May 14, 2026 | During pregnancy, the cells of a fetus make their way into the tissues of mom where they can persist for decades, much like what happens when someone gets an organ transplant but to a far tinier extent. That the maternal-fetal exchange results in chimerism—more specifically, “microchimerism”—is a relatively little-known yet fascinating and profoundly common biological phenomenon, according to Thomas Kroneis, Ph.D., a researcher specializing in single cell analysis in the division of cell biology, histology and embryology at the Medical University of Graz (Austria).
Microchimerism is not new—German pathologist Georg Schmorl first documented that this cell exchange happens back in 1893—but only in recent decades has it fanned the flames of scientific intrigue across a multitude of scientific disciplines that include reproductive and evolutionary biology, immunology, organ transplantation, cancer, and psychology. That breadth of interest is reflected in a perspective piece on the key research questions and challenges, authored by 29 leading microchimerism experts, which was published recently in Advanced Science (DOI: 10.1002/advs.202514969).
These are particularly large cells that get trapped in the first capillary bed they encounter in the maternal lung, he points out, which is where Schmorl found them while doing an autopsy on a pregnant woman who had died from eclampsia. “Nowadays when we talk about microchimerism, we mean cells that integrate into the host body and go on living there ... this is not true for the cells that this pathologist identified.”
The health implications are still a bit murky, Kroneis says, since until recently it was only possible to detect the presence of microchimeric cells in human tissue and not understand anything about their function. When looking at tumor tissue, for example, it might be that these cells caused the mass or that they were somehow attracted to help fight off the tumor. “As long as you cannot say anything about the function, instead you only can associate the presence or absence of these cells with physiological or pathological events.”
But it is noteworthy that in women who have had a Cesarean section, fetal microchimeric cells are called into action, says Kroneis. This has been determined by an analysis of the resected scar tissue, providing one of few functional proofs that these cells can contribute to wound healing.
The mental health benefits would likely extend to survivors who have lost a parent or sibling and feel comforted by the fact that a loved one “lives on” within them, says Kroneis. In addition to maternal-fetal exchange, microchimeric cells can be passed on from older to younger siblings during a subsequent pregnancy as they are from a vanishing twin in early development to the surviving sibling.
Microchimerism is of course a well-established phenomenon in organ transplantation, since post-procedure monitoring frequently involves measuring donor DNA in recipients’ blood, Kroneis says. It inevitably occurs and can help their immune system accept the new organ and reduce the incidence of rejection.
A major turning point in the field emerged from the work of geneticist Ray Owens, who in 1945 discovered naturally acquired chimerism by observing mixed blood cell types in fraternal twin cattle. His work proved that the immune system learns to distinguish self from non-self during early development. Pivotal study findings published in 1954 established that foreign cells could survive and integrate with a host without being attacked by the immune system, foreshadowing modern understanding of maternal-fetal microchimerism.
Janina Walknowska co-authored a seminal paper in 1969 identifying cells with Y chromosomes (male cells) circulating in the blood of pregnant women carrying male fetuses. Her colleague, Finnish geneticist Jaakko Schröder, expanded on this work in the mid-1970s by confirming that fetal leukocytes consistently enter maternal circulation during pregnancy.
But the real watershed moment in microchimerism research occurred in 1996, when physician and medical geneticist Diana Bianchi published evidence that male fetal cells could persist in a mother's blood and bone marrow for decades after childbirth, Kroneis reports. “That’s the highest cited paper in our field” (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.93.2.705).
From here, the idea arose to establish a noninvasive prenatal diagnostic based on maternal blood, he continues. The focus was initially on the common “trisomies,” specifically Down, Edwards, and Patau syndromes.
A key player in this regard is celebrated molecular biologist Dennis Lo Yuk-ming, now head of the Chinese University in Hong Kong, who developed a now-widely-available test to identify the genetic conditions by analyzing the relative concentration of cell-free fetal DNA. To detect chromosomal abnormalities, he developed the massively parallel sequencing technique to distinguish the fetus's DNA from the mother's abundant background DNA.
“At that time [1997], it was not yet clear if this fetal DNA in the blood of pregnant women was from fetal cells that were trafficked to mom and then kind of degraded ... or whether it was a different source,” says Kroneis. “It turned out that the source is the cell turnover of the fetal part of the placenta,” as long ago hypothesized by Schmorl.
