A research team, led by Dr José Silva and Dr Elsa Sousa from the Wellcome – MRC Cambridge Stem Cell Institute, at the University of Cambridge, used mouse embryonic stem cells to reveal that X Chromosome Inactivation starts on all X chromosomes regardless of sex (XX or XY).  Research published in the journal Cell Stem Cell, identify that the cells halt the process in males and allow the inactivation to continue on one of the two X chromosomes in females fundamentally altering the way we think about male and female early development.
 
“Our study shows that initiation of X chromosome inactivation occurs before the cell realizes whether it is male or female” explains Dr Elsa Sousa, who led the current research.  “We previously thought that cells realised their sex by counting their X chromosomes prior to the initiation of X chromosome inactivation if female.  Our research changes this paradigm. We now know we need to look earlier in the process to better understand the mechanisms driving sex-specific development.”

 

 

 

Image: Male embryonic stem cells undergoing transient X chromosome inactivation.  The cell on the right shows X chromosome inactivation has occurred (pink).  The cell on the left has not undergone complete X chromosome inactivation so presence of specific genes on the X chromosome can be seen (white).