Planar cell movements and oriented cell division during early primitive streak formation in the mammalian embryo

Developmental toxicity of 2-bromoacetamide on peri- and early post-implantation mouse embryos in vitro

2-bromoacetamide (BAcAm), a new class of disinfection by-products (DBPs), is widely detected in drinking water across the world. Reports of the high cytogenetic toxicity of BAcAm have aroused public attention concerning its toxic effects on early embryonic development. In this study, we optimized an in vitro culture (IVC) system for peri- and early post-implantation mouse embryos and used this system to determine the developmental toxicity of BAcAm. We found that exposure to BAcAm caused a reduction in egg cylinder formation rate and abnormal lineage differentiation in a dose-dependent manner. Transcriptomic analysis further revealed that BAcAm exposure at early developmental stages altered the abundance of transcripts related to a variety of biological processes including gene expression, metabolism, cell proliferation, cell death and embryonic development, thus indicating its toxic effects on embryonic development. Thus, we developed a robust tool for studying the toxicology of chemicals at the early stages of embryonic development and demonstrated the developmental toxicity of BAcAm in the early embryonic development of mammals.

Defined Microenvironments Trigger In Vitro Gastrulation in Human Pluripotent Stem Cells

Gastrulation is a stage in embryo development where three germ layers arise to dictate the human body plan. In vitro models of gastrulation have been demonstrated by treating pluripotent stem cells with soluble morphogens to trigger differentiation. However, in vivo gastrulation is a multistage process coordinated through feedback between soluble gradients and biophysical forces, with the multipotent epiblast transforming to the primitive streak followed by germ layer segregation. Here, the authors show how constraining pluripotent stem cells to hydrogel islands triggers morphogenesis that mirrors the stages preceding in vivo gastrulation, without the need for exogenous supplements. Within hours of initial seeding, cells display a contractile phenotype at the boundary, which leads to enhanced proliferation, yes-associated protein (YAP) translocation, epithelial to mesenchymal transition, and emergence of SRY-box transcription factor 17 (SOX17)+ T/BRACHYURY+ cells. Molecular profiling and pathway analysis reveals a role for mechanotransduction-coupled wingless-type (WNT) signaling in orchestrating differentiation, which bears similarities to processes observed in whole organism models of development. After two days, the colonies form multilayered aggregates, which can be removed for further growth and differentiation. This approach demonstrates how materials alone can initiate gastrulation, thereby providing in vitro models of development and a tool to support organoid bioengineering efforts.

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

[Figure: see text].

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.

Global pattern of interkinetic nuclear migration in tracheoesophageal epithelia of the mouse embryo: Interorgan and intraorgan regional differences

Interkinetic nuclear migration (INM) is an apicobasal (AB) polarity-based regulatory mechanism of proliferation/differentiation in epithelial stem/progenitor cells. We previously documented INM in the endoderm-derived tracheal/esophageal epithelia at embryonic day (E) 11.5 and suggested that INM is involved in the development of both organs. We here investigated interorgan (trachea vs esophagus) and intraorgan regional (ventral vs dorsal) differences in the INM mode in the tracheal and esophageal epithelia of the mouse embryo. We also analyzed convergent extension (CE) and planar cell movement (PCM) in the epithelia based on cell distribution. The pregnant C57BL/6J mice were intraperitoneally injected with 5-ethynyl-2'-deoxyuridine at E11.5 and E12.5 and were sacrificed 1, 4, 6, 8, and 12 hours later to obtain the embryos. The distribution of labeled cell nuclei along the AB axis was chronologically analyzed in the total, ventral, and dorsal sides of the epithelia. The percentage distribution of the nuclei population was represented by histogram and the chronological change was analyzed statistically using multidimensional scaling. The interorgan comparison of the INM mode during E11.5-E12.0, but not E12.5-E13.0, showed a significant difference. During E11.5-E12.0 the trachea, but not the esophagus, showed a significant difference between ventral and dorsal sides. During E12.5-E13.0 neither organ showed regional differences. CE appeared to occur in both organs during E11.5-E12.0 while PCM was unclear in both organs. These findings suggest a difference between the trachea and esophagus, and a regional difference in the trachea, not in the esophagus, in the INM mode, which may be related with the later differential organogenesis/histogenesis of these organs.

