The cells are presumed to have migrated into the women from a male fetus previously carried (perhaps not to term) by the subjects, as indicated by the 1st parag

The cells are presumed to have migrated into the women from a male fetus previously carried (perhaps not to term) by the subjects, as indicated by the 1st paragraph definition of microchimerism ("the harboring of genetic material and cells that were exchanged between fetus and mother during pregnancy") and the start of the 3rd paragraph: Chan said the study is the first description of male microchimerism in the female human brain. The findings support the likelihood that fetal cells frequently cross the human blood-brain barrier and that microchimerism in the brain is relatively common. Until this study, it was not known whether these cells could cross the barrier in humans. I think it's only the strong proof that such cells reach and persist in the mother's brain that's new about this study; that persistent cells are exchanged in both directions between mother and fetus has been known for a while. See for example this 2005 NYTimes story: http://www.nytimes.com/2005/05/10/health/10bloo.html?pagewanted=all http://www.nytimes.com/2005/05/10/health/10bloo.html?pagewan... Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life. Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it. The most famous cases are those where a child turns out to match a (possibly 'vanished' while in womb) 'aunt' moreso than the birth mother. But I suspect at some point there will be a proven case where a child is found to be, genetically, the direct offspring of their matarnal grandmother's primary genome -- if the mother's egg cell was descended from a persistent grandmother cell rather than the usual mixed maternal-grandfather/matrernal-grandmother cell. (Or even in suitably rare situations, great-grandmother, etc.)