209 Imaging the Molecular and Cell Dynamics that Form the Early Mouse Embryo

Our goal is to reveal how mammalian cells resolve their fate, shape, and position in the body in real time. Understanding how these decisions are made is critical to realise how embryos form and what problems compromise human fertility. Yet, their real-time control in vivo remains unknown. Because fixed specimens cannot capture cell dynamics, we established imaging technologies to study cells directly in live mouse embryos. We recently showed how transcription factors search and bind to the DNA to determine the first cell fates of the embryo. We found that differences in the binding of the transcription factor Sox2 to DNA appear as early as the 4-cell stage of development and predict cell fate. We also discovered that as cell choose their initial fates, they extend long filopodia protrusions to pull their neighbour cells closer, revealing a mechanism for embryo compaction and polarization. Finally, we established techniques to image how dynamic changes in the organisation of the actin and microtubule cytoskeletons regulate the first spatial segregation of inner and outer cells to form the future pluripotent inner mass and placental tissues of the embryo. Together, our findings reveal some of the key dynamic mechanisms that pattern the early mouse embryo.