Amnion formation in the mouse embryo: the single amniochorionic fold model
© Pereira et al; licensee BioMed Central Ltd. 2011
Received: 28 April 2011
Accepted: 1 August 2011
Published: 1 August 2011
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.
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.
Keywordsallantois amniochorionic fold amniotic membrane anterior separation point apoptosis bone morphogenetic proteins chorion epiblast gastrulation
Due to their lordotic position, mouse and rat embryos are peculiar in possessing inverted germ layers in which the ectoderm initially faces the inside of the egg cylinder . Starting at the 9-10-somite stage, mouse embryos undergo axial rotation, and hence achieve the regular flexed foetal position. Consequently, the anterior junction between the embryo proper and the amnion on the one hand, and the embryo and the yolk sac on the other hand shifts progressively from anterior ectoderm over the heart field, to ventrally where the vitelline vein contacts the body wall . When turning is finished (14-16 somite stage), the embryo has become entirely enfolded in the amnion and visceral yolk sac [3, 9] (Figure 1G).
The amniotic membrane has low immunogenicity and hence high potential for regenerative medicine [10, 11]. Indeed, the amnion has been used for a century as a wound dressing . Recently, the amnion has gained attention due to the apparent presence of resident stem cells in term human amniotic ectoderm . Furthermore, cells isolated from human term amniotic ectoderm and mesoderm showed triple lineage differentiation capacity in cell culture [14–17]. Similar studies on rat and mouse amniotic-membrane-derived cells have reported the existence of such pluripotent cells [18, 19]. The origin of the amniotic stem cells is, however, unclear and, in the case of mice and rats, the source used to isolate the so-called amniotic stem cells has sometimes been controversial .
Amnion formation is intimately related to the formation of the primitive streak early during gastrulation, but most investigators have focused on the analysis of the embryonic component of the conceptus, typically discarding the amnion in their studies. Hence, the developmental origin of mouse amnion and its formation have been described fragmentarily [3, 20, 21]. The process of dividing the proamniotic cavity into the exocoelomic and the amniotic cavity by a membrane called the amnion is completed at the late streak/early bud to neural plate stage, depending on the mouse strain investigated.
The primitive streak is the first morphological landmark of gastrulation at late TS 9 (E6.5). It is characterized by a thickening at the posterior side of the epiblast, close to the embryonic-extraembryonic junction. Streak ectoderm undergoes an epithelial-to-mesenchymal transition, and mesoderm emerges [22–25]. Cells from the epiblast, via the streak, give rise to the mesoderm, including embryonic mesoderm and extraembryonic mesoderm of the chorion, amnion, yolk sac and allantois . Moreover, the epiblast gives rise to amniotic ectoderm as well as to embryonic ectoderm, endoderm and primordial germ cells . The extraembryonic ectoderm will form the chorion and, with the ectoplacental cone, the chorionic disk of the placenta. Visceral endoderm (VEnd) becomes the endoderm component of the visceral yolk sac.
Fate mapping studies have revealed that amniotic mesoderm and amniotic ectoderm are derived from different regions of the epiblast. Descendants of epiblast cells located at the posterior and posterolateral sides of the epiblast contribute to amniotic mesoderm . Indeed, labelling cells of the posterior primitive streak showed that the mesoderm derivative is mostly extraembryonic, part of which contributes to the formation of the amnion during early gastrulation (early- and midstreak) [27, 28]. In contrast proximal epiblast that is in the anterior half of the embryo at prestreak and streak stages, gives rise to amniotic ectoderm .
