Rob Stephenson

Understanding the origins of the first 3 cell lineages in the mouse

By embryonic day 4.5 (E4.5), three distinct cell types are apparent in the mouse embryo. Outer cells make up the trophectoderm (TE-later forms the embryonic portion of the placenta), while a clump of cells encompassed by the TE (the ICM) is made up of the primitive endoderm (PE-later becomes the yolk sac) and the epiblast (EPI-later forms the embryo).

The ICM itself was previously believed to be a uniform population of cells prior to E4.5. However recent work in our lab indicates that this may not be the case. Studies of the fates of cells isolated from the early ICM and analysis with markers expressed in the EPI and PE have shown that prior to E4.5 the early ICM consists of two cell types. To further investigate the development of the first 3 cell lineages in the mouse, we are generating fluorescent fusions with markers for each of these lineages in their endogenous loci.

We are using Cdx2 as a marker of the TE, Nanog for the EPI and Gata-6 for the PE. By using different coloured fluorescent proteins, we will be able to visualize the expression of these markers simultaneously in real time in cultured embryos. Interestingly, recent studies examining early cell fates have indicated that not all cells are equivalent at the 4-cell stage and that they may have different developmental potentials. Using antibodies against the markers of cell fate mentioned above, we are also examining the potential of cells isolated from 4-cell embryos cultured in vitro.