Do You Think Your DNA is 100% Uniquely Yours? Think Again.

A chimera is a creature from ancient Greek mythology, usually with parts of a lion, serpent and a goat. For centuries chimera was nothing but a metaphor for something biologically incompossible or fantastical, however, modern genetics revealed a shattering truth about us – humans are literally chimeras. This phenomenon, fittingly, is called microchimerism. 

To understand how people turned to be biological mosaics, we must start from the womb. The placenta, historically, is viewed as an impermeable barrier between a mother and a child, keeping the mother's blood supply chain separate. Now, it is proven that placenta is highly porous, acting like a two-way biological highway. During pregnancy both fetal and maternal cells casually cross-slip into their bloodstreams. Those cells are not temporary visitors, they often take a permanent residency in their host’s organs. In fact, fetal progenitor cells could exist in their mothers' bloodstreams and tissues for almost 27 years postpartum (Bianchi et al., 1996).

The cellular invasion is highly beneficial for mothers, as fetal cells are mostly stem cells. Those cells are pluripotent and possess astonishing precision to identify what type and where particular cells are needed. When a mother experiences tissue damage, fetal cells actively migrate to the site of tear, to speed the recovery. Studies found that fetal microchimeric cells participate actively in maternal wound healing, as they travel to skin wounds, where they take part in inflammations and angiogenesis (Nguyen et al., 2022). Furthermore, as shocking as it is, those microchimeric cells could travel as far as women’s brains. Studies on the brain tissues of a deceased woman revealed that 63% harbored male DNA, presumably from their previous pregnancies with male fetuses (Chan et al., 2012). Those male microchimeric cells could be distributed widely through women’s bodies, surviving the whole lifespan of their hosts, as the oldest positive case of that is reported to be 94 years old. Furthermore, the study also noted that the concentration of those cells was lower in brains of women suffering from Alzheimer’s disease, hinting that microchimerism may start another wave of investigations around it. 

However, microchimerism is a double-edged sword. While those cells provide significant benefit in healing wounds, their presence uncovers another theory for medical mystery: the stark gender gap in autoimmune disorders. Women proved to be significantly more prone to developing conditions like systemic sclerosis (scleroderma), Hashimoto's, and Lupus than men. Microchimerism suggests that long years after pregnancy, women’s bodies may recognize those fetal cells as foreign invaders, setting an immune attack. Pioneer researcher J. Lee Nelson discovered that women with systemic sclerosis have higher concentrations of fetal cells in their blood than healthy control subjects (Nelson, 1998). In that case microchimerism acts analogous to graft-versus-host disease, following a bone marrow transplant, as fetal cells are deeply embedded within the maternal body, catching and destroying all the surrounding tissue in a biological crossfire.

The concept itself truly shows how deeply interrelated human biology is. This phenomenon also resurfaces in later pregnancies as well. According to Kanaan’s study (2017),  microchimerism is able to pass those fetal cells to the fetus of the mother’s second pregnancies, continuing the biological multigenerational mosaic. Furthermore, in that context the concept works both ways. Maternal cells acquired during pregnancy not only trains the developing immune system, but could provide cross-generational immune and disease memory (Kanaan et al., 2017). Therefore, studies detected grandmother’s cells in an umbilical cord of their grandchildren. Interestingly, this busts the myth about bloodline being passed by boys. 

Ultimately, microchimerism provides a stable basis for redefining the true “self”. Individuals are no longer seen as separated islands, genetically people are more “patchworks” of their ancestors, close ones. Thus, when people say that mothers carry their children in their minds, hearts, it is not a poetic sentiment anymore, but a biological fact. 

References:

• Bianchi, D. W., Zickwolf, G. K., Weil, G. J., Sylvester, S., & DeMaria, M. A. (1996). Male fetal progenitor cells persist in maternal blood for as long as 27 years postpartum. Proceedings of the National Academy of Sciences, 93*(2), 705–708. [https://doi.org/10.1073/pnas.93.2.705](https://doi.org/10.1073/pnas.93.2.705)

• Chan, W. F. N., Gurnot, C., Montine, T. J., Sonnen, J. A., Guthrie, K. A., & Nelson, J. L. (2012). Male microchimerism in the human female brain. PLoS ONE, 7(9), e45592. [https://doi.org/10.1371/journal.pone.0045592](https://www.google.com/search?q=https://doi.org/10.1371/journal.pone.0045592)

• Kanaan, S. B., Gammill, H. S., Harrington, W. E., De Rosa, S. C., Stevenson, P. A., Forsyth, A. M., & Nelson, J. L. (2017). Maternal microchimerism is prevalent in cord blood in memory T cells and other cell subsets, and persists post-transplant. Oncoimmunology, 6(5), e1311436. [https://doi.org/10.1080/2162402X.2017.1311436](https://www.google.com/search?q=https://doi.org/10.1080/2162402X.2017.1311436)

• Nelson, J. L. (1998). Microchimerism and autoimmune disease. New England Journal of Medicine, 338(17), 1224–1225. [https://doi.org/10.1056/NEJM199804233381711](https://www.google.com/search?q=https://doi.org/10.1056/NEJM199804233381711)

• Nguyen, T. P., Cohen, P., Khammari, A., Dréno, B., & Khosrotehrani, K. (2022). Contribution of fetal microchimeric cells to maternal wound healing in sickle cell ulcers. Haematologica, 107(7), 1690–1694. [https://doi.org/10.3324/haematol.2022.281140](https://www.google.com/search?q=https%3A%2F%2Fdoi.org%2F10.3324%2Fhaematol.2022.281140)

• (https://www.google.com/search?q=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DVMuZRnuBaoI) (interview)