Cellular flows initiate left-right patterning prior to laterality gene expression in amniotes

A bilateral body plan is predominant throughout the animal kingdom. Bilaterality of amniote embryos becomes recognizable as midline morphogenesis begins at gastrulation, bisecting an embryonic field into the left and right sides. Soon after, left-right asymmetry also starts. While a series of laterality genes expressed after the left-right compartmentalization has been extensively studied, the laterality patterning prior to and during midline morphogenesis has remained unclear. Here, through a biophysical quantification in a high spatial and temporal resolution, applied to a chick model system, we show that a large-scale bilateral counter-rotating cell flow, termed as 'polonaise movements', display left-right asymmetries in early gastrulation. This cell movement starts prior to the formation of the primitive streak, which is the earliest midline structure, and earlier than expression of laterality genes. The cell flow speed and vorticity unravel the location and timing of the left-right asymmetries. The bilateral cell flow exhibited a Left side asymmetry at the beginning, but a transition towards Right dominance. Mitotic arrest that diminishes primitive streak formation resulted in changes in the bilateral flow pattern, but the Right dominance persisted. Our data indicate that the left-right asymmetry in amniote gastrula becomes detectable prior to the point when the asymmetric regulation of the laterality signals at the node leads to the left-right patterning. More broadly, our results suggest that physical processes can play an unexpected but significant role in influencing left-right laterality during embryonic development.

Stem Cell-Derived Microfluidic Amniotic Sac Embryoid (μPASE)

The microfluidic amniotic sac embryoid (μPASE) is a human pluripotent stem cell (hPSC)-derived multicellular human embryo-like structure with molecular and morphological features resembling the progressive development of the early post-implantation human embryonic sac. The microfluidic device is specifically designed to control the formation of hPSC clusters and expose the clusters to different morphogen environments, allowing the development of μPASEs in a highly controllable, reproducible, and scalable fashion. The μPASE model displays human embryonic developmental landmarks such as lumenogenesis of the epiblast, amniotic cavity formation, and the specification of primordial germ cells and gastrulating cells (or mesendoderm cells). Here, we provide detailed instructions needed to reproduce μPASEs, including the immunofluorescence staining and cell retrieval protocols for characterizing μPASEs obtained under different experimental conditions.

Human Pre-gastrulation Embryo Culture in 3D Condition

The development process of human embryo until blastocyst is well understood during the past 30 years, however, embryogenesis from blastocyst to pre-gastrulation was still remained a "black box". Limited by research materials and culture technologies, the "black box" is still unopened. We recently established an extended three-dimensional (3D) culture system of human blastocysts (Xiang et al., Nature 577(7791):537-542, 2020). The 3D embryo culture system could enable human blastocyst growing up to early primitive streak anlage stage in vitro. Here, we introduce the detail protocol and notes of culturing human 3D embryos.

'Neighbourhood watch' model: embryonic epiblast cells assess positional information in relation to their neighbours

In many developing and regenerating systems, tissue pattern is established through gradients of informative morphogens, but we know little about how cells interpret these. Using experimental manipulation of early chick embryos, including misexpression of an inducer (VG1 or ACTIVIN) and an inhibitor (BMP4), we test two alternative models for their ability to explain how the site of primitive streak formation is positioned relative to the rest of the embryo. In one model, cells read morphogen concentrations cell-autonomously. In the other, cells sense changes in morphogen status relative to their neighbourhood. We find that only the latter model can account for the experimental results, including some counter-intuitive predictions. This mechanism (which we name the ‘neighbourhood watch’ model) illuminates the classic ‘French Flag Problem’ and how positional information is interpreted by a sheet of cells in a large developing system.

Summary: In a large developing system, the chick embryo before gastrulation, cells may interpret gradients of positional signals relative to their neighbours to position the primitive streak, establishing bilateral symmetry.

Differentiation of EpiLCs on Micropatterned Substrates Generated by Micro-Contact Printing

During the last decades, signaling pathways responsible for the initiation of gastrulation in mammalian embryos have been identified. However, the physical rules governing the tissue spatial patterning and the extensive morphogenetic movements occurring during that process are still elusive. Progress on these issues is slowed by the difficulty to record or perturb the patterning events in real time, especially in mammalian embryos that develop in utero. Because they permit easy observation and manipulation, in vitro model systems offer an exciting opportunity to dissect the rules governing the organization of the mammalian gastrula. For instance, it is sufficient to cultivate human embryonic stem cells on micropatterned substrates to reveal their self-organization potential. We present here a method to obtain micropatterned mouse Epiblast Like Cells colonies, providing a convenient way to compare spatial organization of mouse and human pluripotent stem cells and to complement the characterization of mutant embryos in a controlled environment.

