Fig. 5. Cell-autonomous Eomes requirements for cell migration from the primitive streak and endoderm specification. (A) Eomes-deficient ES cells were generated by retargeting the remaining wild-type allele in Eomes^N/+ heterozygous ES cells. (B) Eomes^N/N ES cells were introduced into ROSA26^LacZ blastocysts, as depicted and chimeric embryos analysed histologically at E7.5 (C-G) and E9.5 (H-N). (C-G) lacZ-negative Eomes^N/N ES cells (counterstained with Eosin) are found randomly distributed in the epiblast at E7.5 (C',D-G) but fail to exit the PS and accumulate in cell masses in the amniotic cavity. (H) Highly chimeric embryos at E9.5 are developmentally delayed and show a relative paucity of mesoderm derivatives, resulting in gross abnormalities affecting the heart, somites, axis elongation and embryonic turning. (I-N) Eomes^N/N mutant ES cells fail to contribute to definitive endoderm derivatives and cannot be found within the gut tube at any level (arrows in H-N). (N) Chimeric embryos occasionally exhibit posterior neural tube duplications (arrowheads).