The development of the amnion in mice and other amniotes.

The amnion is an extraembryonic tissue that evolutionarily allowed embryos of all amniotes to develop in a transient and local aquatic environment. Despite the importance of this tissue, very little is known about its formation and its molecular characteristics. In this review, we have compared the basic organization of the extraembryonic membranes in amniotes and describe the two types of amniogenesis, folding and cavitation. We then zoom in on the atypical development of the amnion in mice that occurs via the formation of a single posterior amniochorionic fold. Moreover, we consolidate lineage tracing data to better understand the spatial and temporal origin of the progenitors of amniotic ectoderm, and visualize the behaviour of their descendants in the extraembryonic-embryonic junctional region. This analysis provides new insight on amnion development and expansion. Finally, using an online-available dataset of single-cell transcriptomics during the gastrulation period in mice, we provide bioinformatic analysis of the molecular signature of amniotic ectoderm and amniotic mesoderm. The amnion is a tissue with unique biomechanical properties that deserves to be better understood. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Amniotic ectoderm expansion in mouse occurs via distinct modes and requires SMAD5-mediated signalling.

Upon gastrulation, the mammalian conceptus transforms rapidly from a simple bilayer into a multilayered embryo enveloped by its extra-embryonic membranes. Impaired development of the amnion, the innermost membrane, causes major malformations. To clarify the origin of the mouse amnion, we used single-cell labelling and clonal analysis. We identified four clone types with distinct clonal growth patterns in amniotic ectoderm. Two main types have progenitors in extreme proximal-anterior epiblast. Early descendants initiate and expand amniotic ectoderm posteriorly, while descendants of cells remaining anteriorly later expand amniotic ectoderm from its anterior side. Amniogenesis is abnormal in embryos deficient in the bone morphogenetic protein (BMP) signalling effector SMAD5, with delayed closure of the proamniotic canal, and aberrant amnion and folding morphogenesis. Transcriptomics of individual Smad5 mutant amnions isolated before visible malformations and tetraploid chimera analysis revealed two amnion defect sets. We attribute them to impairment of progenitors of the two main cell populations in amniotic ectoderm and to compromised cuboidal-to-squamous transition of anterior amniotic ectoderm. In both cases, SMAD5 is crucial for expanding amniotic ectoderm rapidly into a stretchable squamous sheet to accommodate exocoelom expansion, axial growth and folding morphogenesis.

Enzymatic removal of ribonucleotides from DNA is essential for mammalian genome integrity and development.

The presence of ribonucleotides in genomic DNA is undesirable given their increased susceptibility to hydrolysis. Ribonuclease (RNase) H enzymes that recognize and process such embedded ribonucleotides are present in all domains of life. However, in unicellular organisms such as budding yeast, they are not required for viability or even efficient cellular proliferation, while in humans, RNase H2 hypomorphic mutations cause the neuroinflammatory disorder Aicardi-Goutières syndrome. Here, we report that RNase H2 is an essential enzyme in mice, required for embryonic growth from gastrulation onward. RNase H2 null embryos accumulate large numbers of single (or di-) ribonucleotides embedded in their genomic DNA (>1,000,000 per cell), resulting in genome instability and a p53-dependent DNA-damage response. Our findings establish RNase H2 as a key mammalian genome surveillance enzyme required for ribonucleotide removal and demonstrate that ribonucleotides are the most commonly occurring endogenous nucleotide base lesion in replicating cells.

Clonal and molecular analysis of the prospective anterior neural boundary in the mouse embryo.