Embryo experimentation: is there a case for moving beyond the '14-day rule'

Recent scientific advances have indicated that it may be technically feasible to sustain human embryos in vitro beyond 14 days. Research beyond this stage is currently restricted by a guideline known as the 14-day rule. Since the advances in embryo culturing there have been calls to extend the current limit. Much of the current debate concerning an extension has regarded the 14-day rule as a political compromise and has, therefore, focused on policy concerns rather than assessing the philosophical foundations of the limit. While there are relevant political considerations, I maintain that the success of extension arguments will ultimately depend on the strength of the justifications supporting the current 14-day limit. I argue that the strongest and most prevalent justifications for the 14-day rule-an appeal to individuation and neural development-do not provide adequate support for the limit of 14 days. I instead suggest that an alternative justification based on sentience would constitute a more defensible basis for embryo protection and that a consideration of such grounds appears to support an amendment to the current limit, rather than the retention of it. While these conclusions do not establish conclusively that the current limit should be extended; they do suggest that an extension may be warranted and permissible. As such, this paper offers grounds on which a reassessment of the 14-day rule may be justified.

Caudal duplication syndrome: a literature review and reappraisal of its pathoembryogenesis

INTRODUCTION

The term caudal duplication syndrome (CDS) was first introduced for complex anomalies of the distal caudal end of the trunk. The pathoembryogenesis of CDS is yet unknown, although a few theories have been proposed. We reviewed the previously proposed pathoembryogenetic theories and suggested a new perspective through the common clinical characteristics shown in CDS cases reported in the literature.

METHODS

We conducted a systematic literature search of the online database PubMed from October 1993 to October 2020, using the search term "caudal duplication syndrome", according to the first mention of this entity. A total of 17 articles with 23 patients were reviewed.

RESULTS

The most common manifestations were the duplication of the distal colon, genitourinary organs, and lower spine. Specifically, the duplicated bladders or uteri contacted their counterpart through a septum, and the duplicated bowels ran parallel. More caudal structures, such as the urethra or anus, were formed separately. The duplication seems to be a result of division by an intervening septum or structure in each part. In addition, duplication was not limited to the structures formed from the caudal cell mass (CCM), such as the distal spine and spinal cord, but also included hindgut structures. Moreover, anomalies involving caudal mesenchymal defects were also present. Considering clinical manifestations that are related to all three germ layers and seemingly the overseptation of these germ layers in CDS patients, with supporting data from animal experiments, events such as late-stage errors involving Hensen's node/the primitive streak and the duplication of the CCM with the hyperplasia of the abnormally located central caudal mesenchyme are probable pathoembryogenetic mechanisms for CDS. The "leakage" of the normal growth power of the caudal mesenchyme into the intervening midline space between the two CCMs and consequent weak lateral and caudal pushes of the caudal mesenchyme may explain the association of caudal agenesis or its related anomalies with CDS.

CONCLUSION

We propose a theory that by a molecular interaction, an insult causes late gastrulation phase problems, resulting in ectopic primitive streak formation, and therefore, a duplication of the CCM is induced. Subsequently, the overactivity of abnormally positioned midline mesenchyme between the two CCMs may divide the hindgut derivatives by a central septum. Underactive lateral and caudal pushes of the caudal mesenchyme may lead to an association of features shown in caudal agenesis.

Expression patterns of signalling molecules and transcription factors in the early rabbit embryo and their significance for modelling amniote axis formation

The anterior-posterior axis is a central element of the body plan and, during amniote gastrulation, forms through several transient domains with specific morphogenetic activities. In the chick, experimentally proven activity of signalling molecules and transcription factors lead to the concept of a 'global positioning system' for initial axis formation whereas in the (mammotypical) rabbit embryo, a series of morphological or molecular domains are part of a putative 'three-anchor-point model'. Because circular expression patterns of genes involved in axis formation exist in both amniote groups prior to, and during, gastrulation and may thus be suited to reconcile these models, the expression patterns of selected genes known in the chick, namely the ones coding for the transcription factors eomes and tbx6, the signalling molecule wnt3 and the wnt inhibitor pkdcc, were analysed in the rabbit embryonic disc using in situ hybridisation and placing emphasis on their germ layer location. Peripheral wnt3 and eomes expression in all layers is found initially to be complementary to central pkdcc expression in the hypoblast during early axis formation. Pkdcc then appears - together with a posterior-anterior gradient in wnt3 and eomes domains - in the epiblast posteriorly before the emerging primitive streak is marked by pkdcc and tbx6 at its anterior and posterior extremities, respectively. Conserved circular expression patterns deduced from some of this data may point to shared mechanisms in amniote axis formation while the reshaping of localised gene expression patterns is discussed as part of the 'three-anchor-point model' for establishing the mammalian body plan.

Understanding axial progenitor biology in vivo and in vitro

The generation of the components that make up the embryonic body axis, such as the spinal cord and vertebral column, takes place in an anterior-to-posterior (head-to-tail) direction. This process is driven by the coordinated production of various cell types from a pool of posteriorly-located axial progenitors. Here, we review the key features of this process and the biology of axial progenitors, including neuromesodermal progenitors, the common precursors of the spinal cord and trunk musculature. We discuss recent developments in the in vitro production of axial progenitors and their potential implications in disease modelling and regenerative medicine.

