Epiblast morphogenesis before gastrulation

ETV4 is a mechanical transducer linking cell crowding dynamics to lineage specification

Dynamic changes in mechanical microenvironments, such as cell crowding, regulate lineage fates as well as cell proliferation. Although regulatory mechanisms for contact inhibition of proliferation have been extensively studied, it remains unclear how cell crowding induces lineage specification. Here we found that a well-known oncogene, ETS variant transcription factor 4 (ETV4), serves as a molecular transducer that links mechanical microenvironments and gene expression. In a growing epithelium of human embryonic stem cells, cell crowding dynamics is translated into ETV4 expression, serving as a pre-pattern for future lineage fates. A switch-like ETV4 inactivation by cell crowding derepresses the potential for neuroectoderm differentiation in human embryonic stem cell epithelia. Mechanistically, cell crowding inactivates the integrin-actomyosin pathway and blocks the endocytosis of fibroblast growth factor receptors (FGFRs). The disrupted FGFR endocytosis induces a marked decrease in ETV4 protein stability through ERK inactivation. Mathematical modelling demonstrates that the dynamics of cell density in a growing human embryonic stem cell epithelium precisely determines the spatiotemporal ETV4 expression pattern and, consequently, the timing and geometry of lineage development. Our findings suggest that cell crowding dynamics in a stem cell epithelium drives spatiotemporal lineage specification using ETV4 as a key mechanical transducer.

Development of node architecture and emergence of molecular organizer characteristics in the pig embryo

BACKGROUND

The avian node is the equivalent of the amphibian Spemann's organizer, as indicated by its ability to induce a secondary axis, cellular contribution, and gene expression, whereas the node of the mouse, which displays limited inductive capacities, was suggested to be a part of spatially distributed signaling. Furthermore, the structural identity of the mouse node is subject of controversy, while little is known about equivalent structures in other mammals.

RESULTS

We analyzed the node and emerging organizer in the pig using morphology and the expression of selected organizer genes prior to and during gastrulation. The node was defined according to the "four-quarter model" based on comparative consideration. The node of the pig displays a multilayered, dense structure that includes columnar epithelium, bottle-like cells in the dorsal part, and mesenchymal cells ventrally. Expression of goosecoid (gsc), chordin, and brachyury, together with morphology, reveal the consecutive emergence of three distinct domains: the gastrulation precursor domain, the presumptive node, and the mature node. Additionally, gsc displays a ventral expression domain prior to epiblast epithelialization.

CONCLUSION

Our study defines the morphological and molecular context of the emerging organizer equivalent in the pig and suggests a sequential development of its function.

Loss of TJP1 disrupts gastrulation patterning and increases differentiation toward the germ cell lineage in human pluripotent stem cells

Biological patterning events that occur early in development establish proper tissue morphogenesis. Identifying the mechanisms that guide these patterning events is necessary in order to understand the molecular drivers of development and disease and to build tissues in vitro. In this study, we use an in vitro model of gastrulation to study the role of tight junctions and apical/basolateral polarity in modulating bone morphogenic protein-4 (BMP4) signaling and gastrulation-associated patterning in colonies of human pluripotent stem cells (hPSCs). Disrupting tight junctions via knockdown (KD) of the scaffolding tight junction protein-1 (TJP1, also known as ZO1) allows BMP4 to robustly and ubiquitously activate pSMAD1/5 signaling over time, resulting in loss of the patterning phenotype and marked differentiation bias of pluripotent stem cells to primordial germ cell-like cells (PGCLCs). These findings give important insights into how signaling events are regulated and lead to spatial emergence of diverse cell types in vitro.

Engineering 3D micro-compartments for highly efficient and scale-independent expansion of human pluripotent stem cells in bioreactors

Human pluripotent stem cells (hPSCs) have emerged as the most promising cellular source for cell therapies. To overcome the scale-up limitations of classical 2D culture systems, suspension cultures have been developed to meet the need for large-scale culture in regenerative medicine. Despite constant improvements, current protocols that use microcarriers or generate cell aggregates only achieve moderate amplification performance. Here, guided by reports showing that hPSCs can self-organize in vitro into cysts reminiscent of the epiblast stage in embryo development, we developed a physio-mimetic approach for hPSC culture. We engineered stem cell niche microenvironments inside microfluidics-assisted core-shell microcapsules. We demonstrate that lumenized three-dimensional colonies significantly improve viability and expansion rates while maintaining pluripotency compared to standard hPSC culture platforms such as 2D cultures, microcarriers, and aggregates. By further tuning capsule size and culture conditions, we scale up this method to industrial-scale stirred tank bioreactors and achieve an unprecedented hPSC amplification rate of 277-fold in 6.5 days. In brief, our findings indicate that our 3D culture system offers a suitable strategy both for basic stem cell biology experiments and for clinical applications.

