Modeling X-chromosome inactivation and reactivation during human development

Stem-cell-based embryo models generate much excitement as they offer a window into an early phase of human development that has remained largely inaccessible to scientific investigation. An important epigenetic phenomenon during early embryogenesis is the epigenetic silencing of one of the two X chromosomes in female embryos, which ensures an equal output of X-linked gene expression between the sexes. Xchromosome inactivation (XCI) is thought to be established within the first three weeks of human development, although the inactive X-chromosome is reactivated in primordial germ cells (PGCs) that migrate to the embryonic gonads. Here, we summarize our current understanding of X-chromosome dynamics during human development and comment on the potential of recently established stem-cell-based models to reveal the underlying mechanisms.