Directed Differentiation of Human Pluripotent Stem Cells for the Generation of High-Order Kidney Organoids

Our understanding in the inherent properties of human pluripotent stem cells (hPSCs) have made possible the development of differentiation procedures to generate three-dimensional tissue-like cultures, so-called organoids. Here we detail a stepwise methodology to generate kidney organoids from hPSCs. This is achieved through direct differentiation of hPSCs in two-dimensional monolayer culture toward the posterior primitive streak fate, followed by induction of intermediate mesoderm-committed cells, which are further aggregated and cultured in three-dimensions to generate kidney organoids containing segmented nephron-like structures in a process that lasts 20 days. We also provide a concise description on how to assess renal commitment during the time course of kidney organoid generation. This includes the use of flow cytometry and immunocytochemistry analyses for the detection of specific renal differentiation markers.

Gastrulation : Current Concepts and Implications for Spinal Malformations

It has been recognised for over a century that the events of gastrulation are fundamental in determining, not only the development of the neuraxis but the organisation of the entire primitive embryo. Until recently our understanding of gastrulation was based on detailed histological analysis in animal models and relatively rare human tissue preparations from aborted fetuses. Such studies resulted in a model of gastrulation that neurosurgeons have subsequently used as a means of trying to explain some of the congenital anomalies of caudal spinal cord and vertebral development that present in paediatric neurosurgical practice. Recent advances in developmental biology, in particular cellular biology and molecular genetics have offered new insights into very early development. Understanding the processes that underlie cellular interactions, gene expression and activation/inhibition of signalling pathways has changed the way embryologists view gastrulation and this has led to a shift in emphasis from the ‘descriptive and morphological’ to the ‘mechanistic and functional’. Unfortunately, thus far it has proved difficult to translate this improved knowledge of normal development, typically derived from non-human models, into an understanding of the mechanisms underlying human malformations such as the spinal dysraphisms and anomalies of caudal development. A paediatric neurosurgeons perspective of current concepts in gastrulation is presented along with a critical review of the current hypotheses of human malformations that have been attributed to disorders of this stage of embryogenesis.

Spatially Organized Differentiation of Mouse Pluripotent Stem Cells on Micropatterned Surfaces

Pluripotent stem cells (PSCs) are the in vitro counterpart of the pluripotent epiblast of the mammalian embryo with the capacity to generate all cell types of the adult organism. During development, the three definitive germ layers are specified and simultaneously spatially organized. In contrast, differentiating PSCs tend to generate cell fates in a spatially disorganized manner. This has limited the in vitro study of specific cell-cell interactions and patterning mechanisms that occur in vivo. Here we describe a protocol to differentiate mouse PSCs in a spatially organized manner on micropatterned surfaces. Micropatterned chips comprise many colonies of uniform size and geometry facilitating a robust quantitative analysis of patterned fate specification. Furthermore, multiple factors may be simultaneously manipulated with temporal accuracy to probe the dynamic interactions regulating these processes. The micropattern system is scalable, providing a valuable tool to generate material for large-scale analysis and biochemical experiments that require substantial amounts of starting material, difficult to obtain from early embryos.

Measurement of junctional tension in epithelial cells at the onset of primitive streak formation in the chick embryo via non-destructive optical manipulation

Directional cell intercalations of epithelial cells during gastrulation has, in several organisms, been shown to be associated with a planar cell polarity in the organisation of the actin-myosin cytoskeleton and is postulated to reflect directional tension that drives oriented cell intercalations. We have characterised and applied a recently introduced non-destructive optical manipulation technique to measure the tension in individual epithelial cell junctions of cells in various locations and orientations in the epiblast of chick embryos in the early stages of primitive streak formation. Junctional tension of mesendoderm precursors in the epiblast is higher in junctions oriented in the direction of intercalation than in junctions oriented perpendicular to the direction of intercalation and higher than in junctions of other cells in the epiblast. The kinetic data fit best with a simple viscoelastic Maxwell model, and we find that junctional tension, and to a lesser extent viscoelastic relaxation time, are dependent on myosin activity.

Movements of chick gastrulation

In birds as in all amniotes, the site of gastrulation is a midline structure, the primitive streak. This appears as cells in the one cell-thick epiblast undergo epithelial-to-mesenchymal transition to ingress and form definitive mesoderm and endoderm. Global movements involving tens of thousands of cells in the embryonic epiblast precede gastrulation. They position the primitive streak precursors from a marginal position (equivalent to the situation in anamniotes) along the future antero-posterior axis (typical for amniotes). These epithelial movements continue in modified form during gastrulation, when they are accompanied by collective movements of different class in the forming mesoderm and endoderm. Here I discuss the nature of these collective cell movements shaping the embryo, their interplay with signaling events controlling fate specification and significance in an evolutionary perspective.

