Microchimerism describes the presence of a small population of cells within an individual that originated from another genetically distinct person. These foreign cells are present in low concentrations, often less than 0.1% of the total cell population, yet they can establish long-lasting lineages within the host. This natural phenomenon demonstrates a biological interconnectedness in the human body.
How Microchimerism Arises
The most common source of microchimerism is the bidirectional exchange of cells between a mother and her fetus during pregnancy. This process, known as fetomaternal microchimerism, involves fetal cells traversing the placenta and entering the mother’s circulation, while maternal cells can also cross into the fetus. This cellular trafficking can begin as early as four to six weeks into gestation and continues throughout the pregnancy, with levels increasing significantly closer to birth.
Fetal cells, including stem cells and immune cells, can persist in the mother’s body for decades after childbirth, forming a lasting biological legacy of pregnancy. Maternal cells can also be found within the offspring, influencing their development. This exchange is not limited to successful pregnancies; it also occurs after miscarriages or abortions.
Beyond mother-child interactions, microchimerism can arise from other sources. In instances of multiple births, such as twins sharing a womb, cells can transfer between the fetuses. Additionally, medical interventions can lead to acquired microchimerism. These include blood transfusions, where donor cells can persist in the recipient for months to years. Organ transplantation also results in microchimerism, as donor cells from the transplanted organ can migrate into the recipient’s body, and recipient cells can populate the donor organ.
Where Microchimeric Cells Persist
Once transferred, microchimeric cells can persist and integrate into various tissues and organs throughout the recipient’s body. In mothers, fetal cells have been identified in a wide array of maternal organs, including the brain, heart, lungs, liver, skin, bone marrow, spleen, thyroid, and salivary glands. These cells can establish long-term residence, sometimes lasting for decades or even a recipient’s entire lifespan.
The longevity of these cells suggests they can multiply and maintain their presence over extended periods. While their numbers may decline significantly postpartum, their continued existence highlights a biological connection. This persistence is not confined to blood circulation; microchimeric cells can integrate into the architecture of solid organs, becoming part of the tissue itself.
Studies have shown that these cells can differentiate into various cell types, adapting to their new environment. This adaptability allows them to reside in diverse physiological settings, from the circulatory system to specialized organ tissues. The presence of these genetically distinct cells in multiple bodily locations underscores the pervasive nature of microchimerism.
The Biological Role of Microchimerism
The presence of microchimeric cells is associated with several biological implications, influencing tissue repair, immune system function, and disease susceptibility. Research suggests these foreign cells may contribute to tissue repair and regeneration processes. Fetal cells found in maternal tissues, for example, can differentiate into various cell lineages, including cardiac muscle cells, liver cells, and pancreatic cells, potentially aiding in healing after injury or replacing damaged cells. This regenerative capacity has been observed in animal models, where fetal cells were shown to assist in recovery from conditions like type I diabetes, Parkinson’s disease, and myocardial infarction. They may also play a part in wound healing and neoangiogenesis, the formation of new blood vessels.
Microchimeric cells also influence the immune system, contributing to a complex interplay of tolerance and response. The bidirectional cell transfer during pregnancy can promote immune tolerance between mother and fetus, which is important for a successful pregnancy. However, the presence of these genetically distinct cells is also associated with certain autoimmune conditions, particularly those with a higher prevalence in women after childbirth. Fetal microchimerism has been implicated in conditions such as systemic sclerosis (scleroderma), Hashimoto’s thyroiditis, Graves’ disease, systemic lupus erythematosus, Sjögren’s syndrome, and juvenile idiopathic inflammatory myopathies. In some cases, fetal cells may trigger an immune response against the host’s own tissues.
Despite these associations, the role of microchimerism in autoimmune diseases is nuanced and remains an area of active investigation. Some studies note that certain autoimmune symptoms, such as those of rheumatoid arthritis and multiple sclerosis, may improve during pregnancy when fetal cell levels are highest, suggesting a potential protective or modulatory role. Microchimerism also has disease associations beyond autoimmunity, with some research indicating a potential protective effect against certain cancers, such as breast and ovarian cancers. Conversely, male fetal cells have been observed in lung tumors and linked to the progression of cervical cancer, indicating complex and sometimes contradictory findings. The precise mechanisms underlying these varied effects are still being explored.