assumed never to mix. Nutrients, oxygen and waste crossed through filters. The two rivers stayed in their own beds.
That assumption is now dead. Cells themselves cross the border. They move from one bloodstream to the other, and they do not leave. In the 1970s, researchers found cells carrying male Y chromosomes in the blood of pregnant women — direct evidence that fetal material had entered the mother Lee Nelson at the University of Washington in Seattle described the placenta as having a selective immigration policy — some cells get waved through, and once inside, they stay. [2] The word for this is microchimerism: a body that contains a small population of cells whose DNA belongs to another person. Most of us are walking patchworks — our own cells, plus emissaries from our mother, our children, even our siblings, lodged in our tissues. The question that follows is not whether these cells are there. It is what they are doing.
Counting One Cell in a Hundred Thousand
Seeing these cells has always been the hard part. Brain tissue from both a mother and her child is needed for comparison, and even then the foreign cells hide in plain sight. Sami Kanaan at the Fred Hutchinson Cancer Center in Seattle puts the general frequency at roughly 1 in 100,000 — a single stranger in a stadium of familiar faces. That ratio is why the phenomenon stayed partly invisible for so long.
Kanaan and his colleagues found a way around the problem. They analyzed genetic sequences in brain samples removed from 37 children, from newborns to 14-year-olds, during surgery to treat epilepsy. The team found maternal cells in 26 of the 37 children, with each sample holding 2.2 maternal cells per 100,000 on average. They compared those sequences against mouth swabs taken from the children’s mothers. The technique, single-nucleus RNA sequencing, reads the genetic material of individual cells. Then they ran a computational tool they had built earlier, called Cellector, which scans the sequence data for patterns. As Kanaan describes it, the tool models the data and flags any cell that does not fit the model — so it is probably a foreign cell. More of the children probably carried maternal cells, he says, but the method would have missed cases in which mother and child shared identical genetic variants.
Hina Chaudhry at the Icahn School of Medicine at Mount Sinai in New York, who was not involved in the research, called the work exciting for using sophisticated methods to answer the basic question of whether microchimerism exists at all
The Brain as a Genetic Mosaic
The maternal cells are not idle residents. Kanaan’s team identified them working across many cell types in the brain: neurons, which transmit information; microglia, the brain’s immune cells; astrocytes, which support neurons; oligodendrocytes, which build the protective sheath around neurons; and endothelial cells, which line blood vessels.
Chaudhry’s summary of what this means is blunt: every human brain is likely a genetic mosaic of more than one person. In a second part of the experiment, the team ran Cellector over RNA sequence data from three previously published studies, covering individuals from a 22-week-old fetus to people in their 80s. Microchimeric cells appeared at every age — but their number declined with age, possibly through apoptosis, the programmed cell death that clears damaged and old cells. Kanaan called it crazy that the cells show up in gestational samples, and described harboring them as a widespread human feature that begins in early life and continues throughout the whole life.
That finding lands at the intersection of developmental biology and neuroscience, and it opens questions neither field can answer alone. If a mother’s cells can become neurons and immune cells in her child’s brain, what do they contribute to its wiring and its defense? And if the population shrinks with age, does losing those cells change how the brain ages?
