Amnion formation in the mouse embryo: the single amniochorionic fold model

A posterior fold of extraembryonic ectoderm and associated epiblast is formed early during gastrulation by accumulation of extraembryonic mesoderm posterior to the primitive streak. Previously called the "posterior amniotic fold", we rename it the "amniochorionic fold" (ACF) because it forms both amnion and chorion. Exocoelom formation within the ACF seems not to involve apoptosis within the mesoderm. The ACF and exocoelom expand without disrupting the anterior junction of epiblast, extraembryonic ectoderm and visceral endoderm. No separate anterior fold is formed; its absence was confirmed in 3D reconstructions. Amnion and chorion closure is eccentric, close to the anterior margin of the egg cylinder: we name it the "anterior separation point".Here, we reconcile previous descriptions of amnion formation and provide new nomenclature, as well as an animation, that clarify and emphasize the arrangement of the tissues that contribute to amnion development and the dynamics of the process. According to our data, the amnion and the chorion are formed by a single amniochorionic fold initiated posteriorly. Finally, we give an overview on mutant mouse models with impaired amnion development.To develop and survive in utero, the mammalian conceptus develops a number of extraembryonic tissues and organs to provide nutritional support and protection to the embryo proper. These extraembryonic appendages are shed at birth. The amnion is the innermost extraembryonic membrane that surrounds the foetus of amniotes and delineates the fluid-filled amniotic cavity, thereby providing a confined niche within the conceptus and conferring protection and shock resistance [1]. Of all the extraembryonic membranes it is morphologically the most conserved membrane. In contrast to the visceral yolk sac, the chorion and the allantois, the amnion is a thin transparent membrane (Figure 1E, H) that is avascular in most amniotes. In mouse embryos, the amnion consists throughout gestation of a bi-layer