Embryonic and extraembryonic tissues during mammalian development: shifting boundaries in time and space

The first few days of embryonic development in eutherian mammals are dedicated to the specification and elaboration of the extraembryonic tissues. However, where the fetus ends and its adnexa begins is not always as self-evident during the early stages of development, when the definitive body axes are still being laid down, the germ layers being specified and a discrete form or bodyplan is yet to emerge. Function, anatomy, histomorphology and molecular identities have been used through the history of embryology, to make this distinction. In this review, we explore them individually by using specific examples from the early embryo. While highlighting the challenges of drawing discrete boundaries between embryonic and extraembryonic tissues and the limitations of a binary categorization, we discuss how basing such identity on fate is the most universal and conceptually consistent. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Mesothelial fusion mediates chorioallantoic membrane formation

In amniotic vertebrates (birds, reptiles and mammals), an extraembryonic structure called the chorioallantoic membrane (CAM) functions as respiratory organ for embryonic development. The CAM is derived from fusion between two pre-existing membranes, the allantois, a hindgut diverticulum and a reservoir for metabolic waste, and the chorion which marks the embryo's external boundary. Modified CAM in eutherian mammals, including humans, gives rise to chorioallantoic placenta. Despite its importance, little is known about cellular and molecular mechanisms mediating CAM formation and maturation. In this work, using the avian model, we focused on the early phase of CAM morphogenesis when the allantois and chorion meet and initiate fusion. We report here that chicken chorioallantoic fusion takes place when the allantois reaches the size of 2.5-3.0 mm in diameter and in about 6 hours between E3.75 and E4. Electron microscopy and immunofluorescence analyses suggested that before fusion, in both the allantois and chorion, an epithelial-shaped mesothelial layer is present, which dissolves after fusion, presumably by undergoing epithelial-mesenchymal transition. The fusion process per se, however, is independent of allantoic growth, circulation, or its connection to the developing mesonephros. Mesoderm cells derived from the allantois and chorion can intermingle post-fusion, and chorionic ectoderm cells exhibit a specialized sub-apical intercellular interface, possibly to facilitate infiltration of allantois-derived vascular progenitors into the chorionic ectoderm territory for optimal oxygen transport. Finally, we investigated chorioallantoic fusion-like process in primates, with limited numbers of archived human and fresh macaque samples. We summarize the similarities and differences of CAM formation among different amniote groups and propose that mesothelial epithelial-mesenchymal transition mediates chorioallantoic fusion in most amniotic vertebrates. Further study is needed to clarify tissue morphogenesis leading to chorioallantoic fusion in primates. Elucidating molecular mechanisms regulating mesothelial integrity and epithelial-mesenchymal transition will also help understand mesothelial diseases in the adult, including mesothelioma, ovarian cancer and fibrosis. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Alternative mammalian strategies leading towards gastrulation: losing polar trophoblast (Rauber's layer) or gaining an epiblast cavity

Using embryological data from 14 mammalian orders, the hypothesis is presented that in placental mammals, epiblast cavitation and polar trophoblast loss are alternative developmental solutions to shield the central epiblast from extraembryonic signalling. It is argued that such reciprocal signalling between the edge of the epiblast and the adjoining polar trophoblast or edge of the mural trophoblast or with the amniotic ectoderm is necessary for the induction of gastrulation. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

New insights into the epitranscriptomic control of pluripotent stem cell fate

Each cell in the human body has a distinguishable fate. Pluripotent stem cells are challenged with a myriad of lineage differentiation options. Defects are more likely to be fatal to stem cells than to somatic cells due to the broad impact of the former on early development. Hence, a detailed understanding of the mechanisms that determine the fate of stem cells is needed. The mechanisms by which human pluripotent stem cells, although not fully equipped with complex chromatin structures or epigenetic regulatory mechanisms, accurately control gene expression and are important to the stem cell field. In this review, we examine the events driving pluripotent stem cell fate and the underlying changes in gene expression during early development. In addition, we highlight the role played by the epitranscriptome in the regulation of gene expression that is necessary for each fate-related event.

Characterization of multitype colonies originating from porcine blastocysts produced in vitro

Many types of embryonic stem cells have been induced from pre-implantation blastocysts to study the specification of early lineages. Various cell lines have been established using chemicals, including excessive inhibitory molecules. Previous studies have also aimed to purify cell populations representing a single embryonic lineage from a protocol. In this study, we used a novel culture condition to induce cells from blastocyst seeding and analyzed their characteristics. Next, signaling inhibitors were introduced during the cell culture period. Furthermore, we investigated the cell types using RNA sequencing. Each type of cell population showed a distinct morphology and reactivity with alkaline phosphatase. Marker proteins enabled each cell type to be distinguished by immunocytochemistry, and genes such as Sox17, Gata4, Gata6, T, and Cdx2 showed applicability for the discrimination of cell types. Signaling inhibitors suppressed the production of some cell types, and gene expression and marker protein patterns were collapsed. RNA-sequencing suggested cell-type-specific marker genes and the correlation among samples. In conclusion, four types of cells could be induced from porcine embryos using a single protocol, and they could be isolated manually. Our data will help promote the study of lineage segregation based on embryonic cells.

A hexa-species transcriptome atlas of mammalian embryogenesis delineates metabolic regulation across three different implantation modes

Mammalian embryogenesis relies on glycolysis and oxidative phosphorylation to balance the generation of biomass with energy production. However, the dynamics of metabolic regulation in the postimplantation embryo in vivo have remained elusive due to the inaccessibility of the implanted conceptus for biochemical studies. To address this issue, we compiled single-cell embryo profiling data in six mammalian species and determined their metabolic dynamics through glycolysis and oxidative phosphorylation associated gene expression. Strikingly, we identify a conserved switch from bivalent respiration in the late blastocyst towards a glycolytic metabolism in early gastrulation stages across species, which is independent of embryo implantation. Extraembryonic lineages followed the dynamics of the embryonic lineage, except visceral endoderm. Finally, we demonstrate that in vitro primate embryo culture substantially impacts metabolic gene regulation by comparison to in vivo samples. Our work reveals a conserved metabolic programme despite different implantation modes and highlights the need to optimise postimplantation embryo culture protocols.

