When a woman becomes pregnant with a male child some of his DNA is left behind in her brain. Researchers call this microchimerism. There is little known about the impact of microchimerism on overall health, but some research has shown a connection between these foreign cells, cancer and autoimmune diseases. When cells are passed between mother and fetus it is called fetomaternal microchimerism. While previous research has noted the presence of these cells in maternal organs many months and years after giving birth, this is the first report of male DNA or cells in the female brain.
Researchers autopsied the brains of 59 women between the ages of 32 and 101. Thirty-three of the women experienced Alzheimer's disease and 26 suffered no neurological disease. In total, 63% of the brains autopsied tested positive for male DNA. There was no specific region of the brain noted as cells spread to multiple regions. When researchers found male cells in the brain of a 94-year-old woman, they realized that the cells passed during fetomaternal microchimerism stayed in the female body for life.
Women who suffered from Alzheimer's disease tested positive for male DNA in brain regions less often than non-neurological disease counterparts. Researchers are not in a position to speculate whether this indicates a connection between male DNA in the female brain and neurological disease prevention due to the relatively small study population and lack of pregnancy history on patients.
Previous research into male microchimerism showed positive and negative impacts on cancer risk. Breast cancer risks was lowered in women with male DNA cells while colon cancer risk was increased. There may also be a connection between risk of rheumatoid arthritis and other autoimmune/inflammatory diseases and fetomaternal microchimerism.
Source: Chan WFN, Gurnot C, Montine TJ, Sonnen JA, Guthrie KA, et al. (2012) Male Microchimerism in the Human Female Brain. PLoS ONE 7(9): e45592. doi:10.1371/journal.pone.0045592
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0045592