Mapping cell migrations and fates in a gastruloid model to the human primitive streak

Although fate maps of early embryos exist for nearly all model organisms, a fate map of the gastrulating human embryo remains elusive. Here, we use human gastruloids to piece together a rudimentary fate map for the human primitive streak (PS). This is possible because differing levels of BMP, WNT and NODAL lead to self-organization of gastruloids into homogenous subpopulations of endoderm and mesoderm, and comparative analysis of these gastruloids, together with the fate map of the mouse embryo, allows the organization of these subpopulations along an anterior-posterior axis. We also developed a novel cell tracking technique that detected robust fate-dependent cell migrations in our gastruloids comparable with those found in the mouse embryo. Taken together, our fate map and recording of cell migrations provides a first coarse view of what the human PS may resemble in vivo.

Biophysical phenotypes and determinants of anterior vs. posterior primitive streak cells derived from human pluripotent stem cells

Formation of the primitive streak (PS) marks one of the most important developmental milestones in embryonic development. However, our understanding of cellular mechanism(s) underlying cell fate diversification along the anterior-posterior axis of the PS remains incomplete. Furthermore, differences in biophysical phenotypes between anterior and posterior PS cells, which could affect their functions and regulate their fate decisions, remain uncharacterized. Herein, anterior and posterior PS cells were derived using human pluripotent stem cell (hPSC)-based in vitro culture systems. We observed that anterior and posterior PS cells displayed significantly different biophysical phenotypes, including cell morphology, migration, and traction force generation, which was further regulated by different levels of Activin A- and BMP4-mediated developmental signaling. Our data further suggested that intracellular cytoskeletal contraction could mediate anterior and posterior PS differentiation and phenotypic bifurcation through its effect on Activin A- and BMP4-mediated intracellular signaling events. Together, our data provide new information about biophysical phenotypes of anterior and posterior PS cells and reveal an important role of intracellular cytoskeletal contractility in regulating anterior and posterior PS differentiation of hPSCs. STATEMENT OF SIGNIFICANCE: Formation of the primitive streak (PS) marks one of the most important developmental milestones in embryonic development. However, molecular and cellular mechanism(s) underlying functional diversification of embryonic cells along the anterior-posterior axis of the PS remains incompletely understood. This work describes the first study to characterize the biophysical properties of anterior and posterior PS cells derived from human pluripotent stem cells (hPSCs). Importantly, our data showing the important role of cytoskeleton contraction in controlling anterior vs. posterior PS cell phenotypic switch (through its effect on intracellular Smad signaling activities downstream of Activin A and BMP4) should shed new light on biomechanical regulations of the development and anterior-posterior patterning of the PS. Our work will contribute significantly to uncovering new biophysical principles and cellular mechanisms driving cell lineage diversification and patterning during the PS formation.

Scalable Cardiac Differentiation of Pluripotent Stem Cells Using Specific Growth Factors and Small Molecules

The envisioned routine application of human pluripotent stem cell (hPSC)-derived cardiomyocytes (CMs) for therapies and industry-compliant screening approaches will require efficient and highly reproducible processes for the mass production of well-characterized CM batches.On their way toward beating CMs, hPSCs initially undergo an epithelial-to-mesenchymal transition into a primitive-streak (PS)-like population that later gives rise to all endodermal and mesodermal lineages, including cardiovascular progenies (CVPs). CVPs are multipotent and possess the capability to give rise to all major cell types of the heart, including CMs, endothelial cells, cardiac fibroblasts, and smooth muscle cells. This article provides an historical overview and describes the stepwise development of protocols that typically result in the appearance of beating CMs within 7-12 days of hPSC differentiation.We describe the development of directed and closely controlled cardiomyogenic differentiation, which now enables the induction of >90% CM purity without further lineage enrichment. Although secreted lineage specifiers (revealed from developmental biology) were initially used, we outline the advantages of chemical pathway modulators, as defined by more recent screening approaches. Subsequently, we discuss the use of defined culture media for upscaling the production of hPSC-CMs in controlled bioreactors and how this, in principle, unlimited source of human CMs can be used to progress heart regeneration and stimulate the drug discovery pipeline. Graphical Abstract.

Paracrine mechanisms in early differentiation of human pluripotent stem cells: Insights from a mathematical model

With their capability to self-renew and differentiate into derivatives of all three germ layers, human pluripotent stem cells (hPSCs) offer a unique model to study aspects of human development in vitro. Directed differentiation towards mesendodermal lineages is a complex process, involving transition through a primitive streak (PS)-like stage. We have recently shown PS-like patterning from hPSCs into definitive endoderm, cardiac as well as presomitic mesoderm by only modulating the bulk cell density and the concentration of the GSK3 inhibitor CHIR99021, a potent activator of the WNT pathway. The patterning process is modulated by a complex paracrine network, whose identity and mechanistic consequences are poorly understood. To study the underlying dynamics, we here applied mathematical modeling based on ordinary differential equations. We compared time-course data of early hPSC differentiation to increasingly complex model structures with incremental numbers of paracrine factors. Model simulations suggest at least three paracrine factors being required to recapitulate the experimentally observed differentiation kinetics. Feedback mechanisms from both undifferentiated and differentiated cells turned out to be crucial. Evidence from double knock-down experiments and secreted protein enrichment allowed us to hypothesize on the identity of two of the three predicted factors. From a practical perspective, the mathematical model predicts optimal settings for directing lineage-specific differentiation. This opens new avenues for rational stem cell bioprocessing in more advanced culture systems, e.g. in perfusion-fed bioreactors enabling cell therapies.