Epithelial dynamics during early mouse development

The first epithelia to arise in an organism face the challenge of maintaining the integrity of the newly formed tissue, while exhibiting the behavioral flexibility required for morphogenetic processes to occur effectively. Epithelial cells integrate biochemical and biomechanical cues, both intrinsic and extrinsic, in order to bring about the molecular changes which determine their morphology, behavior and fate. In this review we highlight recent advances in our understanding of the various dynamic processes that the emergent epithelial cells undergo during the first seven days of mouse development and speculate what the future holds in understanding the mechanistic bases for these processes through integrative approaches.

Modeling Epiblast Shape in Implanting Mammalian Embryos

An indispensable prerequisite of mammalian development is successful morphogenesis in the epiblast, the embryonic tissue that gives rise to all differentiated cells of the adult mammal. The right control of both epiblast morphogenesis and the events that regulate its shape in particular during implantation is henceforth of tremendous importance. However, monitoring the process of development in implanting human embryos is ethically and technically challenging, making it difficult to troubleshoot when things go wrong, as it is unfortunately the case with over 30% of pregnancy failures. Although modern in vitro techniques have proven very insightful lately, more tools are needed in the quest to elucidate mammalian and human development. Mathematical and computational modeling position themselves as helpful complementary tools in the biologist's toolbox, enabling the exploration of the living in silico, beyond the boundaries set by ethical concerns and the potential limitations of wet lab techniques. Here, we show how mathematical modeling and computer simulations can be used to emulate and investigate mechanisms driving epiblast shape changes in mouse and human embryos during implantation.

The primitive streak and cellular principles of building an amniote body through gastrulation

[Figure: see text].

Pluripotent stem cells related to embryonic disc exhibit common self-renewal requirements in diverse livestock species

Despite four decades of effort, robust propagation of pluripotent stem cells from livestock animals remains challenging. The requirements for self-renewal are unclear and the relationship of cultured stem cells to pluripotent cells resident in the embryo uncertain. Here, we avoided using feeder cells or serum factors to provide a defined culture microenvironment. We show that the combination of activin A, fibroblast growth factor and the Wnt inhibitor XAV939 (AFX) supports establishment and continuous expansion of pluripotent stem cell lines from porcine, ovine and bovine embryos. Germ layer differentiation was evident in teratomas and readily induced in vitro. Global transcriptome analyses highlighted commonality in transcription factor expression across the three species, while global comparison with porcine embryo stages showed proximity to bilaminar disc epiblast. Clonal genetic manipulation and gene targeting were exemplified in porcine stem cells. We further demonstrated that genetically modified AFX stem cells gave rise to cloned porcine foetuses by nuclear transfer. In summary, for major livestock mammals, pluripotent stem cells related to the formative embryonic disc are reliably established using a common and defined signalling environment.

This article has an associated ‘The people behind the papers’ interview.

Summary: We report the derivation of similar, stable and continuously expandable pluripotent stem cells related to embryonic disc epiblast from embryos of pig, sheep and cow.

Blastocoel morphogenesis: A biophysics perspective

The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.

Bioengineered embryoids mimic post-implantation development in vitro

The difficulty of studying post-implantation development in mammals has sparked a flurry of activity to develop in vitro models, termed embryoids, based on self-organizing pluripotent stem cells. Previous approaches to derive embryoids either lack the physiological morphology and signaling interactions, or are unconducive to model post-gastrulation development. Here, we report a bioengineering-inspired approach aimed at addressing this gap. We employ a high-throughput cell aggregation approach to simultaneously coax mouse embryonic stem cells into hundreds of uniform epiblast-like aggregates in a solid matrix-free manner. When co-cultured with mouse trophoblast stem cell aggregates, the resulting hybrid structures initiate gastrulation-like events and undergo axial morphogenesis to yield structures, termed EpiTS embryoids, with a pronounced anterior development, including brain-like regions. We identify the presence of an epithelium in EPI aggregates as the major determinant for the axial morphogenesis and anterior development seen in EpiTS embryoids. Our results demonstrate the potential of EpiTS embryoids to study peri-gastrulation development in vitro.

Epiblast and trophoblast morphogenesis in the pre-gastrulation blastocyst of the pig. A light- and electron-microscopical study

The epiblast of the amniote embryo is of paramount importance during early development as it gives rise to all tissues of the embryo proper. In mammals, it emerges through segregation of the hypoblast from the inner cell mass and subsequently undergoes transformation into an epithelial sheet to create the embryonic disc. In rodents and man, the epiblast cell layer is covered by the polar trophoblast which forms the placenta. In mammalian model organisms (rabbit, pig, several non-human primates), however, the placenta is formed by mural trophoblast whereas the polar trophoblast disintegrates prior to gastrulation and thus exposes the epiblast to the microenvironment of the uterine cavity. Both, polar trophoblast disintegration and epiblast epithelialization, thus pose special cell-biological requirements but these are still rather ill-understood when compared to those of gastrulation morphogenesis. This study therefore applied high-resolution light and transmission electron microscopy and three-dimensional (3D) reconstruction to 8- to 10-days-old pig embryos and defines the following steps of epiblast transformation: (1) rosette formation in the center of the ball-shaped epiblast, (2) extracellular cavity formation in the rosette center, (3) epiblast segregation into two subpopulations - addressed here as dorsal and ventral epiblast - separated by a "pro-amniotic" cavity. Ventral epiblast cells form between them a special type of desmosomes with a characteristic dense felt of microfilaments and are destined to generate the definitive epiblast. The dorsal epiblast remains a mass of non-polarized cells and closely associates with the disintegrating polar trophoblast, which shows morphological features of both apoptosis and autophagocytosis. Morphogenesis of the definitive epiblast in the pig may thus exclude a large portion of bona fide epiblast cells from contributing to the embryo proper and establishes contact de novo with the mural trophoblast at the junction between the two newly defined epiblast cell populations.

