In our mothers' hearts and minds

For the most part, we love our mothers and miss them when we are away. In turn, most mothers love and care for their children and feel a strong connection with each child. This bond is based in the intimate familiarity of pregnancy, the trauma and wonder of birth and the years of nurturing. Our mothers may even remind us that we are always in their hearts and minds. Ongoing research suggests the physical connection with our mothers is more tangible and deeper than previously envisioned.

The placenta provides the first and most obvious connection between mother and child. This complex organ, purpose-built of cells from the fetus and the mother, serves as a channel for the exchange of nutrients, gasses and wastes. In addition to blood and other fluids, body cells migrate through the placenta between mother and child, taking up residence in many of each other’s organs. The presence of these ‘foreign’ cells in our bodies challenges the concept that we are wholly autonomous individuals made up of our own unique cells. We each carry parts of our parents, not just in the genetic sense, but as part of the cells incorporated into ourselves.

Mixing of cells from genetically distinct individuals is called chimerism (after the mythological fire-breathing Chimera that was part serpent, lion and goat). Naturally occurring chimeras include slime molds, corals and lichens, each of which thrives as a melding of genetically distinct species. Mythical chimeras include mermaids, centaurs and the disturbing puppy-monkey-baby.

The presence of groups of genetically distinct cells in an individual, or microchimerism, has been known for a while. Recently, microchimeric cells were not only found circulating in the blood, they were also found taking up residence in the brain. Researchers examined the brains of deceased women and found cells containing male “Y” chromosomes in various regions of more than 60 percent of the brains studied. The cells are genetically male and likely originated from their babies during gestation.

Because Alzheimer’s disease is more common in women who have multiple children, the hypothesis was that the number of chimeric brain cells would be greater in women with Alzheimer’s than those who had no such neurological disease. The result was the opposite: women with Alzheimer’s had fewer fetal-derived cells. While this correlation is intriguing, it does not quite mean that more fetal cells in a woman’s brain can reduce Alzheimer’s. However, it is some comfort to think that each of us may contribute in this small way to our mother’s elderly wellbeing.

There are many ways to share or acquire microchimeric cells. In addition to the placental route, a mother may transfer cells to her child through nursing and twins in utero may exchange cells. Cells residing in the mother from an older sibling may travel across the placenta to a younger sibling during the latter’s development in the womb. Accordingly, a woman may carry microchimeric cells from her mother as well as from her own children, and each of us likely carries such cells from our own mothers.

Fetal microchimeric cells are analogous to stem cells in their ability to develop into a variety of tissues. One group of researchers studied fetal microchimeric cells placed into a mother rat after her heart was damaged. The fetal cells traveled to the mother rat’s heart and developed into cells that repaired the injury. Microchimeric cells identified in the brains of some animals had become nerve cells which appeared to be functionally integrated into the brain. This may possibly be true of such cells in the brains of human mothers.

Microchimeric cells may also impact the immune system. Fetal microchimeric cells are recognized by the mother’s immune system partly as hers, since genetically the child is half-identical to the mother, but also viewed as half-foreign, based on the father’s genes. This may sensitize the mother’s immune system to handle cells not quite identical to the self. Cancer cells caused by genetic mutations are such “not-quite-self” cells and studies suggest that microchimeric cells pre-adapt the immune system to curtail the growth of tumors. Interestingly, more microchimeric cells are found in the blood of healthy women as compared to those with breast cancer.

But in other circumstances, the “not-quite-self” aspect of chimerical cells may turn the immune system against the self. People with multiple sclerosis have been found to have more microchimeric cells than their siblings without the disease. Again, the correlation is interesting and may suggest chimeric cells play a role in MS, perhaps by setting off an autoimmune attack of the self against the “not-quite-self.”

More research and discoveries are coming, but what we have already learned validates the truth that we are more closely connected, each to the other, as child to mother, than we have ever imagined. We, or at least some part of our cellular beings, truly may be in our mothers' hearts and minds. Our mothers may always be with us, not just as that little voice in our heads, but as the very neurons that carry thoughts of each other.