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Female human embryos adjust the balance of X chromosomes before implantation
Date:6/29/2009

Amsterdam, The Netherlands: Dutch researchers have found the first evidence that a process of inactivating the X chromosome during embryo development and implantation, which was known to occur in mice but unknown in humans, does, in fact, take place in human female embryos prior to implantation in the womb.

Ms Ilse van den Berg told the 25th annual meeting of the European Society of Human Reproduction and Embryology in Amsterdam today (Monday) that her findings may have implications for the laboratory cultures that embryos are grown in before transfer to a woman's womb during fertility treatment, as well as for embryo stem cell research.

Males and females have two sex chromosomes: X and Y. While females have two X chromosomes and no Y chromosome, males have one of each. As the X chromosome is much larger then the Y chromosome, males and females also differ in their numbers of genes and gene expression. To equalise this difference in gene expression, females need to silence one X chromosome in every cell a process known as X chromosome inactivation (or XCI).

In mice, XCI occurs before embryo implantation when the X chromosome inherited from the father is turned off, while the maternal X chromosome remains turned on. As the cells carry on dividing and reach the blastocyst stage, the cells that will go on to form the placenta continue to have the paternal X chromosome switched off, but it is switched back on in the cells that are going to form the inner cell mass that develops into the foetus.

"It is from these cells that mice embryonic stem cell lines are made; these ES cells have two active X chromosomes and are capable of becoming any kind of cell in the body. As soon as the cells are going to differentiate into any kind of specialised cell, one X chromosome is turned off again, but this time it is a random process and it can be either the maternal or paternal X chromosome that is switched off," explained Ms van den Berg,
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Contact: Mary Rice
mary@mrcommunication.org
European Society for Human Reproduction and Embryology
Source:Eurekalert

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