Embryo size regulates the timing and mechanism of pluripotent tissue morphogenesis

Mammalian embryogenesis is a paradigm of regulative development as mouse embryos show plasticity in the regulation of cell fate, cell number, and tissue morphogenesis. However, the mechanisms behind embryo plasticity remain largely unknown. Here, we determine how mouse embryos respond to an increase in cell numbers to regulate the timing and mechanism of embryonic morphogenesis, leading to the formation of the pro-amniotic cavity. Using embryos and embryonic stem cell aggregates of different size, we show that while pro-amniotic cavity formation in normal-sized embryos is achieved through basement membrane-induced polarization and exocytosis, cavity formation of increased-size embryos is delayed and achieved through apoptosis of cells that lack contact with the basement membrane. Importantly, blocking apoptosis, both genetically and pharmacologically, alters pro-amniotic cavity formation but does not affect size regulation in enlarged embryos. We conclude that the regulation of embryonic size and morphogenesis, albeit concomitant, have distinct molecular underpinnings.

Progress and challenges in developing organoids in farm animal species for the study of reproduction and their applications to reproductive biotechnologies

Within the past decades, major progress has been accomplished in isolating germ/stem/pluripotent cells, in refining culture medium and conditions and in establishing 3-dimensional culture systems, towards developing organoids for organs involved in reproduction in mice and to some extent in humans. Haploid male germ cells were generated in vitro from primordial germ cells. So were oocytes, with additional support from ovarian cells and subsequent follicle culture. Going on with the female reproductive tract, spherical oviduct organoids were obtained from adult stem/progenitor cells. Multicellular endometrial structures mimicking functional uterine glands were derived from endometrial cells. Trophoblastic stem cells were induced to form 3-dimensional syncytial-like structures and exhibited invasive properties, a crucial point for placentation. Finally, considering the embryo itself, pluripotent embryonic cells together with additional extra-embryonic cells, could self-organize into a blastoid, and eventually into a post-implantation-like embryo. Most of these accomplishments have yet to be reached in farm animals, but much effort is devoted towards this goal. Here, we review the progress and discuss the specific challenges of developing organoids for the study of reproductive biology in these species. We consider the use of such organoids in basic research to delineate the physiological mechanisms involved at each step of the reproductive process, or to understand how they are altered by environmental factors relevant to animal breeding. We evaluate their potential in reproduction of animals with a high genetic value, from a breeding point of view or in the context of preserving local breeds with limited headcounts.

Epithelial-Mesenchymal Transition Drives Three-Dimensional Morphogenesis in Mammalian Early Development

From fertilization to onset of gastrulation, a mammalian embryo goes through several rounds of cellular morphogenesis resembling phenomena of epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET), collectively referred to as EMTs. How these EMT events play a role in shaping the three-dimensional (3-D) architecture of the developing embryo is not well-understood. In this review, we present a model in which cellular morphogenesis, represented primarily by dynamic changes in its epithelialization status, is the driving force of embryonic 3-D organization. This is achieved through the integration of three key components of mammalian early development, the pluripotency regulation, morphogenetic signaling, and biomechanical force anisotropy. Although cells in an early embryo do not exhibit full mesenchymal characteristics, our model underscores the importance of investigating molecular regulation of epithelial cell polarity and partial EMT/MET in understanding mammalian early development.

Cell differentiation events in pre-implantation mouse and bovine embryos

Early mammal embryogenesis starts with oocyte fertilization, giving rise to the zygote. The events that the newly formed zygote surpasses are crucial to the embryo developmental success. Shortly after activation of its genome, cells of the embryo segregate into the inner cell mass (ICM) or the trophectoderm (TE). The first will give rise to the embryo while the latter will become the placenta. This first segregation involves cellular and molecular processes that include cell polarity linked to intracellular pathway activation, which will regulate the transcription of trophectoderm-related genes. Then, cells of the ICM undergo the second event of mammalian cell differentiation, which consists of the separation between epiblast (EPI) and hypoblast or primitive endoderm (PrE). This second segregation involves paracrine signaling, leading to differential expression of key genes that will dictate the fate of the cell. Although these processes are described in detail in the mouse, recent studies suggest that the bovine embryo could also be an interesting model for early development, since there are differences to the mouse and similarities with early human embryogenesis. In this review, we gathered the main data available in the literature upon bovine and mouse early development events, suggesting that both models should be analyzed and studied in a complementary way, to better model early events occurring in human development.

Mechanisms of human embryo development: from cell fate to tissue shape and back

Gene regulatory networks and tissue morphogenetic events drive the emergence of shape and function: the pillars of embryo development. Although model systems offer a window into the molecular biology of cell fate and tissue shape, mechanistic studies of our own development have so far been technically and ethically challenging. However, recent technical developments provide the tools to describe, manipulate and mimic human embryos in a dish, thus opening a new avenue to exploring human development. Here, I discuss the evidence that supports a role for the crosstalk between cell fate and tissue shape during early human embryogenesis. This is a critical developmental period, when the body plan is laid out and many pregnancies fail. Dissecting the basic mechanisms that coordinate cell fate and tissue shape will generate an integrated understanding of early embryogenesis and new strategies for therapeutic intervention in early pregnancy loss.