Amnion formation begins with the accumulation of extraembryonic mesoderm leading to the formation of a posterior amniotic fold [3, 21, 29, 30] followed by folds along the sides of the egg cylinder like the progression of the lateral mesoderm wings . Bonnevie (1950) disputed the role and existence of a posterior fold, but highlighted that the extraembryonic ectoderm at the anterior margin of the egg cylinder remains closely associated with the visceral endoderm, despite the eventual intercalation by extraembryonic mesoderm. Snell & Stevens (1966) emphasized that extraembryonic mesoderm may accumulate at the anterior margin and regarded it as a small anterior fold. The exocoelomic cavity is then formed by the accumulation and coalescence of "small cavities", or "small closed lumina" , within the posterior and lateral folds. According to Snell & Stevens (1966), the posterior, lateral and anterior folds should, however, be thought of as a continuous constriction around the middle of the egg cylinder that tightens as the folds develop. This description differs from Kaufman's authoritative description (1992), which proposes the existence of separate posterior and anterior amniotic folds, each with an exocoelomic cavity. Kaufman (1992) described and illustrated the subsequent amnion expansion as follows: "The rapid expansion of the posterior amniotic fold and its apposition and eventual fusion with the considerably smaller anterior amniotic fold results in the formation of the chorion and amnion, which divide the proamniotic cavity into ectoplacental, exocoelomic and amniotic cavities, respectively" (plate 5 in The Atlas of Mouse Development ). Several phenotypes observed in the amnion of mutant mouse models have been interpreted according to Kaufman's description [31–34]. However, during routine analysis of serial sections, we came to the conclusion that this description might be inaccurate because we never observed an anterior amniotic fold with exocoelom. Therefore, we re-examined the process of amnion formation in the mouse based on histological analysis of mouse embryos between the prestreak (E6.0) and the neural plate stage (E7.5). Computer reconstruction of histological sections used for The Atlas of Mouse Development confirmed the absence of an anterior fold. Finally, we provide an animation that illustrates the single amniochorionic fold model and emphasizes the dynamics and arrangement of the tissues that contribute to amnion development.
The proamniotic canal becomes localized anteriorly (Figure 2F; Figure 3E), close to where the lateral wings of the exocoelom converge. Here, the extraembryonic and embryonic ectoderm from the ACF will contact their counterparts at the anterior side of the egg cylinder resulting in the closure of the amniotic cavity and the separation of embryonic and extraembryonic ectoderm. We propose to name the latter region the anterior separation point (ASP) (Figure 2G; Figure 3F). The ectoderm of the embryo proper and the amniotic ectoderm now delineate the amniotic cavity completely, and the extraembryonic ectoderm is now called chorionic ectoderm. The junction between presumptive chorionic ectoderm and amniotic ectoderm remains distinct, without apparent cell mingling across the visible anatomical junction between extraembryonic ectoderm and embryonic ectoderm in the amniochorionic fold, as shown by the complementary expression patterns of Eomes (extraembryonic ectoderm) and presence of Oct3/4 (ectoderm layer of the amnion) (Figure 5D).
Since exocoelom formation and its consequence for amnion and chorion formation are highly dynamic processes which are difficult to envision, we clarify the process in an animation (Additional file 1).
Earlier descriptions of amnion and exocoelom formation in the mouse have been partial and conflicting. Both Snell & Stevens (1966) and Kaufman (1992) describe a small anterior amniotic fold. However, Snell & Stevens consider this anterior fold a continuation of the posterior and lateral amniotic folds, and they do not describe it having lacunae. Conversely, Kaufman describes an anterior fold with an independent exocoelom. Our present data demonstrates the absence of an independent, exocoelom containing, anterior fold. On the other hand, we show the presence of a single fold that is initiated posteriorly, and which we redefine as the amniochorionic fold (ACF). The fold expands laterally around the egg cylinder, like the progression of the lateral mesodermal wings. The lateral extensions converge on the anterior midline. The expansion of the exocoelomic cavity of the ACF accompanies the lateral expansion of the fold around the egg cylinder, but does not reach the anterior side of the embryo. Instead, a local accumulation of mesoderm can occur, forming what could be interpreted as a small anterior fold (Figure 6A; Figure 7A). We, however, propose not to call this small bulge an anterior fold because it risks being confusing. Interestingly, while the epiblast grows directionally towards the primitive streak [25, 39], the proamniotic canal remains localized anteriorly, close to where the exocoelom wings converge, and maintains a relatively constant diameter before closure at the level of the embryonic-extraembryonic junction (Figure 3C-3E). This may promote the formation and expansion of the exocoelomic cavity within the extraembryonic mesoderm. Compared with the growing embryo, relatively little cellular material is required in the developing amnion, chorion and yolk sac by virtue of exocoelom formation. Ultimately, amnion closure is eccentric, close to the anterior margin of the egg cylinder, which we define as the anterior separation point (ASP).
