In the case of more children being born, this can lead to some of the older child's DNA to be transferred from the mother to the younger child's fetus. In an attempt to study microchimerism, J. Lee Nelson, an immunologist from the Fred Hutchinson Cancer Research Center in Seattle conducted an experiment with assistance from his colleague Natalie. Blood samples from 32 healthy women revealed over 20% of them to bear white blood cells which belonged to their mother. It is hard to understand how these cells reside there instead of being rejected immediately by the host body. One assumption stated by Nelson indicates this to be a way to enhance the mother's immunity to prepare for the development of the fetus which can be deemed as a foreign organ on its own. They might even actively participate in the reproductive process by repairing some tissues helping the embryo in the womb to survive. The study also revealed how cells stored from previous generations might enter the placenta as well, resulting in changes in the baby's cells. This kind of cell alteration is termed as 'transdifferentiaton'. The cell constitutions of the mother and the child are usually organized such that they endure each other's immunological oppositions. If it fails to work that way, the fetus can face problems in survival.

Chimerism can be classified as true and confined. True Chemerism occurs when more than one zygote unite pretty early in the life of the embryo, causing every tissue to develop chimerically. Confined Chimerism is said to occur when the mixed characteristics of the zygotes is restricted to a single tissue kind ( for instance in the placenta, termed as confined placental Chimerism and blood, termed confined blood Chimerism ). Human Chimerism has always been deemed to be a rare occurrence or infeasible. Recent studies and experiments have revealed its occurrence to be more than previously anticipated. Advanced techniques of controlled reproduction have improved the chances of true or...

CBC can result in tricky situations which show up things like the twin-twin transfusion syndrome. It can cause future problems such as blood grouping issues, HLA typing, fetal amelia and cutis aplasia. Research in Placental Chimerism has been boosted by cases of prenatal analysis mistakes from medico-legal villious sampling (CVS) in Confined Blood Chimerism. Studies of such kinds of Chimerism are difficult as it requires the placenta along with the embryo to show any results. A study was conducted at the All India Institute of Medical Sciences in India which aimed at analyzing Confined Placental Chimerism using molecular research methods. The ones which were used are -- FISH (interphase fluorescent in situ hybridization) and QF PCR (quantitative fluorescent PCR) along with STR (short tandem repeats marker). It brought forth some results which indicated chances of placental chimerism in sex chromosomes to be a fairly probable. Their likelyhood would increase if autosomes are brought into consideration as well. In cases where the reproductive process is supported with the different kinds of placenta coming in contact, chimerism can occur in the placenta rather than the embryo. The rates of successful placental chimerism in early pregnancy are found to go as high as 40%.
References

Halder Ashutosh " Placental chimerism in early human pregnancy." Original Communication 11.2(2005): 84-88 Web. Web 8 Apr.2010

Nicholas, J.W.,Jenkins W.J. And Marsh W.L "Human Blood Chimeras -- A study of surviving twins" The British Medical Journal 1.5033(1957): 1458-1460 Web 8 Apr.2010

"What is Chimerism ?" Wisegeek. Web. 8 Apr.2010.

"The Stranger Within." Katewerk. Web. 8 Apr.2010

"Mosaicism and Chimerism." Colostate. Web 8 Apr.2010

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