Fig. 4. Definition of embryonic stages and identification of pluripotent cell populations by analysis of Oct4 expression. Whole mount in situ hybridization analysis of 4.5 d.p.c. (A), 4.75 d.p.c. (B), 5.0 d.p.c. (C), 5.25 d.p.c. (D) and 5.5 d.p.c. (E) mouse embryos using antisense digoxygenin-labelled Oct4 riboprobes. (F) A 5.0 d.p.c. embryo analysed by whole mount in situ hybridization using antisense digoxygenin-labelled PRCE riboprobes. (G) A 5.25 d.p.c. embryo and (H) a 5.5 d.p.c. embryo analysed by whole mount in situ hybridization using sense digoxygenin-labelled PRCE riboprobes. (F-H) These images are included to illustrate the formation of the proamniotic cavity (PAC) and pseudostratified epithelial layer of primitive ectoderm (PE). Abbreviations: EEE, extra-embryonic ectoderm; ICM, inner cell mass; TE, trophectoderm; TGC, trophoblast giant cells; VEn, visceral endoderm. Bar, 50 µm. The colouration in the trophectoderm of the 4.5 d.p.c. blastocyst is not cellular but can be attributed to a shadow rather than gene expression.