Mesenchymal-epithelial transition regulates initiation of pluripotency exit before gastrulation

The pluripotent epiblast gives rise to all tissues and organs in the adult body. Its differentiation starts at gastrulation, when the epiblast generates mesoderm and endoderm germ layers through epithelial-mesenchymal transition (EMT). Although gastrulation EMT coincides with loss of epiblast pluripotency, pluripotent cells in development and in vitro can adopt either mesenchymal or epithelial morphology. The relationship between epiblast cellular morphology and its pluripotency is not well understood. Here, using chicken epiblast and mammalian pluripotency stem cell (PSC) models, we show that PSCs undergo a mesenchymal-epithelial transition (MET) prior to EMT-associated pluripotency loss. Epiblast MET and its subsequent EMT are two distinct processes. The former, a partial MET, is associated with reversible initiation of pluripotency exit, whereas the latter, a full EMT, is associated with complete and irreversible pluripotency loss. We provide evidence that integrin-mediated cell-matrix interaction is a key player in pluripotency exit regulation. We propose that epiblast partial MET is an evolutionarily conserved process among all amniotic vertebrates and that epiblast pluripotency is restricted to an intermediate cellular state residing between the fully mesenchymal and fully epithelial states.

Blastula stage specification of avian neural crest

Cell fate specification defines the earliest steps towards a distinct cell lineage. Neural crest, a multipotent stem cell population, is thought to be specified from the ectoderm, but its varied contributions defy canons of segregation potential and challenges its embryonic origin. Aiming to resolve this conflict, we have assayed the earliest specification of neural crest using blastula stage chick embryos. Specification assays on isolated chick epiblast explants identify an intermediate region specified towards the neural crest cell fate. Furthermore, low density culture suggests that the specification of intermediate cells towards the neural crest lineage is independent of contact mediated induction and Wnt-ligand induced signaling, but is, however, dependent on transcriptional activity of β-catenin. Finally, we have validated the regional identity of the intermediate region towards the neural crest cell fate using fate map studies. Our results suggest a model of neural crest specification within a restricted epiblast region in blastula stage chick embryos.

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

Genetic basis for primordial germ cells specification in mouse and human: Conserved and divergent roles of PRDM and SOX transcription factors

Primordial germ cells (PGCs) are embryonic precursors of sperm and egg that pass on genetic and epigenetic information from one generation to the next. In mammals, they are induced from a subset of cells in peri-implantation epiblast by BMP signaling from the surrounding tissues. PGCs then initiate a unique developmental program that involves comprehensive epigenetic resetting and repression of somatic genes. This is orchestrated by a set of signaling molecules and transcription factors that promote germ cell identity. Here we review significant findings on mammalian PGC biology, in particular, the genetic basis for PGC specification in mice and human, which has revealed an evolutionary divergence between the two species. We discuss the importance and potential basis for these differences and focus on several examples to illustrate the conserved and divergent roles of critical transcription factors in mouse and human germline.

Biomechanical regulation of EMT and epithelial morphogenesis in amniote epiblast

Epiblast is composed of pluripotent cells which will give rise to all cell lineages in a human body. It forms a single-cell layered epithelium conserved among all amniotic vertebrates (birds, reptiles and mammals) and undergoes complex morphogenesis both before and during gastrulation. Our knowledge of the amniote epiblast is based on data acquired through cellular and molecular analyses of early chick and mouse embryos in vivo and mammalian pluripotent stem cells (PSCs) in vitro. Very few studies have been published on biomechanical characteristics of the amniote epiblast, largely due to lack of experimental tools for measuring and perturbing biomechanical properties. Also missing is a conceptual framework that can integrate both biomechanical and molecular parameters of the epiblast. This review is aimed at providing a background based on which epiblast morphogenesis, including its transition between the epithelial and mesenchymal states, can be understood from a biomechanical perspective. This simple developmental biology system is suitable for testing a multitude of theoretical models in biomechanics, leading to a better understanding of biomechanical logics and constraints governing multicellular organization.

Capacitation of human naïve pluripotent stem cells for multi-lineage differentiation

Human naïve pluripotent stem cells (PSCs) share features with the pre-implantation epiblast. They therefore provide an unmatched opportunity for characterising the developmental programme of pluripotency in Homo sapiens Here, we confirm that naïve PSCs do not respond directly to germ layer induction, but must first acquire competence. Capacitation for multi-lineage differentiation occurs without exogenous growth factor stimulation and is facilitated by inhibition of Wnt signalling. Whole-transcriptome profiling during this formative transition highlights dynamic changes in gene expression, which affect many cellular properties including metabolism and epithelial features. Notably, naïve pluripotency factors are exchanged for postimplantation factors, but competent cells remain devoid of lineage-specific transcription. The gradual pace of transition for human naïve PSCs is consistent with the timespan of primate development from blastocyst to gastrulation. Transcriptome trajectory during in vitro capacitation of human naïve cells tracks the progression of the epiblast during embryogenesis in Macaca fascicularis, but shows greater divergence from mouse development. Thus, the formative transition of naïve PSCs in a simple culture system may recapitulate essential and specific features of pluripotency dynamics during an inaccessible period of human embryogenesis.

