Hidden Guests review: This book is a great insight into the new science of microchimerism

Hidden guests
Lisa Barneud, translated by Bronwyn Haslam, Greystone Books

My children were conceived using donor eggs, so you would be forgiven for assuming that we do not share any genetic material. However, science has proven that this is not entirely true.

We now know that during pregnancy, fetal cells penetrate the placenta into the mother’s body, invading all studied organs. Likewise, my mother's cells, and even those passed from my mother to me, can enter my children. And things could get even weirder: I have older sisters, so their cells that passed into my mom during pregnancy could then end up in me and, in turn, in my children.

This fascinating idea that we are a holobiont, composed not only of human cells and microbes, but also of fragments of other cells, and its implications lie at the heart of Hidden Guests: Cell Migration and How the New Science of Microchimerism Redefines Human Identity Liz Barneud.

Barnet traces not only the happy discovery of these microchimeric cellsbut also how their interpretation is shaped by culture and politics. The idea of ​​fragments of parents, siblings, and even fetuses being implanted into our bodies and brains can stimulate a wide variety of feelings.

“Some people find comfort in connecting with loved ones they've lost… while others decry yet another way for men to expand their control,” she says, referring to the fact that some anti-abortion activists claim fetal cells “harass” women who have abortions, causing illness to punish them.

Luckily, Barnet mostly sticks to the science. She follows researchers through their mistakes, doubts and eureka moments, showing how embryonic-derived cells can both help and harm. They appear to help in tissue repair and fight tumors, but are also involved in autoimmune diseases. Barneud approaches more emotional areas with sensitivity, explaining, for example, how miscarriage cells can become embedded in mothers' bodies for decades.

She also reveals how their presence breaks the classical rules of genetic inheritance, creating extraordinary biological mysteries. For example, there is a woman who shares genetics with only one of her three sons, despite conceiving them all from her own eggs. Or a woman with hepatitis C whose liver is riddled with cells whose DNA matches that of two previous partners, likely the result of a pregnancy terminated decades ago. Or the Olympic cyclist who tried to blame the “vanishing twin” (a fraternal twin whose DNA merges with the other in the womb) for his suspiciously mixed blood type.

Hidden guests The writing is clear and full of useful metaphors and analogies. Journalist Barnet compares the microchimera cells in our body to stars from other galaxies that “carry molecular signatures different from ours.” And when they are found in tumors, she compares the untested assumption that the cells cause the growth to blaming firefighters for starting the fires.

There are more than a few surprises. Spoiler alert: Barnet encourages readers to consider that cells from a partner's seminal fluid can enter the blood and lymphatic vessels surrounding the vagina and then invade areas of the body and brain, much as donor cells can migrate from transplanted organs to other parts of the recipient's body.

This blurring of heredity as cells move up and down the family tree can be confusing. But Barnet does an excellent job of explaining the state of this new field and its profound implications for medicine and human nature without going too far into hypotheticals. She simply and cheerfully breaks down the age-old equation of “one person, one genome.”

As a mother who once believed that I did not share biology with my children, I have found that Hidden guests is both scientifically fascinating and deeply comforting. Barnet shows us that we all carry traces of others within us. She made me want to see how this field will develop in the future.

Helen Thomson writer living in London and New scientist reviewer