Most moms I know would agree that they feel profoundly altered by motherhood. As a witty friend of mine said, “it’s like Kafka’s ‘The Metamorphosis’ without the exoskeleton.” We can all identify the scars, stretch marks or sagging appendages that are our badges of motherhood, but the change is more profound than that. I felt my sense of self was shattered, reassembled and seemed to expand to include my children. While I often day dream about spending a weekend as my former carefree, kid-free self, when I actually get that weekend off kid duty I wander around feeling out of sorts. I enjoy myself, but I have this surreal feeling that I have left behind something really important- like a limb. It maybe a cliché to say that your children will always be a part of you, but there is a reason for the cliché: it rings true for a lot of people. It turns out it may be true in a more literal way than you’d imagine.
For a long time scientists thought of the placenta as a selective barrier between mom and baby that allowed certain things, like oxygen and nutrients to be exchanged, but never cells. Now we know that this is not the case. In fact, mother and baby exchange small numbers of cells during pregnancy. The astonishing thing is that these cells persist for decades and often a lifetime. Researchers have found baby’s cells in mother’s heart, liver, kidney, brain and other organs long after she has given birth. Likewise, babies receive cells from their mothers, for instance some types of immune cells, although this phenomenon is less well studied. This means that both mother and baby are technically “chimeras” which is the medical term for an organism that is composed of two or more genetically different populations of cells. The term originates in Greek mythology in which the Chimera was a fire-breathing female creature that was part lion, snake and goat. Because we are talking about very small numbers of cells in these cases, the term “microchimerism” is used.
One of the remarkable things about microchimerism is that it challenges the conventional notion of how the immune system works. A simple, but not totally inaccurate way to sum up the modus operandi of the immune system is to say that it identifies agents as “self” or “non-self” and then destroys anything that is “non-self.” But if a small number of baby’s cells live on in mother’s body, why doesn’t her immune system react to them? In some cases it might. Scientists are researching the possibility that microchimerism is the reason women are more prone to autoimmune diseases than men, and the incidence of some of these diseases increases in their post-reproductive years. Perhaps, because exchange of cells between mother and baby is normal, the immune system has mechanisms to avoid attacking these “non-self” cells, but in some cases these mechanisms fail, leading to diseases like scleroderma or multiple sclerosis. Researchers are also investigating possible links between microchimerism and certain cancers, as well as Alzheimer’s disease. But microchimerism probably isn’t all bad news for mom. There is some evidence that suggests that babies’ cells may help protect mothers from breast cancer and repair injured tissues. The study of microchimerism is really just beginning and more research needs to be done before the role of microchimerism in health and disease can be understood. But if microchimerism does play a role in disease, this research may help scientists develop more effective treatments.
I trained as a biologist and I still tend to look at the world as a biologist does. When I think about “who I am” I think of myself as a product of my genes and my experiences. But now I know that as a mother, my body harbors the cells and genes of my children. Through their cells I have also acquired some of my husband’s genes. Of course I am also a daughter and therefore I contain cells from my mother who passed away too young. I am excited to see what science discovers about what role, if any, these cells play in my body, but microchimerism fascinates me for sentimental reasons as well. All of these people that I love are not only figuratively “in my heart,” but tiny bits of them are literally in my body. They will always be a part of me- no matter what happens. There is something so satisfying about that.
You can learn more about microchimerism research by visiting www.microchimerism.org