The differences in interpretation of amnion formation may be partly explained by the difficulties in correctly orienting and staging mouse embryos when sectioned within the deciduum, but also to slight variations in the expansion of the exocoelom on the left and right sides and to residual adjustment of axial symmetry of the embryo [40, 41]. In our study, we analysed whole-mount embryos dissected free from the deciduum to better control the plane of section at the embryonic-extraembryonic junction. For instance, to visualize the ASP in a midline section, it is crucial to examine a section in which the endodermal furrow and the base of the allantois are both present.
We propose a model of amnion formation in the mouse involving a single ACF growing and expanding laterally from the posterior side of the embryo: the single amniochorionic fold model. The 3D-reconstructions of Kaufman's (1992) original serial sections support our model further. The new material was in a CD1 background (Figure 2 to Figure 6), and the model was confirmed in an F1 (C57B6xCBA) background (Figure 7). However strain dependent differences in the formation of the amnion cannot be fully excluded.
Amnion development in the mouse is intimately related to exocoelom expansion. The initial establishment of the exocoelom is intriguing. The question remains as to what cellular and molecular mechanisms drive the formation of the lacunae in the extraembryonic mesoderm. Selective cell survival and programmed cell death have been implicated in causing the cavitation in epiblast leading to the formation of the proamniotic cavity . Should a similar mechanism drive the formation of the exocoelomic cavity, cells at multiple sites throughout the extraembryonic mesoderm would have to undergo programmed cell death to generate the scattered small individual cavities. However, we did not detect apoptosis in the mesoderm of the fold, indicating that programmed cell death is likely not involved in the process of exocoelom formation. Perhaps the formation of the exocoelomic cavity reflects merely the enlargement of extracellular spaces, or depends on the continuous rearrangement of cell adhesion molecules and extracellular matrix, allowing the formation of spaces in-between the mesodermal cells of the ACF, similar to vascular lumen formation in invertebrates and vertebrates [42, 43]. The accumulation and coalescence of these extracellular spaces or lacunae leads to the formation of a large extraembryonic coelom - the exocoelom - lined by extraembryonic mesoderm. To our knowledge, there are no mutants reported with explicitly impaired exocoelom formation in the newly formed extraembryonic mesoderm. Nevertheless, the ectopic appearance of the cell adhesion molecule VCAM and its receptor α1-integrin on the extraembryonic mesoderm lining the exocoelom (amniotic, chorionic and yolk sac mesoderm component) in FoxF1-deficient mice leads to a compressed/ruffled exocoelom boundary . Conversely, reduced expression of a component of the extracellular matrix, Fibronectin-1, in CHATO-deficient mice results in expansion of the exocoelomic cavity . This suggests that rearrangements of cell-cell and cell-extracellular matrix contacts may play a role in the formation/maintenance of the exocoelomic cavity and the tissues lining it.
