Fetal-Fetal and Fetal-Maternal Microchimerism: Insights from Mammalian Placental Biology.

Feto-maternal microchimerism (Mc) refers to the exchange of cells between the fetus and mother, and fetal-fetal Mc to the exchange between fetuses during pregnancy. This phenomenon occurs across mammalian species, including humans, mice, and cattle. Key data on Mc cells and theoretical considerations regarding the presence of fetal-derived material, such as trophoblast cells, cell-free fetal DNA (cffDNA), and exosomes in maternal blood are summarized. This review aims to first, synthesize current knowledge on feto-maternal and fetal-fetal Mc across mammals, second, address three core questions: how and where Mc has been demonstrated in animals, what techniques have been used over time to detect fetal-derived material and Mc, and how placental structures influence the frequency of Mc. Finally, it aims to identify gaps in the literature for species such as horses, goats, and pigs. This article concludes that Mc is a widespread phenomenon among mammals, but detection methods and reported frequencies vary significantly by species and placental type. A biological model is presented in this article in which multinucleated trophoblast cells undergo apoptosis, releasing cffDNA that enters the maternal blood circulation after multinucleated trophoblast invasion. Advances in molecular biology technology have improved the ability to detect fetal-derived material, cells, DNA, and exosomes in maternal blood. However, notable research gaps remain for Mc in horses, goats, and pigs, highlighting the need for targeted studies to better understand species-specific patterns or a general biological model.