TCF7L1 suppresses primitive streak gene expression to support human embryonic stem cell pluripotency

Human embryonic stem cells (hESCs) are exquisitely sensitive to WNT ligands, which rapidly cause differentiation. Therefore, hESC self-renewal requires robust mechanisms to keep the cells in a WNT inactive but responsive state. How they achieve this is largely unknown. We explored the role of transcriptional regulators of WNT signaling, the TCF/LEFs. As in mouse ESCs, TCF7L1 is the predominant family member expressed in hESCs. Genome-wide, it binds a gene cohort involved in primitive streak formation at gastrulation, including NODAL, BMP4 and WNT3 Comparing TCF7L1-bound sites with those bound by the WNT signaling effector β-catenin indicates that TCF7L1 acts largely on the WNT signaling pathway. TCF7L1 overlaps less with the pluripotency regulators OCT4 and NANOG than in mouse ESCs. Gain- and loss-of-function studies indicate that TCF7L1 suppresses gene cohorts expressed in the primitive streak. Interestingly, we find that BMP4, another driver of hESC differentiation, downregulates TCF7L1, providing a mechanism of BMP and WNT pathway intersection. Together, our studies indicate that TCF7L1 plays a major role in maintaining hESC pluripotency, which has implications for human development during gastrulation.

From the primitive streak to the somitic mesoderm: labeling the early stages of chick embryos using EGFP transfection

Mesoderm is derived from the primitive streak. The rostral region of the primitive streak forms the somitic mesoderm. We have previously shown the developmental origin of each level of the somitic mesoderm using DiI fluorescence labeling of the primitive streak. We found that the more caudal segments were derived from the primitive streak during the later developmental stages. DiI labeled several pairs of somites and showed the distinct rostral boundary; however, the fluorescence gradually disappeared in the caudal region. This finding can be explained in two ways: the primitive streak at a specific developmental stage is primordial of only a certain number of pairs of somites, or the DiI fluorescent dye was gradually diluted within the primitive streak by cell division. Here, we traced the development of the primitive streak cells using enhanced green fluorescent protein (EGFP) transfection. We confirmed that, the later the EGFP transfection stage, the more caudal the somites labeled. Different from DiI labeling, EGFP transfection performed at any developmental stage labeled the entire somitic mesoderm from the anterior boundary to the tail bud in 4.5-day-old embryos. Furthermore, the secondary neural tube was also labeled, suggesting that not only the somite precursor cells but also the axial stem cells were labeled.

Visceral endoderm and the primitive streak interact to build the fetal-placental interface of the mouse gastrula

Hypoblast/visceral endoderm assists in amniote nutrition, axial positioning and formation of the gut. Here, we provide evidence, currently limited to humans and non-human primates, that hypoblast is a purveyor of extraembryonic mesoderm in the mouse gastrula. Fate mapping a unique segment of axial extraembryonic visceral endoderm associated with the allantoic component of the primitive streak, and referred to as the "AX", revealed that visceral endoderm supplies the placentae with extraembryonic mesoderm. Exfoliation of the AX was dependent upon contact with the primitive streak, which modulated Hedgehog signaling. Resolution of the AX's epithelial-to-mesenchymal transition (EMT) by Hedgehog shaped the allantois into its characteristic projectile and individualized placental arterial vessels. A unique border cell separated the delaminating AX from the yolk sac blood islands which, situated beyond the limit of the streak, were not formed by an EMT. Over time, the AX became the hindgut lip, which contributed extensively to the posterior interface, including both embryonic and extraembryonic tissues. The AX, in turn, imparted antero-posterior (A-P) polarity on the primitive streak and promoted its elongation and differentiation into definitive endoderm. Results of heterotopic grafting supported mutually interactive functions of the AX and primitive streak, showing that together, they self-organized into a complete version of the fetal-placental interface, forming an elongated structure that exhibited A-P polarity and was composed of the allantois, an AX-derived rod-like axial extension reminiscent of the embryonic notochord, the placental arterial vasculature and visceral endoderm/hindgut.