Mutations affecting amnion formation
Type I transmembrane protein
Amnion absent while extraembryonic structures like chorion, yolk sac blood islands, and allantois develop normally
E 6.5 - Prestreak, no amniotic fold
Amnion present but lacks the fold covering the loops of the gut in the umbilical ring region
Amnion has a delayed fusion or fails to fuse; heart is formed within the exocoelomic cavity as a result of lack of amnion fusion
E 7.0 - Late streak, large exocoelomic cavity
E8.5 to E9.5
Ds (Ds mutation)
Semidominant mutation associated with early amnion rupture or amniotic band sequence (ABS)
Transcription factor with zinc finger motifs
Amnion filled with fluid; unbalanced amniotic ectoderm and mesoderm
E 7.5 - Neural plate, amnion and chorion segregated
High molecular-weight glycoprotein
Undersized amnion; amniotic cavity with pressure deficit
E 7.5 - Neural plate, amnion and chorion segregated
E9.5 to E10.5
Winged helix transcription
Undersized amnion tightens and restricts embryo growth; ectopic VCAM and receptor α1-integrin expression in amniotic mesoderm
E 7.5 - Neural plate, amnion and chorion segregated
E9.5 to E10.5
LIM domain-binding protein
Undersized amnion leading to constricted embryonic-extraembryonic junction
E 7.5 - Neural plate, amnion and chorion segregated
E9.5 to E10
Focal adhesion molecule
Undersized amnion; amniotic cavity with pressure deficit
E 7.5 - Neural plate, amnion and chorion segregated
E8.5 to E9
Bmp signaling intermediate
Delayed fusion of the amnion; Thickenings containing ectopic haematopoietic, endothelial and PGC-like cells
E 7.0 - Late streak/early bud, amnion fusion
E9.5 to E10.5
In chimeras, Flk1 null cells fail to form blood islands and accumulate in amnion
E 7.5 - Neural plate, chorion and ectoplacental cone fuse
No mesoderm and amniochorionic fold forms, and consequently no amnion
E 6.5 - Prestreak, no amniotic fold
We have provided here a morphological description and an animation of the poorly understood process of amnion formation. Nevertheless, we are still a long way from understanding how the process is regulated at the molecular level. Given the poor documentation of gene expression patterns in the amnion, it is at present also unclear if the amnion itself is differentially patterned in anterior versus posterior or lateral amnion. It is unclear what defines or distinguishes progressively embryonic and amniotic ectoderm, or yolk sac and amniotic mesoderm, at the molecular level. Moreover, little is known about the amnion with respect to its impact on the development of the embryo and its surrounding extraembryonic tissues e.g. allantois, yolk sac and chorion, or vice versa. Does the amnion then function exclusively as a container and filter for the amniotic fluid and as a shock absorber? Or does it also signal actively to the surrounding tissues, and hence influences the patterning of the embryo?
Stem cell-like cells have been reported in the human amnion [16, 34, 51–53] and recently also in the rat . So far, their origin has been speculative. The origin, presence and potential of an amniotic stem cell-like population may differ in primate and rodent embryos because of the difference in topology between the disc-shaped primate and the cup-shaped rodent embryo, and the differences in developmental origin of amniotic layers . However, if amniotic stem cell-like cells exist in mice, mouse genetic models will be extremely valuable for investigating the developmental origin of these cells, as well as in unravelling the complex cascade of molecular events that lead to the appearance of this cell population. The single amniochorionic fold model and the comprehensive animation reported here provide a new framework to investigate this cell population and to examine complex defects in the amnion of mouse mutants.
Our histomorphological analysis revealed that only one amniotic fold is present in the mouse embryo, which we rename the "amniochorionic fold" (ACF). The ACF emerges at the posterior side of the egg cylinder and expands laterally around the egg cylinder. Exocoelom formation within this fold seems not to involve apoptosis. Here we show that the ACF and exocoelom do not expand through the anterior side of the embryo. Amnion closure is eccentric and occurs close to the anterior margin of the egg cylinder, which we define as the "anterior separation point" (ASP). The 3D reconstructions of historical sections of E7.5 embryos from Kaufman (1992) confirm the single amniochorionic fold model. This model and the comprehensive animation provide a new framework for interpreting fate-map data, investigating amniotic stem cell populations and complex defects in the amnion of mouse mutants.