In the mouse embryo the anterior ectoderm undergoes extensive growth and morphogenesis to form the forebrain and cephalic non-neural ectoderm. We traced descendants of single ectoderm cells to study cell fate choice and cell behaviour at late gastrulation. In addition, we provide a comprehensive spatiotemporal atlas of anterior gene expression at stages crucial for anterior ectoderm regionalisation and neural plate formation. Our results show that, at late gastrulation stage, expression patterns of anterior ectoderm genes overlap significantly and correlate with areas of distinct prospective fates but do not define lineages. The fate map delineates a rostral limit to forebrain contribution. However, no early subdivision of the presumptive forebrain territory can be detected. Lineage analysis at single-cell resolution revealed that precursors of the anterior neural ridge (ANR), a signalling centre involved in forebrain development and patterning, are clonally related to neural ectoderm. The prospective ANR and the forebrain neuroectoderm arise from cells scattered within the same broad area of anterior ectoderm. This study establishes that although the segregation between non-neural and neural precursors in the anterior midline ectoderm is not complete at late gastrulation stage, this tissue already harbours elements of regionalisation that prefigure the later organisation of the head.

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

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.

Histone H4K20me3 and HP1α are late heterochromatin markers in development, but present in undifferentiated embryonic stem cells.

We report here that the formation of heterochromatin in cell nuclei during mouse development is characterised by dynamic changes in the epigenetic modifications of histones. Our observations reveal that heterochromatin in mouse preimplantation embryos is in an immature state that lacks the constitutive heterochromatin markers histone H4 trimethyl Lys20 (H4K20me3) and chromobox homolog 5 (HP1α, also known as CBX5). Remarkably, these somatic heterochromatin hallmarks are not detectable--except in mural trophoblast--until mid-gestation, increasing in level during foetal development. Our results support a developmentally regulated connection between HP1α and H4K20me3. Whereas inner cell mass (ICM) and epiblast stain negative for H4K20me3 and HP1α, embryonic stem (ES) cell lines, by contrast, stain positive for these markers, indicating substantial chromatin divergence. We conclude that H4K20me3 and HP1α are late developmental epigenetic markers, and slow maturation of heterochromatin in tissues that develop from ICM is ectopically induced during ES cell derivation. Our findings suggest that H4K20me3 and HP1α are markers for cell type commitment that can be triggered by developmental or cell context, independently of the differentiation process.

Smad5 determines murine amnion fate through the control of bone morphogenetic protein expression and signalling levels.

Smad5 is an intracellular mediator of bone morphogenetic protein (Bmp) signalling. It is essential for primordial germ cell (PGC) development, for the development of the allantois and for amnion closure, as demonstrated by loss of Bmp signalling. By contrast, the appearance of ectopic PGC-like cells and regionalized ectopic vasculogenesis and haematopoiesis in thickened Smad5(m1/m1) amnion are amnion defects that have not been associated with loss of Bmp signalling components. We show that defects in amnion and allantois can already be detected at embryonic day (E) 7.5 in Smad5 mutant mice. However, ectopic Oct4-positive (Oct4(+)) and alkaline phosphatase-positive (AP(+)) cells appear suddenly in thickened amnion at E8.5, and at a remote distance from the allantois and posterior primitive streak, suggesting a change of fate in situ. These ectopic Oct4(+), AP(+) cells appear to be Stella negative and hence cannot be called bona fide PGCs. We demonstrate a robust upregulation of Bmp2 and Bmp4 expression, as well as of Erk and Smad activity, in the Smad5 mutant amnion. The ectopic expression of several Bmp target genes in different domains and the regionalized presence of cells of several Bmp-sensitive lineages in the mutant amnion suggest that different levels of Bmp signalling may determine cell fate. Injection of rBMP4 in the exocoelom of wild-type embryos can induce thickening of amnion, mimicking the early amnion phenotype in Smad5 mutants. These results support a model in which loss of Smad5 results paradoxically in gain of Bmp function defects in the amnion.

How is the mouse germ-cell lineage established?

Six cells have been detected in the early mouse embryo that express the transcriptional repressor Blimp1--as also do all the 40 or so cells that constitute the founder germ cell pool a day later. Are these half-dozen cells the ancestors of the entire mouse germ cell lineage?

Nuclear re-organisation of the Hoxb complex during mouse embryonic development.

