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BMC Developmental Biology

, 11:48

First Online: 01 August 2011Received: 28 April 2011Accepted: 01 August 2011


BackgroundDespite 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.

ResultsA 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-.

ConclusionsHere, 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.

Keywordsallantois amniochorionic fold amniotic membrane anterior separation point apoptosis bone morphogenetic proteins chorion epiblast gastrulation List of abbreviations3D three dimensional

AC amniotic cavity

ACF amniochorionic fold

Al allantois

Al-bud allantoic bud

Am amnion

AmEc amniotic ectoderm

AmM amniotic mesoderm

Amn amnionless

ASP anterior separation point

AVE anterior visceral endoderm

BMP bone morphogenetic protein

Ch chorion

De deciduum

DS disorganization mutation

E7.5 embryonic day 7.5

EC exocoelomic cavity

Em embryo

EMAP Edinburgh Mouse Atlas Project

En-frw endodermal furrow

Eomes Eomesodermin

EPC ectoplacental cavity

EP-Cn ectoplacental cone

Evi1 ecotropic viral integration site 1

ExEc extraembryonic ectoderm

ExM extraembryonic mesoderm

Flk1 protein-tyrosine kinase receptor

FoxF1 Forkhead box protein F1

IHC immunohistochemistry

Ldb1 LIM domain binding 1

Oct4 octamer-4

PAC proamniotic cavity

PBS phosphate buffered saline

PEnd parietal endoderm

Pl Placenta

PS primitive streak

PYS parietal yolk sac

RM Reichert-s membrane

Smad5 mothers against decapentaplegic homolog 5

TE trophectoderm

TS Theiler stage

UC umbilical cord

VEnd visceral endoderm

VYS visceral yolk sac

YSC yolk sac cavity.

Electronic supplementary materialThe online version of this article doi:10.1186-1471-213X-11-48 contains supplementary material, which is available to authorized users.

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Autor: Paulo NG Pereira - Mariya P Dobreva - Liz Graham - Danny Huylebroeck - Kirstie A Lawson - AN Zwijsen


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