Pitx2 and nodal as conserved early markers of the anterior-posterior axis in the rabbit embryo

Attaining molecular and morphological axial polarity during gastrulation is a fundamental early requirement for normal development of the embryo. In mammals, the first morphological sign of the anterior-posterior axis appears anteriorly in the form of the anterior marginal crescent (or anterior visceral endoderm) while in the avian the first such sign is the Koller's sickle at the posterior pole of the embryonic disc. Despite this inverse mode of axis formation many genes and molecular pathways involved in various steps of this process seem to be evolutionarily conserved amongst amniotes, the nodal gene being a well-known example with its functional involvement prior and during gastrulation. The pitx2 gene, however, is a new candidate described in the chick as an early marker for anterior-posterior polarity and as a regulator of axis formation including twinning. To find out whether pitx2 has retained its inductive and early marker function during the evolution of mammals this study analyses pitx2 and nodal expression at parallel stages during formation of the anterior-posterior polarity in the early rabbit embryo using whole-mount in situ hybridization and serial light-microscopical sections. At a late pre-gastrulation stage a localized reduction of nodal expression presages the position of the anterior pole of the embryonic disc and thus serves as the earliest molecular marker of anterior-posterior polarity known so far. Pitx2 is expressed in a polarized manner in the anterior marginal crescent and in the posterior half of the embryonic disc during further development. In the anterior segment of the posterior pitx2 expression domain, the anterior streak domain (ASD) is defined by nodal expression as a hypothetical progenitor region of the anterior half of the primitive streak. The expression patterns of both genes thus serve as signs of a conserved involvement in early axis formation in amniotes and, possibly, in twinning in mammals as well.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Use of the Coelomic Grafting Technique for Prolonged ex utero Cultivation of Late Preprimitive Streak-Stage Rabbit Embryos

Due to its morphological similarity with the early human embryo, the pregastrulation-stage rabbit may represent an appropriate mammalian model for studying processes involved in early human development. The usability of mammalian embryos for experimental studies depends on the availability of whole embryo culture methods facilitating prolonged ex utero development. While currently used culture methods yield high success rates for embryos from primitive streak stages onward, the success rate of extended cultivation of preprimitive streak-stage mammalian embryos is low for all previously established methods and for all studied species. This limits the usability of preprimitive streak-stage rabbit embryos in experimental embryology. We have tested whether the extraembryonic coelom of 4-day-old chick embryos may be used for prolonged ex utero culture of preprimitive streak-stage rabbit embryos (stage 2, 6.2 days post coitum). We found that, within this environment, stage 2 rabbit blastocysts can be cultured at decreasing success rates (55% after 1 day, 35% after 2 days, 15% after 3 days) up to a maximum of 72 h. Grafted blastocysts can continue development from the onset of gastrulation to early organogenesis and thereby form all structures characterizing age-matched controls (e.g. neural tube, somites, beating heart). Compared to normal controls, successfully cultured embryos developed at a slower rate and finally showed some structural and gross morphological anomalies. The method presented here was originally developed for whole embryo culture of mouse embryos by Gluecksohn-Schoenheimer in 1941. It is a simple and inexpensive method that may represent a useful extension to presently available ex utero culture systems for rabbit embryos.

Tissue morphodynamics shaping the early mouse embryo

Generation of the elongated vertebrate body plan from the initially radially symmetrical embryo requires comprehensive changes to tissue form. These shape changes are generated by specific underlying cell behaviors, coordinated in time and space. Major principles and also specifics are emerging, from studies in many model systems, of the cell and physical biology of how region-specific cell behaviors produce regional tissue morphogenesis, and how these, in turn, are integrated at the level of the embryo. New technical approaches have made it possible more recently, to examine the morphogenesis of the mouse embryo in depth, and to elucidate the underlying cellular mechanisms. This review focuses on recent advances in understanding the cellular basis for the early fundamental events that establish the basic form of the embryo.

Conserved and divergent expression patterns of markers of axial development in eutherian mammals

BACKGROUND

Mouse embryos are cup shaped, but most nonrodent eutherian embryos are disk shaped. Extraembryonic ectoderm (ExEc), which may have essential roles in anterior-posterior (A-P) axis formation in mouse embryos, does not develop in many eutherian embryos. To assess A-P axis formation in eutherians, comparative analyses were made on rabbit, porcine, and Suncus embryos.

RESULTS

All embryos examined expressed Nodal initially throughout epiblast and visceral endoderm; its expression became restricted to the posterior region before gastrulation. Anterior visceral endoderm (AVE) genes were expressed in Otx2-positive visceral endoderm, with Dkk1 expression being most anterior. The mouse pattern of AVE formation was conserved in rabbit embryos, but had diverged in porcine and Suncus embryos. No structure that was molecularly equivalent to Bmp-positive ExEc, existed in rabbit or pig embryos. In Suncus embryos, A-P axis was determined at prehatching stage, and these embryos attached to uterine wall at future posterior side.

CONCLUSIONS

Nodal, but not Bmp, functions in epiblast and visceral endoderm development may be conserved in eutherians. AVE functions may also be conserved, but the pattern of its formation has diverged among eutherians. Roles of BMP and NODAL gradients in AVE formation seem to have been established in a subset of rodents.

