“According to traditional thinking, the placenta acts as a barrier between a mother and a child in the womb, preventing an exchange of cells between them. But recent research has revealed that the placenta is more porous than previously believed, says Kirby Johnson, a biologist at Tufts University. ‘Now we know a mother and her baby have to be linked. Cell-based communication is essential for a healthy pregnancy.’
“Overall, the placenta allows for a lot of two-way traffic, with fetal cells stealing into Mom, and maternal cells slipping into Child. (Even tumor cells can cross over, and there are a few well-documented cases of mothers giving cancer to fetuses.) After cells cross over, some get rounded up and killed by the new host’s immune system. Many, however, take root in the other body, burrowing into the heart, liver, kidneys, spleen, skin, pancreas, gallbladder, and intestines, among other places. Most of these organs house tens to hundreds of interlopers per million normal cells, but the lungs can tolerate thousands of foreign cells per million. Fetal cells do an especially good job of colonizing Mom’s body since they often have the power, much like stem cells, to turn into multiple types of tissue, depending on where they find themselves.
“At first, researchers assumed that microchimeric transplants would harm the recipient. Most scientists who study microchimerism also study autoimmune diseases, which occur in women three times more often than in men. Scientists have reasoned that perhaps a mother’s immune system, in trying to exterminate fetal cells inside her, inadvertently causes collateral damage to her own tissue. Or perhaps the fetal cells, surrounded by foreign tissue, rebel and attack the mother. Studies have indeed linked high levels of microchimeric cells to some forms of lupus, cirrhosis, and thyroid disease. Twin studies have also found higher levels of microchimerism in females with multiple sclerosis….
“On a more general level, given all the two-way cellular traffic, ‘the dogma of every cell in our body being genetically identical has to be revised,’ (says Gerald Udolph, a biologist at the Institute of Medical Biology in Singapore).“