Amnion formation in mouse embryos, illustrated by longitudinal sections. Sections at the midline in the extraembryonic-embryonic region of embryos between E6.0 and E7.5 illustrate the different stages of amnion development. The tissue layers in the extraembryonic and extraembryonic-embryonic junction region were artificially painted based on histomorphological differences and the analysis of layer-specific markers (Figure 4). (A) Prestreak stage: no mesoderm, no amniochorionic fold (ACF). (B) Early streak stage: extraembryonic and embryonic mesoderm emerges at the primitive streak (PS). (C) Midstreak stage: extracellular spaces accumulate within the extraembryonic mesoderm. (D) Late streak/no bud stage: the ACF protrudes into the proamniotic cavity (PAC) and coalescence of spaces in the extraembryonic mesoderm generates the exocoelomic cavity (EC). The endodermal furrow (En-frw) marks the anterior midline at the extraembryonic-embryonic junction. (E) Between late streak/no bud and late streak/early bud stage: expansion of the EC. (F) Late streak/early bud stage: a large ACF extends from the posterior. The allantoic bud (Al-bud) has become visible. (G) Between late streak/early bud and neural plate stage: the lateral extensions of the EC meet at the focal anterior separation point (ASP). Closure and separation of the ectodermal lineages occurs. (H) Neural plate stage: the segregated amniotic (Am) and chorionic (Ch) membranes divide the PAC into the amniotic cavity (AC), EC and ectoplacental cavity (EPC). The allantois (Al) and visceral yolk sac (VYS) become clear. The amniotic ectoderm (AmEc) and mesoderm (AmM) acquire their squamous architecture. Sections do not go through the midline at the posterior (E) or anterior (H) side at the extraembryonic-embryonic junction of the conceptus. Scale bar: 200 μm.