Rho kinase activity controls directional cell movements during primitive streak formation in the rabbit embryo

During animal gastrulation, the specification of the embryonic axes is accompanied by epithelio-mesenchymal transition (EMT), the first major change in cell shape after fertilization. EMT takes place in disparate topographical arrangements, such as the circular blastopore of amphibians, the straight primitive streak of birds and mammals or in intermediate gastrulation forms of other amniotes such as reptiles. Planar cell movements are prime candidates to arrange specific modes of gastrulation but there is no consensus view on their role in different vertebrate classes. Here, we test the impact of interfering with Rho kinase-mediated cell movements on gastrulation topography in blastocysts of the rabbit, which has a flat embryonic disc typical for most mammals. Time-lapse video microscopy, electron microscopy, gene expression and morphometric analyses of the effect of inhibiting ROCK activity showed – besides normal specification of the organizer region – a dose-dependent disruption of primitive streak formation; this disruption resulted in circular, arc-shaped or intermediate forms, reminiscent of those found in amphibians, fishes and reptiles. Our results reveal a crucial role of ROCK-controlled directional cell movements during rabbit primitive streak formation and highlight the possibility that temporal and spatial modulation of cell movements were instrumental for the evolution of gastrulation forms.

Summary: ROCK regulates cell motility, polarisation, intercalation and division in gastrulating rabbit embryos, revealing a role for Wnt-PCP signalling in primitive streak formation.

Cadherins and epithelial-to-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a process whereby epithelial cells are transcriptionally reprogrammed, resulting in decreased adhesion and enhanced migration or invasion. EMT occurs during different stages of embryonic development, including gastrulation and neural crest cell delamination, and is induced by a panel of specific transcription factors. These factors comprise, among others, members of the Snail, ZEB, and Twist families, and are all known to modulate cadherin expression and, in particular, E-cadherin. By regulating expression of the cadherin family of proteins, EMT-inducing transcription factors dynamically modulate cell adhesion, allowing many developmental processes to take place. However, during cancer progression EMT can be utilized by cancer cells to contribute to malignancy. This is also reflected at the level of the cadherins, where the cadherin switch between E- and N-cadherins is a classical example seen in cancer-related EMT. In this chapter, we give a detailed overview of the entanglement between EMT-inducing transcription factors and cadherin modulation during embryonic development and cancer progression. We describe how classical cadherins such as E- and N-cadherins are regulated during EMT, as well as cadherin 7, -6B, and -11.

Formation and segmentation of the vertebrate body axis

Body axis elongation and segmentation are major morphogenetic events that take place concomitantly during vertebrate embryonic development. Establishment of the final body plan requires tight coordination between these two key processes. In this review, we detail the cellular and molecular as well as the physical processes underlying body axis formation and patterning. We discuss how formation of the anterior region of the body axis differs from that of the posterior region. We describe the developmental mechanism of segmentation and the regulation of body length and segment numbers. We focus mainly on the chicken embryo as a model system. Its accessibility and relatively flat structure allow high-quality time-lapse imaging experiments, which makes it one of the reference models used to study morphogenesis. Additionally, we illustrate conservation and divergence of specific developmental mechanisms by discussing findings in other major embryonic model systems, such as mice, frogs, and zebrafish.

Rabbit as a reproductive model for human health

The renaissance of the laboratory rabbit as a reproductive model for human health is closely related to the growing evidence of periconceptional metabolic programming and its determining effects on offspring and adult health. Advantages of rabbit reproduction are the exact timing of fertilization and pregnancy stages, high cell numbers and yield in blastocysts, relatively late implantation at a time when gastrulation is already proceeding, detailed morphologic and molecular knowledge on gastrulation stages, and a hemochorial placenta structured similarly to the human placenta. To understand, for example, the mechanisms of periconceptional programming and its effects on metabolic health in adulthood, these advantages help to elucidate even subtle changes in metabolism and development during the pre- and peri-implantation period and during gastrulation in individual embryos. Gastrulation represents a central turning point in ontogenesis in which a limited number of cells program the development of the three germ layers and, hence, the embryo proper. Newly developed transgenic and molecular tools offer promising chances for further scientific progress to be attained with this reproductive model species.

Mouse primitive streak forms in situ by initiation of epithelial to mesenchymal transition without migration of a cell population

BACKGROUND

During gastrulation, an embryo acquires the three primordial germ layers that will give rise to all of the tissues in the body. In amniote embryos, this process occurs via an epithelial to mesenchymal transition (EMT) of epiblast cells at the primitive streak. Although the primitive streak is vital to development, many aspects of how it forms and functions remain poorly understood.

RESULTS

Using live, 4 dimensional imaging and immunohistochemistry, we have shown that the posterior epiblast of the pre-streak murine embryo does not display convergence and extension behavior or large scale migration or rearrangement of a cell population. Instead, the primitive streak develops in situ and elongates by progressive initiation EMT in the posterior epiblast. Loss of basal lamina (BL) is the first step of this EMT, and is strictly correlated with ingression of nascent mesoderm. Once the BL is lost in a given region, cells leave the epiblast by apical constriction in order to enter the primitive streak.

CONCLUSIONS

This is the first description of dynamic cell behavior during primitive streak formation in the mouse embryo, and reveals mechanisms that are quite distinct from those observed in other amniote model systems. Unlike chick and rabbit, the murine primitive streak arises in situ by progressive initiation of EMT beginning in the posterior epiblast, without large-scale movement or convergence and extension of epiblast cells.