We have previously found that certain stem cells that are derived from rat blastocysts and named extraembryonic endoderm precursor (XEN-P) cells show a unique molecular signature sharing some of the characteristics of embryonic stem cells (ES), trophoblast stem cells (TS), and extraembryonic endoderm stem cells (XEN). These XEN-P cells are positive for AP, SSEA1, Oct4, and Rex1 markers similar to ES cells and also express signature markers of TS-eomesodermin (Eomes) and XEN-Gata6. Here we show that these cells integrate into the visceral and parietal extraembryonic endoderm lineages as well as into the inner cell mass (ICM), the primitive endoderm, and the polar and mural trophectoderm (TE) of cultured embryos. In addition, we find that the XEN-P cells colonize yolk sac and contribute to trophoblast lineages of postimplantation embryos following transfer to surrogate mothers. We also find that the XEN-P cell culture propagates by shedding cell clusters into the media in addition to typical expansion of colonies. Interestingly, the cell cultures exist as mixed populations of two interconvertible phenotypes of flat and round cells with preferential expression of stem cell markers Oct4 and SSEA1 in round cells. We believe these cells represent a metastable stage during ICM cellular segregation. These results are important for developing hypotheses of cell fate plasticity in the ICM and provide a model for the study of development and differentiation along the extraembryonic lineages.