The spatial and temporal co-linear expression of Hox genes during development is an exquisite example of programmed gene expression. The precise mechanisms underpinning this are not known. Analysis of Hoxb chromatin structure and nuclear organisation, during the differentiation of murine ES cells, has lent support to the idea that there is a progressive 'opening' of chromatin structure propagated through Hox clusters from 3'to 5', which contributes to the sequential activation of gene expression. Here, we show that similar events occur in vivo in at least two stages of development. The first changes in chromatin structure and nuclear organisation were detected during gastrulation in the Hoxb1-expressing posterior primitive streak region: Hoxb chromatin was decondensed and the Hoxb1 locus looped out from its chromosome territory, in contrast to non-expressing Hoxb9, which remained within the chromosome territory. At E9.5, when differential Hox expression along the anteroposterior axis is being established, we found concomitant changes in the organisation of Hoxb. Hoxb organisation differed between regions of the neural tube that had never expressed Hoxb [rhombomeres (r) 1 and 2], strongly expressed Hoxb1 but not b9 (r4), had downregulated Hoxb1 (r5), expressed Hoxb9 but not Hoxb1 (spinal cord), and expressed both genes (tail bud). We conclude that Hoxb chromatin decondensation and nuclear re-organisation is regulated in different parts of the developing embryo, and at different developmental stages. The differential nuclear organisation of Hoxb along the anteroposterior axis of the developing neural tube is coherent with co-linear Hox gene expression. In early development nuclear re-organisation is coupled to Hoxb expression, but does not anticipate it.

BMP signaling mediated by ALK2 in the visceral endoderm is necessary for the generation of primordial germ cells in the mouse embryo.

Deletion of various bone morphogenetic proteins (BMPs) and their downstream Smads in mice have clearly shown that BMP signaling is essential for the formation of primordial germ cells (PGCs). However, the molecular mechanism through which this takes place is still unclear. Here, we demonstrate that BMP4 produced in the extraembryonic ectoderm signals through ALK2, a type I BMP receptor, in the visceral endoderm (VE) to induce formation of PGCs from the epiblast. Firstly, embryonic day 5.5-6.0 (E5.5-E6.0) embryos cultured on fibronectin formed PGCs in the presence of VE, but not in its absence. Secondly, Alk2-deficient embryos completely lacked PGCs and the heterozygotes had reduced numbers, resembling Bmp4-deficient phenotypes. Thirdly, expression of constitutively active ALK2 in the VE, but not in the epiblast, was sufficient to rescue the PGC phenotype in Bmp4-deficient embryos. In addition, we show that the requirement for the VE at E5.5-E6.0 can be replaced by culturing embryos stripped of VE on STO cells, indicating that STO cells provide or transduce signals necessary for PGC formation that are normally transmitted by the VE. We propose a model in which direct signaling to proximal epiblast is supplemented by an obligatory indirect BMP-dependent signal via the VE.

Acquisition of Hox codes during gastrulation and axial elongation in the mouse embryo.

Early sequential expression of mouse Hox genes is essential for their later function. Analysis of the relationship between early Hox gene expression and the laying down of anterior to posterior structures during and after gastrulation is therefore crucial for understanding the ontogenesis of Hox-mediated axial patterning. Using explants from gastrulation stage embryos, we show that the ability to express 3' and 5' Hox genes develops sequentially in the primitive streak region, from posterior to anterior as the streak extends, about 12 hours earlier than overt Hox expression. The ability to express autonomously the earliest Hox gene, Hoxb1, is present in the posterior streak region at the onset of gastrulation, but not in the anterior region at this stage. However, the posterior region can induce Hoxb1 expression in these anterior region cells. We conclude that tissues are primed to express Hox genes early in gastrulation, concomitant with primitive streak formation and extension, and that Hox gene inducibility is transferred by cell to cell signalling. Axial structures that will later express Hox genes are generated in the node region in the period that Hox expression domains arrive there and continue to spread rostrally. However, lineage analysis showed that definitive Hox codes are not fixed at the node, but must be acquired later and anterior to the node in the neurectoderm, and independently in the mesoderm. We conclude that the rostral progression of Hox gene expression must be modulated by gene regulatory influences from early on in the posterior streak, until the time cells have acquired their stable positions along the axis well anterior to the node.