In a phenomenon called fetomaternal microchimerism some foetal cells stay back within the mother’s body after a pregnancy. But how does it change her?
Women do not stay the same after a pregnancy. And we are not talking of just mental, physical or hormonal changes here. Those changes are a lived reality for every woman who has gone through the different stages of a pregnancy. There’s no doubt about it. But pregnancies also bring about changes that goes down to the DNA strands contained within the cell nucleus. We are talking of genetic changes. Yes, pregnancies change the genetic constitution of women. Which means, if you are a woman, you will never be the same – genetically – after each pregnancy.
The change is not in the maternal DNA sequence but in the cells constituting her tissues. A mother acquires genetically distinct foetal cells after each pregnancy. The phenomenon is called fetomaternal microchimerism.
Foetal cells enter the mother through the placenta and settle in the mother’s body. These cells survive for decades after the pregnancy. These foetal cells also proliferate within the mother’s body turning her into a microchimera with patches of foetal cells comprising various organs like the bone marrow, blood, liver, skin and brain. Fetomaternal microchimerism last lifelong in childbearing women.
Foetal stem cells that cross over into the mother’s body has various implications. They repair damaged tissues in the mother’s body by specifically migrating to injury sites. This benefits both the mother and the foetus. The mother regains her vigour which enables her to nurture her offspring better. But apart from this benefit these cells are also the culprits behind the various autoimmune diseases that affect women specifically.
Interestingly, one needn’t be a mother to experience the implications of fetomaternal microchimerism. Even terminated pregnancies supplies the woman with a batch of foetal cells to kick off the process. The results are similar to that found in an event of blood transfusion or organ transplantation.
The journey across the placenta
Trophoblasts are the cells constituting the outer boundary of a blastocyst that form about five to six days after fertilization of the egg by the sperm. The trophoblasts then go on to form the placenta and other supportive foetal membranes. They are the ones to first cross over into the mother’s body. Trophoblasts are followed by other cells derived from foetal blood.
Foetal DNA first appear in the mother’s blood circulation as early as four weeks after conception. The foetal stage, when the vital organ systems develop, starts only after the 12th week of gestation. Plugs of trophoblasts that block maternal blood from entering foetal placenta gradually starts dissolving from the 10th week of pregnancy. As maternal blood breaks into foetal placenta the dislodged trophoblasts enter maternal circulation.
It is in the foetal placenta that the mother’s blood comes in direct contact with the trophoblast cells and it is just where the migration begins. The placenta is also rich in foetal hematopoetic stem cells and they migrate into the mother’s circulation from there.
Fate of foetal cells entering the mother’s body
Foetal cells persist within the mother’s body decades after they enter. The longest recorded time span for persistence of fetomaternal microchimerism was that found in a woman who had borne her last child, a male, some 27 years ago. These foetal cells engraft themselves in the maternal bone marrow and other organs often including sites of injury that need repairing.
During pregnancy the immune system of the mother remains suppressed. The suppression is lifted once the baby is delivered. So the degree to which foetal cells persist within the mother’s body depends on the immuno-compatibility between the foetal and the maternal cells – much like immuno-compatibility between host and the donor in a case of organ transplantation. Brain is a very good site where foetal cells persist because the brain stays protected from the immune system by the blood-brain barrier.
It has been found that foetal cells specifically seek out sites of injury in the mother’s body and migrate to lodge there. Since foetal cells are mostly multipotent by nature (stem cells) they help repair damages in the mother’s body. Thus, it can be assumed that fetomaternal microchimerism develops as a means to increase the physical fitness of the mother to help cope with the rigors of pregnancy.
In cases of spontaneous and induced abortions there is a higher chance of fetomaternal microchimerism occurring due to trauma and injury caused by the act of abortion itself.
Microchimerism and the healing effect of foetal cells
Studies have shown that presence of foetal cells or fetomaternal microchimerism is higher in diseased tissues than in healthy tissues. Due to the damage repairing function of foetal cells they are more prone to migrate to sites of injury in the mother’s body. They also persist longer in injured tissues than at uninjured sites.
The higher presence of fetal cells at sites of injury may also be a natural outcome rather than a choice. Damaged blood vessels cause foetal cells in maternal circulation to leak into the surrounding tissues and hence their higher presence at sites of injury.
It is hard to distinguish foetal cells from maternal cells even when they form a large chunk of an organ mass, years after pregnancy. In case the child is male, foetal cells carry the Y-chromosome and it has become a marker to help researchers differentiate foetal cells from the mother’s own cells. Foetal cells completely integrate into the mother’s body and bear organ specific markers. This is only possible when the migrating foetal cells are progenitor cells.
Microchimerism and auto-immune disorders in women
Foetal cells also seem to be the pivotal factor behind the prevalence of autoimmune disorders in women. Researchers have found circulating immune cells in women that have originated from foetal hematopoietic stem cells which may have migrated from their foetuses during former pregnancies. These foetal hematopoietic stem cells produce foetal immune cells like B-cells, T-cells, natural killer cells and monocytes that circulate in the mother’s blood. Being foetal by origin they naturally recognize the mother’s tissue as something foreign hence worth attacking. Once they start the offensive the mother’s own immune cells jump in into the bandwagon attacking her own cells in turn.
Pregnancy changes a woman’s body
Foetal cells that migrate into the mother’s body during pregnancy can both build and destroy tissues. There is much to know in this field and much more research to be done. But, one thing is for certain – after nine months of stay within their mother’s womb children don’t depart from their mother’s bodies with the severing of the umbilical cord. They always leave a part of themselves with their mother that transforms her in many curious ways, like turning her into a chimera for example!