“Microchimerism blurs the boundaries of time and death”

[Dans « Les Cellules buissonnières », à paraître le 21 septembre, la journaliste scientifique Lise Barnéoud, qui collabore régulièrement au « Monde », explore le microchimérisme. Un phénomène fascinant par lequel nous pouvons abriter dans nos organismes des cellules de nos mères, de nos frères ou sœurs, de nos enfants, ou même d’embryons qui ne sont pas développés. D’autres ADN que le « nôtre ».]

We, the human species, spend on average nine months warm, housed and fed. Nine months during which, from a single, tiny cell, the fusion of an egg and a sperm, we become an organism rich in several hundred billion cells. More than stars in our galaxy.

There is something astounding and pleasant at the same time in imagining that all of these cells that make us up, from head to toe, heart to brain, come from the same fertilized egg. An egg cell so small that it could fit within the diameter of a hair, and which contains a unique combination of 23 maternal chromosomes and 23 paternal chromosomes. They carry our DNA, considered our genetic identity for the rest of our lives.


Except things are more complicated than that. At the turn of the millennium, scientists already dealt the first blow to this egotistical conception of our identities by teaching us that this “I”, which we hoped to be pure and unique, was in reality a “we”, half of which constituents did not belong to us. Intertwined with our human cells live an equivalent number of microbial cells without which we could not survive. Bacteria, viruses, fungi, yeasts… so many micro-organisms embedded in our tissues and which influence not only our metabolism, our immunity, but also our moods, our behaviors…


Now another revolution is underway: even this half of humanity that we are is not only made up of this “I”. This last unity to which we could cling is ing. It is also plural. The trillions of human cells that make up us as adults do not all come from our original nucleus. Similar to stars from elsewhere, some of them carry other chemical signatures than ours, they hide different DNA. And for good reason: they come from other human beings…


Such is the extent of the upheavals brought to light by microchimerism. Funny name for a field of scientific research, right? In Greek mythology, Chimera was an evil creature with the head of a lion, the body of a goat and the tail of a serpent, which came from its mother, the viper goddess Echidna. There are no extraordinary creatures here: we are all microchimeric. The borrowing from the fantastic imagination reflects the astonishment and fascination of scientists faced with what they first interpreted as a monstrosity. Mixing other people’s cells with your own? The cells of another who sometimes no longer lives, or even never lived? Is each of us therefore multiple? What a reversal of perspective! We who thought we were pure, equipped with an effective territorial defense tool, capable of recognizing our “self” and rejecting the “non-self”.

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