After reading this article, I think my sister and I are much closer than we ever thought. Not only are we genetically related, and best friends, but perhaps she actually has some of my very own cells!!!
Male cells have been found in the umbilical cord blood of baby girls who have older brothers, suggesting that the transfer of cells between mother and baby may be more extensive than previously imagined. We may all be walking chimeras. A chimera is a single organism composed of two genetically distinct groups of cells, which originated from different zygotes (the initial single cell formed after fertilization).
Previous studies have shown that cells from both mother and fetus can cross the placenta during pregnancy, and survive for decades in the skin, liver, brain and spleen - a phenomenon called fetal microchimerism (click for more on fetal microchimerism). There is even evidence that fetal cells may repair damage to the mother's heart during pregnancy.
Other studies have suggested that fetal cells might contribute to autoimmune diseases, supporting the notion that fetal cells disperse more widely, possibly passing between siblings and even across generations. To investigate this, Miranda Dierselhuis of Leiden University Medical Center in the Netherlands and a group of researchers analyzed umbilical cord blood from 23 newborn girls, 17 of whom had older brothers. In some of the samples, they specifically looked for immune cells directed against the male Y chromosome.
Of the 12 girls in the group with elder brothers, 11 had cord blood containing immune cells against the Y chromosome, suggesting that male cells had somehow crossed the placenta from the mother, most likely entering her body from a male fetus during an earlier pregnancy. In some of the girls, DNA testing revealed direct evidence of male cells in the cord blood (Blood, bloodjournal.hematologylibrary.org).
Amazingly, however, a small amount of male cells were also found in one of the girls with no older brother, raising the possibility that these cells were from her uncle, passing to her mother during her grandmother's pregnancy. Another possibility is that they originated from an earlier miscarriage of which the mother was unaware.
The discovery of the transfer of cells between siblings and across generations raises the possibility that these cells may influence the course of health or disease in all of us. Several diseases including asthma, type 1 diabetes and certain cancers are less common in younger siblings. Meanwhile, an autoimmune disease called scleroderma has also been linked to the presence of fetal cells in the blood, and seems to be more common in younger siblings. "The further you are down the line in birth order, the greater the risk," says Maureen Mayes at the University of Texas in Houston, who has studied the phenomenon (Arthritis Care and Research, onlinelibrary.wiley.com).
The entire issue being raised is so fascinating to me, and really makes me wonder. After studying so much about diversity and variety within species, it is interesting to look at another perspective showing the interconnectedness and specific linkage. The linkage not only from the parents passing their genes on, but also the unique genetic combinations from one sibling somehow passing on to a younger sibling. Alas, despite the fundamental difference between my sister and I, she carries a part of me within her, whether she likes it or not!
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