Characterizing Epiblast and Primitive Endoderm Cell Sorting in Pre- Implantation Embryos via Live Imaging

Abstract

In the preimplantation mouse embryo, the inner cell mass (ICM) stochastically differentiates into Epiblast (EPI) and Primitive Endoderm (PE) cells which in turn will develop into cells that build the body of the embryo and the yolk sac, respectively. These two cell types then sort themselves into two distinct compartments prior to implantation, which is essential for correct development. While it is known that the sorting process occurs between the E3.5 and E4.5 stages, very little is known about the mechanisms of EPI and PE cell sorting. This lack of knowledge is in part due to limited tools available to visualize it. For my senior thesis research, I wish to use fluorescent reporter embryos in conjunction with live imaging using light sheet microscopy to answer key questions behind EPI/PE sorting, such as: how is sorting initiated? Does a threshold number of EPI or PE cells need to be differentiated for sorting to start? Is a specific threshold of nuclear marker expression needed for sorting to occur? Do PE cells closer to the lumen sort earlier than PE cells farther away? Do both EPI and PE cells sort or do PE cells sort to form the layer around the lumen? Do cells move in a continuous beeline to their final positions? Characterizing the start and dynamics of EPI and PE cell sorting will be a crucial foundation for understanding the mechanisms which regulate this important process.