Dynamic changes in Sox2 spatio-temporal expression promote the second cell fate decision through/signaling in preimplantation mouse embryos.

Oct4 and Sox2 regulate the expression of target genes such as, and, by binding to their respective regulatory motifs. Their functional cooperation is reflected in their ability to heterodimerize on adjacentregulatory motifs, the composite Sox/Oct motif. Given that Oct4 and Sox2 regulate many developmental genes, a quantitative analysis of their synergistic action on different Sox/Oct motifs would yield valuable insights into the mechanisms of early embryonic development. In the present study, we measured binding affinities of Oct4 and Sox2 to different Sox/Oct motifs using fluorescence correlation spectroscopy. We found that the synergistic binding interaction is driven mainly by the level of Sox2 in the case of theSox/Oct motif. Taking into accountexpression levels fluctuate more than, our finding provides an explanation on how Sox2 controls the segregation of the epiblast and primitive endoderm populations within the inner cell mass of the developing rodent blastocyst.