This is an image of a female mouse blastocyst. The red colored mass of cells is called the inner cell mass and will give rise to the embryo proper. The cells surrounding the inner cell mass are trophectoderm cells that will produce the placenta. During blastocyst development in females, one of the two-X-chromosomes undergoes silencing (X inactivation). This phenomenon equalizes X-linked gene expression in males and females. X-inactivation is thought to be triggered by the expression of the X inactive-specific transcript (Xist) from (paradoxically) the inactive-X. This image of a female mouse blastocyst shows that Xist expression (denoted by green dots detecting the inactive X) is detectable in most cells. When X inactivation does not occur properly, the double dose of X-linked genes can cause multiple types of problems from fertility to mental retardation and growth issues.
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