To understand cell fate specification and maintenance during development, it is essential to visualize both lineage markers and cell behaviors in real time using endogenous markers to report cell fate. We have generated a reporter line in which eGFP is fused to the endogenous locus of Cdx2, a transcription factor essential for trophectoderm specification, allowing us to visualize cell fate decisions in the preimplantation mouse embryo. We used two-photon laser scanning microscopy to visualize expression of the endogenous Cdx2 fusion protein and show that Cdx2 undergoes phases of upregulation. Additionally, we show that as late as the 32-cell stage, outer trophectoderm cells may change their fates by migrating inward and losing Cdx2 expression. Furthermore, the tools and techniques we report allow for dual-colored imaging, which will greatly facilitate the study of not only preimplantation development, but later stages of development and tissues where Cdx2 plays an important role.
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