Figure 5

Embryonic and extraembryonic ectoderm demarcation in the amniochorionic fold. Embryonic ectoderm was marked by IHC for Oct3/4 (brown), while extraembryonic ectoderm was marked by the expression of Eomes (blue). The latter is also detected in nascent embryonic and extraembryonic mesoderm. (A) Prior to the formation of the exocoelomic cavity (EC), the amniochorionic fold (ACF) is recognized by the extraembryonic mesoderm accumulation between extraembryonic ectoderm and visceral endoderm. (B) Soon after formation of the EC, the extraembryonic ectoderm is the major contributor for the ectoderm of the ACF. (C) The contribution of embryonic ectoderm to the ACF increases progressively as the embryo continues to grow, and the EC enlarges. The black arrow indicates the transient non-alignment that often appears between the mesoderm and extraembryonic ectoderm of the ACF. (D) At the anterior separation point (ASP), embryonic ectoderm is aligned with extraembryonic mesoderm in the lower half of the ACF to form the amnion, while the extraembryonic ectoderm apposed to extraembryonic mesoderm in the upper half of the fold forms the chorion. Scale bar: 200 μm.