During pregnancy, cells cross the placenta in both directions. The mother’s immune cells enter the fetus; the fetus’s cells enter the mother. This phenomenon, known as microchimerism, has been studied for years, primarily in blood and peripheral tissues.
A new study posted on the preprint server bioRxiv suggests it may go much further. Researchers have found maternal cells in human brain tissue, not just in infants, but in adults well into their 90s. The cells had differentiated into neurons and other brain cell types, raising questions about what role they might play in development and lifelong brain function.
What the study found
The research team, led by scientists including S.B. Kanaan at the University of Washington, analyzed brain tissue from two groups: 37 children with severe epilepsy who had undergone surgical resection, and 32 healthy individuals whose brains were collected at autopsy.
Using quantitative PCR to detect maternal DNA among millions of cells in each sample, the team found maternal cells in 70 percent of the epilepsy group (26 of 37 children) and 78 percent of the healthy group (25 of 32 brains). The healthy cohort included three men in their 80s and 90s, showing that maternal cells can persist for decades.
Single-nucleus RNA sequencing revealed what types of cells the maternal cells had become. In younger brains they were most often neurons, specifically layer 2/3 cortical neurons, and in older brains they were more commonly microglia, the brain’s resident immune cells. They also appeared as oligodendrocytes (which produce myelin), astrocytes (which support neurons), and endothelial cells (which line blood vessels).
The highest concentration was found in the hippocampus of one child, with 459 maternal cells per 100,000 cells, a small fraction but a measurable one.
Firstborn effect
One pattern stood out in the data. Among children who had maternal cells in their brains, 14 were firstborns and 12 were later-born. Among those without detectable maternal cells, only 1 was a firstborn, compared to 10 later-born children. This suggests that firstborns may receive a larger transfer of maternal cells during gestation, possibly because the placental barrier is more permeable during a first pregnancy.
The average density in the epilepsy group was about 2.2 maternal cells per 100,000 brain cells, an extremely small fraction, but one that could be functionally significant if these cells occupy specific roles rather than being distributed randomly.
Open questions
Amy Boddy, co-director of the Microchimerism, Human Health and Evolution Project at the University of California, Santa Barbara, who was not involved in the study, called the findings exciting. “What’s exciting here is that it’s tissue, not blood; it’s real human data, not an animal model; and the methods are cutting-edge.”
But significant questions remain. The healthy cohort could not be definitively confirmed as containing maternal cells because maternal DNA samples were not available for comparison, the foreign cells could theoretically come from other sources, such as an older sibling (through a phenomenon called fetal microchimerism), a vanished twin, a past pregnancy or miscarriage in female subjects, or even the maternal grandmother.
Boddy noted that the detection method has limits. “Due to the limits of detecting rare cells at low frequency with this method,” the true prevalence may be higher than reported.
The study is a preprint and has not yet been peer-reviewed.
Sources
- Kanaan, S.B., McDonough, A., Gentil, C., et al. “Microchimerism in the human brain, quantitative assessment and single nuclei profiling establish cell types and diversity.” bioRxiv (2026). DOI: 10.64898/2026.06.05.730225
- Live Science: “Cells from your mother likely infiltrated your brain in the womb” (July 19, 2026)
Research methodology note: This study is posted as a preprint on bioRxiv and has not yet completed peer review.