For histological analysis, CD1 embryos were collected between E6.0 and E7.5. All experiments were approved by the ethical commission from Katholieke Universiteit Leuven (097/2008). We used the staging nomenclature of embryos that is described in the Edinburgh Mouse Atlas Project . After overnight fixation in 4% paraformaldehyde in PBS at 4°C, the embryos were washed with saline, dehydrated and stored in 70% ethanol at 4°C. The embryos were further dehydrated and embedded in Technovit 8100 (Heraeus Kulzer), sectioned (transverse sections at 7 μm, longitudinal sections at 4 μm) and stained with 0.05% Neutral Red solution. Serial sections of at least 13 embryos per stage were analysed for Figures 2, 3 and 5.
Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay
TUNEL assays were performed using the In Situ Cell Death Detection Kit, Fluorescein (Roche). Serial sections of 5 paraffin embedded embryos with an emerging amniochorionic fold were analyzed. Sections were deparaffinised using Xylene (VWR) and rehydrated through an ethanol series to distilled water. Permeabilization was done by incubation with 10 μg ProteinaseK/mL (Invitrogen) in 10 mM Tris/HCl pH 7.4 at 30°C during 20 minutes. TUNEL reactions were performed according to manufacturer's instructions. DAPI (Invitrogen) was used to counterstain nuclei.
In situ hybridization
Whole mount in situ hybridization with an antisense probe for Eomes  was performed as described elsewhere , with minor modifications. The embryos were processed afterwards for plastic embedding and sectioning as described above.
Immunohistochemistry (IHC) was performed on 4 μm thick paraffin sections of 4% paraformaldehyde fixed embryos using an automated platform (Ventana Discovery, Ventana Medical Systems). We used a rabbit antibody to Oct3/4 (N19, Santa Cruz).
Images of the original, serial, longitudinal sections of embryos in the collection used for The Atlas of Mouse Development (Kaufman 1992) were obtained and stacked using the methods and software developed for the Edinburgh Mouse Atlas Project . The image stacks were sliced at the desired level and orientation to obtain perfectly sagittal and transverse slices using MAPaint .
List of abbreviations
- 3D :
- AC :
- ACF :
- Al :
- Al-bud :
- Am :
- AmEc :
- AmM :
- Amn :
- ASP :
anterior separation point
- AVE :
anterior visceral endoderm
- BMP :
bone morphogenetic protein
- Ch :
- De :
- DS :
- E7.5 :
embryonic day 7.5
- EC :
- Em :
- EMAP :
Edinburgh Mouse Atlas Project
- En-frw :
- Eomes :
- EPC :
- EP-Cn :
- Evi1 :
ecotropic viral integration site 1
- ExEc :
- ExM :
- Flk1 :
protein-tyrosine kinase receptor
- FoxF1 :
Forkhead box protein F1
- IHC :
- Ldb1 :
LIM domain binding 1
- Oct4 :
- PAC :
- PBS :
phosphate buffered saline
- PEnd :
- Pl :
- PS :
- PYS :
parietal yolk sac
- RM :
- Smad5 :
mothers against decapentaplegic homolog 5
- TE :
- TS :
- UC :
- VEnd :
- VYS :
visceral yolk sac
- YSC :
yolk sac cavity.
The authors are very grateful to Professor Kaufman for making available the original sections used for The Atlas of Mouse Development and for his willingness to re-examine and revisit his description of amnion development. They also wish to thank colleagues in the Edinburgh Mouse Atlas Project for practical support. The authors wish to express appreciation for challenging discussions with Susana Chuva de Sousa Lopes and Anne Camus, and to all lab members for support. Jeroen Korving is thanked sincerely for expert help with plastic embedding and sectioning, and Anne Camus for whole mount in situ support.
This work was supported by OT05/09/053 from the research Council of the University of Leuven and the Interuniversity Attraction Poles Program IUAP-6/20. P.N.G.P. is a predoctoral fellow of FCT (SFRH/BD/15901/2005) from GABBA PhD program, M.D. is a predoctoral fellow sponsored by VIB11.
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