Single cell transcriptome analysis of human, marmoset and mouse embryos reveals common and divergent features of preimplantation development

The mouse embryo is the canonical model for mammalian preimplantation development. Recent advances in single cell profiling allow detailed analysis of embryogenesis in other eutherian species, including human, to distinguish conserved from divergent regulatory programs and signalling pathways in the rodent paradigm. Here, we identify and compare transcriptional features of human, marmoset and mouse embryos by single cell RNA-seq. Zygotic genome activation correlates with the presence of polycomb repressive complexes in all three species, while ribosome biogenesis emerges as a predominant attribute in primate embryos, supporting prolonged translation of maternally deposited RNAs. We find that transposable element expression signatures are species, stage and lineage specific. The pluripotency network in the primate epiblast lacks certain regulators that are operative in mouse, but encompasses WNT components and genes associated with trophoblast specification. Sequential activation of GATA6, SOX17 and GATA4 markers of primitive endoderm identity is conserved in primates. Unexpectedly, OTX2 is also associated with primitive endoderm specification in human and non-human primate blastocysts. Our cross-species analysis demarcates both conserved and primate-specific features of preimplantation development, and underscores the molecular adaptability of early mammalian embryogenesis.

Highlighted Article: Analysis of stage-matched, single-cell gene expression data from three mammalian species reveals conserved and primate-specific regulation of early embryogenesis and lineage specification.

Symmetry Breaking in the Mammalian Embryo

We present an overview of symmetry breaking in early mammalian development as a continuous process from compaction to specification of the body axes. While earlier studies have focused on individual symmetry-breaking events, recent advances enable us to explore progressive symmetry breaking during early mammalian development. Although we primarily discuss embryonic development of the mouse, as it is the best-studied mammalian model system to date, we also highlight the shared and distinct aspects between different mammalian species. Finally, we discuss how insights gained from studying mammalian development can be generalized in light of self-organization principles. With this review, we hope to highlight new perspectives in studying symmetry breaking and self-organization in multicellular systems.

On the origin of the human germline

In mice, primordial germ cells (PGCs), the precursors of eggs and sperm, originate from pregastrulation postimplantation embryos. By contrast, the origin of human PGCs (hPGCs) has been less clear and has been difficult to study because of the technical and ethical constraints that limit direct studies on human embryos. In recent years, however, in vitro simulation models using human pluripotent stem cells, together with surrogate non-rodent mammalian embryos, have provided insights and experimental approaches to address this issue. Here, we review these studies, which suggest that the posterior epiblast and/or the nascent amnion in pregastrulation human embryos is a likely source of hPGCs, and that a different gene regulatory network controls PGCs in humans compared with in the mouse. Such studies on the origins and mechanisms of hPGC specification prompt further consideration of the somatic cell fate decisions that occur during early human development.

Prosurvival effect of cumulus prostaglandin G/H synthase 2/prostaglandin2 signaling on bovine blastocyst: impact on in vivo posthatching development

Apoptotic activity is a common physiological process which culminates at the blastocyst stage in the preimplantation embryo of many mammals. The degree of embryonic cell death can be influenced by the oocyte microenvironment. However, the prognostic significance of the incidence of apoptosis remains undefined. Prostaglandin E2 (PGE2) derived from prostaglandin G/H synthase-2 (PTGS2) activity is a well-known prosurvival factor that is mainly studied in oncology. PGE2 is the predominant PTGS2-derived prostaglandin present in the oocyte microenvironment during the periconceptional period. Using an in vitro model of bovine embryo production followed by transfer and collection procedures, we investigated the impact of periconceptional PGE2 on the occurrence of spontaneous apoptosis in embryos and on subsequent in vivo posthatching development. Different periconceptional PGE2 environments were obtained using NS-398, a specific inhibitor of PTGS2 activity, and exogenous PGE2. We assessed the level of embryonic cell death in blastocysts at day 8 postfertilization by counting total cell numbers, by the immunohistochemical staining of active caspase-3, and by quantifying terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling signals and apoptosis regulator (BCL-2/BAX) mRNA expression. Morphometric parameters were used to estimate the developmental stage of the embryonic disk and the extent of trophoblast elongation on day 15 conceptuses. Our findings indicate that periconceptional PGE2 signaling durably impacts oocytes, conferring increased resistance to spontaneous apoptosis in blastocysts and promoting embryonic disk development and the elongation process during preimplantation development.

Systematic analysis of transcription start sites in avian development

Cap Analysis of Gene Expression (CAGE) in combination with single-molecule sequencing technology allows precision mapping of transcription start sites (TSSs) and genome-wide capture of promoter activities in differentiated and steady state cell populations. Much less is known about whether TSS profiling can characterize diverse and non-steady state cell populations, such as the approximately 400 transitory and heterogeneous cell types that arise during ontogeny of vertebrate animals. To gain such insight, we used the chick model and performed CAGE-based TSS analysis on embryonic samples covering the full 3-week developmental period. In total, 31,863 robust TSS peaks (>1 tag per million [TPM]) were mapped to the latest chicken genome assembly, of which 34% to 46% were active in any given developmental stage. ZENBU, a web-based, open-source platform, was used for interactive data exploration. TSSs of genes critical for lineage differentiation could be precisely mapped and their activities tracked throughout development, suggesting that non-steady state and heterogeneous cell populations are amenable to CAGE-based transcriptional analysis. Our study also uncovered a large set of extremely stable housekeeping TSSs and many novel stage-specific ones. We furthermore demonstrated that TSS mapping could expedite motif-based promoter analysis for regulatory modules associated with stage-specific and housekeeping genes. Finally, using Brachyury as an example, we provide evidence that precise TSS mapping in combination with Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-on technology enables us, for the first time, to efficiently target endogenous avian genes for transcriptional activation. Taken together, our results represent the first report of genome-wide TSS mapping in birds and the first systematic developmental TSS analysis in any amniote species (birds and mammals). By facilitating promoter-based molecular analysis and genetic manipulation, our work also underscores the value of avian models in unravelling the complex regulatory mechanism of cell lineage specification during amniote development.