Patterning and gastrulation defects caused by the tw18 lethal are due to loss of Ppp2r1a

The mouse t haplotype, a variant 20 cM genomic region on Chromosome 17, harbors 16 embryonic control genes identified by recessive lethal mutations isolated from wild mouse populations. Due to technical constraints so far only one of these, the t^w5 lethal, has been cloned and molecularly characterized. Here we report the molecular isolation of the t^w18 lethal. Embryos carrying the t^w18 lethal die from major gastrulation defects commencing with primitive streak formation at E6.5. We have used transcriptome and marker gene analyses to describe the molecular etiology of the t^w18 phenotype. We show that both WNT and Nodal signal transduction are impaired in the mutant epiblast, causing embryonic patterning defects and failure of primitive streak and mesoderm formation. By using a candidate gene approach, gene knockout by homologous recombination and genetic rescue, we have identified the gene causing the t^w18 phenotype as Ppp2r1a, encoding the PP2A scaffolding subunit PR65alpha. Our work highlights the importance of phosphatase 2A in embryonic patterning, primitive streak formation, gastrulation, and mesoderm formation downstream of WNT and Nodal signaling.

STELLA collaborates in distinct mesendodermal cell subpopulations at the fetal-placental interface in the mouse gastrula

The allantois-derived umbilical component of the chorio-allantoic placenta shuttles fetal blood to and from the chorion, thereby ensuring fetal-maternal exchange. The progenitor populations that establish and supply the fetal-umbilical interface lie, in part, within the base of the allantois, where the germ line is claimed to segregate from the soma. Results of recent studies in the mouse have reported that STELLA (DPPA-3, PGC7) co-localizes with PRDM1 (BLIMP1), the bimolecular signature of putative primordial germ cells (PGCs) throughout the fetal-placental interface. Thus, if PGCs form extragonadally within the posterior region of the mammal, they cannot be distinguished from the soma on the basis of these proteins. We used immunohistochemistry, immunofluorescence, and confocal microscopy of the mouse gastrula to co-localize STELLA with a variety of gene products, including pluripotency factor OCT-3/4, mesendoderm-associated T and MIXl1, mesendoderm- and endoderm-associated FOXa2 and hematopoietic factor Runx1. While a subpopulation of cells localizing OCT-3/4 was always found independently of STELLA, STELLA always co-localized with OCT-3/4. Despite previous reports that T is involved in specification of the germ line, co-localization of STELLA and T was detected only in a small subset of cells in the base of the allantois. Slightly later in the hindgut lip, STELLA+/(OCT-3/4+) co-localized with FOXa2, as well as with RUNX1, indicative of definitive endoderm and hemangioblasts, respectively. STELLA was never found with MIXl1. On the basis of these and previous results, we conclude that STELLA identifies at least five distinct cell subpopulations within the allantois and hindgut, where they may be involved in mesendodermal differentiation and hematopoiesis at the posterior embryonic-extraembryonic interface. These data provide a new point of departure for understanding STELLA's potential roles in building the fetal-placental connection.

Brachyury drives formation of a distinct vascular branchpoint critical for fetal-placental arterial union in the mouse gastrula

How the fetal-placental arterial connection is made and positioned relative to the embryonic body axis, thereby ensuring efficient and directed blood flow to and from the mother during gestation, is not known. Here we use a combination of genetics, timed pharmacological inhibition in living mouse embryos, and three-dimensional modeling to link two novel architectural features that, at present, have no status in embryological atlases. The allantoic core domain (ACD) is the extraembryonic extension of the primitive streak into the allantois, or pre-umbilical tissue; the vessel of confluence (VOC), situated adjacent to the ACD, is an extraembryonic vessel that marks the site of fetal-placental arterial union. We show that genesis of the fetal-placental connection involves the ACD and VOC in a series of steps, each one dependent upon the last. In the first, Brachyury (T) ensures adequate extension of the primitive streak into the allantois, which in turn designates the allantoic-yolk sac junction. Next, the streak-derived ACD organizes allantoic angioblasts to the axial junction; upon signaling from Fibroblast Growth Factor Receptor-1 (FGFR1), these endothelialize and branch, forming a sprouting VOC that unites the umbilical and omphalomesenteric arteries with the fetal dorsal aortae. Arterial union is followed by the appearance of the medial umbilical roots within the VOC, which in turn designate the correct axial placement of the lateral umbilical roots/common iliac arteries. In addition, we show that the ACD and VOC are conserved across Placentalia, including humans, underscoring their fundamental importance in mammalian biology. We conclude that T is required for correct axial positioning of the VOC via the primitive streak/ACD, while FGFR1, through its role in endothelialization and branching, further patterns it. Together, these genetic, molecular and structural elements safeguard the fetus against adverse outcomes that can result from vascular mispatterning of the fetal-placental arterial connection.

