Background: Despite the detailed knowledge obtained over the last decade on the molecular regulation of
gastrulation in amniotes, the process of amnion development has been poorly described and illustrated in mice,
and conflicting descriptions exist. Understanding the morphogenesis and development not only of the early
mouse embryo, but also of its extraembryonic tissues, is crucial for correctly interpreting fate-mapping data and
mouse mutants with gastrulation defects. Moreover, the recent isolation from amnion of cells with stem cell
features further argues for a better understanding of the process of amnion formation. Here, we revisit the highly
dynamic process of amnion formation in the mouse. Amnion development starts early during gastrulation and is
intimately related to the formation of the exocoelom and the expansion of the amniotic fold. The authoritative
description involves the fusion of two amniotic folds, a big posterior and a smaller anterior fold. We challenged
this ‘two amniotic folds’ model by performing detailed histomorphological analyses of dissected, staged embryos
and 3D reconstructions using historical sections.
Results: 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”.
Conclusions: 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.