Capturing Human Naïve Pluripotency in the Embryo and in the Dish

Although human embryonic stem cells (hESCs) were first derived almost 20 years ago, it was only recently acknowledged that they share closer molecular and functional identity to postimplantation lineage-primed murine epiblast stem cells than to naïve preimplantation inner cell mass-derived mouse ESCs (mESCs). A myriad of transcriptional, epigenetic, biochemical, and metabolic attributes have now been described that distinguish naïve and primed pluripotent states in both rodents and humans. Conventional hESCs and human induced pluripotent stem cells (hiPSCs) appear to lack many of the defining hallmarks of naïve mESCs. These include important features of the naïve ground state murine epiblast, such as an open epigenetic architecture, reduced lineage-primed gene expression, and chimera and germline competence following injection into a recipient blastocyst-stage embryo. Several transgenic and chemical methods were recently reported that appear to revert conventional human PSCs to mESC-like ground states. However, it remains unclear if subtle deviations in global transcription, cell signaling dependencies, and extent of epigenetic/metabolic shifts in these various human naïve-reverted pluripotent states represent true functional differences or alternatively the existence of distinct human pluripotent states along a spectrum. In this study, we review the current understanding and developmental features of various human pluripotency-associated phenotypes and discuss potential biological mechanisms that may support stable maintenance of an authentic epiblast-like ground state of human pluripotency.

Epithelial-mesenchymal transition (EMT): A biological process in the development, stem cell differentiation, and tumorigenesis

The lineage transition between epithelium and mesenchyme is a process known as epithelial-mesenchymal transition (EMT), by which polarized epithelial cells lose their adhesion property and obtain mesenchymal cell phenotypes. EMT is a biological process that is often involved in embryogenesis and diseases, such as cancer invasion and metastasis. The EMT and the reverse process, mesenchymal-epithelial transition (MET), also play important roles in stem cell differentiation and de-differentiation (or reprogramming). In this review, we will discuss current research progress of EMT in embryonic development, cellular differentiation and reprogramming, and cancer progression, all of which are representative models for researches of stem cell biology in normal and in diseases. Understanding of EMT and MET may help to identify specific markers to distinguish normal stem cells from cancer stem cells in future.

Early embryonic development and spatiotemporal localization of mammalian primordial germ cell-associated proteins in the basal rodent Lagostomus maximus

The gene network controlling primordial germ cell (PGC) specification in eutherian mammals has been exhaustively investigated in mice. The egg-cylinder morphology of the mouse embryo is the key event enabling inductive signals from the extra-embryonic ectoderm (ExE) to specify epiblast cells as PGCs early on. We investigated the embryonic development and the spatiotemporal localization of PGC-associated proteins in the basal Hystricognathi rodent Lagostomus maximus. L. maximus develops through a flat-disc epiblast far apart from the ExE. In the primitive streak stage, OCT4-positive cells are detected in the posterior pole of the embryo disc in the mesoderm of the proximal epiblast. In the neural plate stage, a reduced 8 to 12 OCT4-positive cell population transiently expresses FRAGILIS, STELLA and SOX17 in the posterior streak. Soon after translocation to the hindgut, pluripotent OCT4 cells start expressing VASA, and then, STELLA and FRAGILIS are turned on during migration toward the genital ridge. L. maximus shows a spatiotemporal pattern of PGC-associated markers divergent from the early PGC restriction model seen in mice. This pattern conforms to alternative models that are based on a pluripotent population in the embryonic axis, where PGCs are specified later during development.

The genome-wide molecular regulation of mouse gastrulation embryo

The diverse morphologies among vertebrate species stems from the evolution of a basic body plan that is constituted by a spatially organized ensemble of tissue lineage progenitors. At gastrulation, this body plan is established through a coordinated morphogenetic process and the delineation of tissue lineages that are driven by the activity of the genome. To explore the molecular mechanisms, in a comprehensive context, it is imperative to glean an understanding of the region- and population-specific genetic activity underpinning this fundamental developmental process. In this review, we outline the recent progresses and the future directions in studies of genome activity for the regulation of mouse embryogenesis at gastrulation.

Establishment of the Vertebrate Germ Layers

The process of germ layer formation is a universal feature of animal development. The germ layers separate the cells that produce the internal organs and tissues from those that produce the nervous system and outer tissues. Their discovery in the early nineteenth century transformed embryology from a purely descriptive field into a rigorous scientific discipline, in which hypotheses could be tested by observation and experimentation. By systematically addressing the questions of how the germ layers are formed and how they generate overall body plan, scientists have made fundamental contributions to the fields of evolution, cell signaling, morphogenesis, and stem cell biology. At each step, this work was advanced by the development of innovative methods of observing cell behavior in vivo and in culture. Here, we take an historical approach to describe our current understanding of vertebrate germ layer formation as it relates to the long-standing questions of developmental biology. By comparing how germ layers form in distantly related vertebrate species, we find that highly conserved molecular pathways can be adapted to perform the same function in dramatically different embryonic environments.