Making a Kidney Organoid Using the Directed Differentiation of Human Pluripotent Stem Cells

An organoid can be defined as a three-dimensional organ-like structure formed from organ-specific progenitor cells. Organ progenitor cells were empirically found to self-organize three-dimensional tissues when they were aggregated and cultivated in vitro. While this nature power of progenitor cells has an amazing potential to recreate artificial organs in vitro, there had been difficulty to apply this technology to human organs due to the inaccessibility to human progenitor cells until human-induced pluripotent stem cell (hiPSC) was invented by Takahashi and Yamanaka in 2007. As embryonic stem cells do, hiPSCs also have pluripotency to give rise to any organs/tissues cell types, including the kidney, via directed differentiation. Here, we provide a detailed protocol for generating kidney organoids using human pluripotent stem cells. The protocol differentiates human pluripotent stem cells into the posterior primitive streak. This is followed by the simultaneous induction of posterior and anterior intermediate mesoderm that are subsequently aggregated and undergo self-organization into the kidney organoid. Such kidney organoids are comprised of all anticipated kidney cell types including nephrons segmented into the glomerulus, proximal tubule, loop of Henle, and distal tubule as well as the collecting duct, endothelial network, and renal interstitium.

Epiblast-specific loss of HCF-1 leads to failure in anterior-posterior axis specification

Mammalian Host-Cell Factor 1 (HCF-1), a transcriptional co-regulator, plays important roles during the cell-division cycle in cell culture, embryogenesis as well as adult tissue. In mice, HCF-1 is encoded by the X-chromosome-linked Hcfc1 gene. Induced Hcfc1(cKO/+) heterozygosity with a conditional knockout (cKO) allele in the epiblast of female embryos leads to a mixture of HCF-1-positive and -deficient cells owing to random X-chromosome inactivation. These embryos survive owing to the replacement of all HCF-1-deficient cells by HCF-1-positive cells during E5.5 to E8.5 of development. In contrast, complete epiblast-specific loss of HCF-1 in male embryos, Hcfc1(epiKO/Y), leads to embryonic lethality. Here, we characterize this lethality. We show that male epiblast-specific loss of Hcfc1 leads to a developmental arrest at E6.5 with a rapid progressive cell-cycle exit and an associated failure of anterior visceral endoderm migration and primitive streak formation. Subsequently, gastrulation does not take place. We note that the pattern of Hcfc1(epiKO/Y) lethality displays many similarities to loss of β-catenin function. These results reveal essential new roles for HCF-1 in early embryonic cell proliferation and development.

Extra-embryonic Wnt3 regulates the establishment of the primitive streak in mice

The establishment of the head to tail axis at early stages of development is a fundamental aspect of vertebrate embryogenesis. In mice, experimental embryology, genetics and expression studies have suggested that the visceral endoderm, an extra-embryonic tissue, plays an important role in anteroposterior axial development. Here we show that absence of Wnt3 in the posterior visceral endoderm leads to delayed formation of the primitive streak and that interplay between anterior and posterior visceral endoderm restricts the position of the primitive streak. Embryos lacking Wnt3 in the visceral endoderm, however, appear normal by E9.5. Our results suggest a model for axial development in which multiple signals are required for anteroposterior axial development in mammals.

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.

Assessing the bipotency of in vitro-derived neuromesodermal progenitors

Retrospective clonal analysis in the mouse has demonstrated that the posterior spinal cord neurectoderm and paraxial mesoderm share a common bipotent progenitor. These neuromesodermal progenitors (NMPs) are the source of new axial structures during embryonic rostrocaudal axis elongation and are marked by the simultaneous co-expression of the transcription factors T(Brachyury) (T(Bra)) and Sox2. NMP-like cells have recently been derived from pluripotent stem cells in vitro following combined stimulation of Wnt and fibroblast growth factor (FGF) signaling. Under these conditions the majority of cultures consist of T(Bra)/Sox2 co-expressing cells after 48-72 hours of differentiation. Although the capacity of these cells to generate posterior neural and paraxial mesoderm derivatives has been demonstrated at the population level, it is unknown whether a single in vitro-derived NMP can give rise to both neural and mesodermal cells. Here we demonstrate that T(Bra) positive cells obtained from mouse epiblast stem cells (EpiSCs) after culture in NMP-inducing conditions can generate both neural and mesodermal clones. This finding suggests that, similar to their embryonic counterparts, in vitro-derived NMPs are truly bipotent and can thus be exploited as a model for studying the molecular basis of developmental cell fate decisions.

Assessing the bipotency of in vitro-derived neuromesodermal progenitors

Retrospective clonal analysis in the mouse has demonstrated that the posterior spinal cord neurectoderm and paraxial mesoderm share a common bipotent progenitor. These neuromesodermal progenitors (NMPs) are the source of new axial structures during embryonic rostrocaudal axis elongation and are marked by the simultaneous co-expression of the transcription factors T(Brachyury) (T(Bra)) and Sox2. NMP-like cells have recently been derived from pluripotent stem cells in vitro following combined stimulation of Wnt and fibroblast growth factor (FGF) signaling. Under these conditions the majority of cultures consist of T(Bra)/Sox2 co-expressing cells after 48-72 hours of differentiation. Although the capacity of these cells to generate posterior neural and paraxial mesoderm derivatives has been demonstrated at the population level, it is unknown whether a single in vitro-derived NMP can give rise to both neural and mesodermal cells. Here we demonstrate that T(Bra) positive cells obtained from mouse epiblast stem cells (EpiSCs) after culture in NMP-inducing conditions can generate both neural and mesodermal clones. This finding suggests that, similar to their embryonic counterparts, in vitro-derived NMPs are truly bipotent and can thus be exploited as a model for studying the molecular basis of developmental cell fate decisions.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