Kroneis started his microchimerism research in 2003 believing that circulating intact fetal cells would be the important players in noninvasive prenatal diagnosis. “It turned out I was wrong,” he says. “The numbers are really, really low.”
In the years that followed, researchers found microchimeric cells in tissues and organs throughout the body, including the liver, lung, and brain, says Kroneis. “This is true for mothers and cells of their kids, but this is also true for kids and cells of their mothers, so this works both ways.”
Attention is now turning to the function of these cells as well as how they get trafficked to different parts of the body, which is a main interest of Kroneis currently. He and his team are seeking to identify sites within the placenta where the cells are sent and the circumstances of tissues that permit their entry.
In terms of determining the function of microchimeric cells, it is impractical to sample whole individuals and, while it is easy enough to look in the blood of the living, that’s not necessarily where the cells are to be found, says Kroneis. “So, what we do is scale down our samples and use mice.”
The plan now is to use a technique that makes the animals so transparent that researchers could read a newspaper underneath the clear samples. Light-sheet microscopy will enable screening of microchimeric cells throughout the body and produce a 3D image of their distribution. “This is in our opinion the only way we can start thinking about the holistic function of these cells.”
Distribution of the cells in the body “matters a lot,” he stresses. For example, they might be in the bone marrow trying to hide from the immune system of the mother mouse or located close to lymphatic organs and interacting with the immune system.
“There will be areas in the animals that are enriched for the cells and areas that have hardly any cells,” he notes. “The big advantage here is that we can use very different mouse models” recapitulating various diseases and genetic backgrounds.
In an article that was published earlier this year, Kroneis and his colleagues describe a new technique for the detection and verification of microchimeric cells that overcomes the sex bias of the traditional approach where X and Y chromosomes are used to identify male cells in women (Clinical Chemistry, DOI: 10.1093/clinchem/hvaf119). The detection of male cells in female tissue is the “low-hanging fruit,” but completely ignores multiple potential scenarios such as those involving mothers and daughters.
With the novel “padlock probes” method, microchimeric cells are still identified based on differences in DNA but using a panel of human leukocyte antigen genes associated with tissue histocompatibility instead of the Y chromosome, he explains. The same approach could be adapted to assess if an organ or stem cell transplant is working, although PCR blood testing is “easier and more robust.”
The 27-assay panel targets a small but likely expressed number of molecules (several thousand variants in total) present on every cell with a nucleus, which excludes only red blood cells, continues Kroneis. The molecules signal to the immune system that they are self and not foreign cells, and babies inherit them from their mother and father. If the dad molecules are found in the tissue of the mother, “it must be from the kids.” Importantly, the approach also makes it possible to study mother-daughter systems.
“The drawback is you need to know what you’re going to target, so you need to know upfront what these molecules look like,” he says, “because there exists about 10,000 different variants of these molecules. Luckily for us, just a couple dozen of these molecules are very highly present in the population ... that we can use for testing.”
Microchimerism experts from around the world, led by authors of the Advanced Science perspective piece, have organized the first-ever international conference and public symposium around their field of research that takes place at the Medical University of Graz at the end of this month. The event was one of the deliverables tied to a grant from the John Templeton Foundation supporting a multidisciplinary research initiative aimed at understanding how microchimeric cells persist in a host and affect human health, immunity, and evolution.
All different aspects of microchimerism will be covered and shared among about 60 world-leading experts, Kroneis shares. A series of joint grants will be pursued as a next step to fund functional analysis studies.
One topic of repeated discussions over the past few years has been the psychological effects of microchimerism, he says. His former boss, a medical doctor and psychologist, reports that women are helped in their process of grieving a pregnancy loss with the knowledge that they might retain living cells of their deceased child within them. “This is something we hear on a regular basis,” he adds, including from midwives.
A key opinion leader in the fields of microchimerism and microscopy, who heads an institute in Germany and is attending the upcoming conference, shares the potentially viral nature of the phenomenon. He first told his wife, a high school biology teacher, about how these cells live in other bodies, and she was so thrilled she told her pupils. One of those students had just lost her mother to cancer and reported feeling a sense of relief to know they would forever have a biological connection.