Preattachment Embryos of Domestic Animals: Insights into Development and Paracrine Secretions

In mammalian species, endometrial receptivity is driven by maternal factors independently of embryo signals. When pregnancy initiates, paracrine secretions of the preattachment embryo are essential both for maternal recognition and endometrium preparation for implantation and for coordinating development of embryonic and extraembryonic tissues of the conceptus. This review mainly focuses on domestic large animal species. We first illustrate the major steps of preattachment embryo development, including elongation in bovine, ovine, porcine, and equine species. We next highlight conceptus secretions that are involved in the communication between extraembryonic and embryonic tissues, as well as between the conceptus and the endometrium. Finally, we introduce experimental data demonstrating the intimate connection between conceptus secretions and endometrial activity and how adverse events perturbing this interplay may affect the progression of implantation that will subsequently impact pregnancy outcome, postnatal health, and expression of production traits in livestock offspring.

Germline development in amniotes: A paradigm shift in primordial germ cell specification

In the field of germline development in amniote vertebrates, primordial germ cell (PGC) specification in birds and reptiles remains controversial. Avians are believed to adopt a predetermination or maternal specification mode of PGC formation, contrary to an inductive mode employed by mammals and, supposedly, reptiles. Here, we revisit and review some key aspects of PGC development that channelled the current subdivision, and challenge the position of birds and reptiles as well as the 'binary' evolutionary model of PGC development in vertebrates. We propose an alternative view on PGC specification where germ plasm plays a role in laying the foundation for the formation of PGC precursors (pPGC), but not necessarily of PGCs. Moreover, inductive mechanisms may be necessary for the transition from pPGCs to PGCs. Within this framework, the implementation of data from birds and reptiles could provide new insights on the evolution of PGC specification in amniotes.

Fibronectin assembly during early embryo development: A versatile communication system between cells and tissues

BACKGROUND

Fibronectin extracellular matrix is essential for embryogenesis. Its assembly is a cell-mediated process where secreted fibronectin dimers bind to integrin receptors on receiving cells, which actively assemble fibronectin into a fibrillar matrix. During development, paracrine communication between tissues is crucial for coordinating morphogenesis, typically being mediated by growth factors and their receptors. Recent reports of situations where fibronectin is produced by one tissue and assembled by another, with implications on tissue morphogenesis, suggest that fibronectin assembly may also be a paracrine communication event in certain contexts.

RESULTS

Here we addressed which tissues express fibronectin (Fn1) while also localizing assembled fibronectin matrix and determining the mRNA expression and/or protein distribution pattern of integrins α5 and αV, α chains of the major fibronectin assembly receptors, during early chick and mouse development. We found evidence supporting a paracrine system in fibronectin matrix assembly in several tissues, including immature mesenchymal tissues, components of central and peripheral nervous system and developing muscle.

CONCLUSIONS

Thus, similarly to growth factor signaling, fibronectin matrix assembly during early development can be both autocrine and paracrine. We therefore propose that it be considered a cell-cell communication event at the same level and significance as growth factor signaling during embryogenesis.

Determinant molecular markers for peri-gastrulating bovine embryo development

Peri-gastrulation defines the time frame between blastocyst formation and implantation that also corresponds in cattle to elongation, pregnancy recognition and uterine secretion. Optimally, this developmental window prepares the conceptus for implantation, placenta formation and fetal development. However, this is a highly sensitive period, as evidenced by the incidence of embryo loss or early post-implantation mortality after AI, embryo transfer or somatic cell nuclear transfer. Elongation markers have often been used within this time frame to assess developmental defects or delays, originating either from the embryo, the uterus or the dam. Comparatively, gastrulation markers have not received great attention, although elongation and gastrulation are linked by reciprocal interactions at the molecular and cellular levels. To make this clearer, this peri-gastrulating period is described herein with a focus on its main developmental landmarks, and the resilience of the landmarks in the face of biotechnologies is questioned.

Morphological and Gene Expression Changes in Cattle Embryos from Hatched Blastocyst to Early Gastrulation Stages after Transfer of In Vitro Produced Embryos

A detailed morphological staging system for cattle embryos at stages following blastocyst hatching and preceding gastrulation is presented here together with spatiotemporal mapping of gene expression for BMP4, BRACHYURY, CERBERUS1 (CER1), CRIPTO, EOMESODERMIN, FURIN and NODAL. Five stages are defined based on distinct developmental events. The first of these is the differentiation of the visceral hypoblast underlying the epiblast, from the parietal hypoblast underlying the mural trophoblast. The second concerns the formation of an asymmetrically positioned, morphologically recognisable region within the visceral hypoblast that is marked by the presence of CER1 and absence of BMP4 expression. We have termed this the anterior visceral hypoblast or AVH. Intra-epiblast cavity formation and the disappearance of the polar trophoblast overlying the epiblast (Rauber’s layer) have been mapped in relation to AVH formation. The third chronological event involves the transition of the epiblast into the embryonic ectoderm with concomitant onset of posterior NODAL, EOMES and BRACHYURY expression. Lastly, gastrulation commences as the posterior medial embryonic ectoderm layer thickens to form the primitive streak and cells ingress between the embryonic ectoderm and hypoblast. At this stage a novel domain of CER1 expression is seen whereas the AVH disappears. Comparison with the mouse reveals that while gene expression patterns at the onset of gastrulation are well conserved, asymmetry establishment, which relies on extraembryonic tissues such as the hypoblast and trophoblast, has diverged in terms of both gene expression and morphology.