Mixl1 localizes to putative axial stem cell reservoirs and their posterior descendants in the mouse embryo

Mixl1 is thought to play important roles in formation of mesoderm and endoderm. Previously, Mixl1 expression was reported in the posterior primitive streak and allantois, but the precise spatiotemporal whereabouts of Mixl1 protein throughout gastrulation have not been elucidated. To localize Mixl1 protein, immunohistochemistry was carried out at 2-4 h intervals on mouse gastrulae between primitive streak and 16-somite pair (s) stages (~E6.5-9.5). Mixl1 localized to the entire primitive streak early in gastrulation. However, by headfold stages (~E7.75-8.0), Mixl1 diminished within the mid-streak but remained concentrated at either end of the streak, and localized throughout midline posterior visceral endoderm. At the streak's anterior end, Mixl1 was confined to the posterior crown cells of Hensen's node, which contribute to dorsal hindgut endoderm, and the posterior notochord. In the posterior streak, Mixl1 localized to the Allantoic Core Domain (ACD), which is the source of most of the allantois and contributes to the posterior embryonic-extraembryonic interface. In addition, Mix1 co-localized with the early hematopoietic marker, Runx1, in the allantois and visceral yolk sac blood islands. During hindgut invagination (4-16s, ~E8.5-9.5), Mixl1 localized to the hindgut lip, becoming concentrated within the midline anastomosis of the splanchnopleure, which appears to create the ventral component of the hindgut and omphalomesenteric artery. Surrounding the distal hindgut, Mixl1 identified midline cells within tailbud mesoderm. Mixl1 was also found in the posterior notochord. These findings provide a critical systematic, and tissue-level understanding of embryonic Mixl1 localization, and support its role in regulation of crucial posterior axial mesendodermal stem cell niches during embryogenesis.

Regulation of pluripotent cell differentiation by a small molecule, staurosporine

Research in the embryo and in culture has resulted in a sophisticated understanding of many regulators of pluripotent cell differentiation. As a consequence, protocols for the differentiation of pluripotent cells generally rely on a combination of exogenous growth factors and endogenous signalling. Little consideration has been given to manipulating other pathways to achieve pluripotent cell differentiation. The integrity of cell:cell contacts has been shown to influence lineage choice during pluripotent cell differentiation, with disruption of cell:cell contacts promoting mesendoderm formation and maintenance of cell:cell contacts resulting in the preferential formation of neurectoderm. Staurosporine is a broad spectrum inhibitor of serine/threonine kinases which has several effects on cell function, including interruption of cell:cell contacts, decreasing focal contact size, inducing epithelial to mesenchyme transition (EMT) and promoting cell differentiation. The possibility that staurosporine could influence lineage choice from pluripotent cells in culture was investigated. The addition of staurosporine to differentiating mouse EPL resulted in preferential formation of mesendoderm and mesoderm populations, and inhibited the formation of neurectoderm. Addition of staurosporine to human ES cells similarly induced primitive streak marker gene expression. These data demonstrate the ability of staurosporine to influence lineage choice during pluripotent cell differentiation and to mimic the effect of disrupting cell:cell contacts. Staurosporine induced mesendoderm in the absence of known inducers of formation, such as serum and BMP4. Staurosporine induced the expression of mesendoderm markers, including markers that were not induced by BMP4, suggesting it acted as a broad spectrum inducer of molecular gastrulation. This approach has identified a small molecule regulator of lineage choice with potential applications in the commercial development of ES cell derivatives, specifically as a method for forming mesendoderm progenitors or as a culture adjunct to prevent the formation of ectoderm progenitors during pluripotent cell differentiation.

Decoupling of amniote gastrulation and streak formation reveals a morphogenetic unity in vertebrate mesoderm induction

Mesoderm is formed during gastrulation. This process takes place at the blastopore in lower vertebrates and in the primitive streak (streak) in amniotes. The evolutionary relationship between the blastopore and the streak is unresolved, and the morphogenetic and molecular changes leading to this shift in mesoderm formation during early amniote evolution are not well understood. Using the chick model, we present evidence that the streak is dispensable for mesoderm formation in amniotes. An anamniote-like circumblastoporal mode of gastrulation can be induced in chick and three other amniote species. The induction requires cooperative activation of the FGF and Wnt pathways, and the induced mesoderm field retains anamniote-like dorsoventral patterning. We propose that the amniote streak is homologous to the blastopore in lower vertebrates and evolved from the latter in two distinct steps: an initial pan-amniote posterior restriction of mesoderm-inducing signals; and a subsequent lineage-specific morphogenetic modification of the pre-ingression epiblast.