Clonal analysis of cell fate during gastrulation and early neurulation in the mouse

The role of CoQ10 in embryonic development

Coenzyme Q10 (CoQ10) is a natural component widely present in the inner membrane of mitochondria. CoQ10 functions as a key cofactor for adenosine triphosphate (ATP) production and exhibits antioxidant properties in vivo. Mitochondria, as the energy supply center of cells, play a crucial role in germ cell maturation and embryonic development, a complicated process of cell division and cellular differentiation that transforms from a single cell (zygote) to a multicellular organism (fetus). Here, we discuss the effects of CoQ10 on oocyte maturation and the important role of CoQ10 in the growth of various organs during different stages of fetal development. These allowed us to gain a deeper understanding of the pathophysiology of embryonic development and the potential role of CoQ10 in improving fertility quality. They also provide a reference for further developing its application in clinical treatments.

Cre recombinase microinjection for single-cell tracing and localised gene targeting

Tracing and manipulating cells in embryos are essential to understand development. Lipophilic dye microinjections, viral transfection and iontophoresis have been key to map the origin of the progenitor cells that form the different organs in the post-implantation mouse embryo. These techniques require advanced manipulation skills and only iontophoresis, a demanding approach of limited efficiency, has been used for single-cell labelling. Here, we perform lineage tracing and local gene ablation using cell-permeant Cre recombinase (TAT-Cre) microinjection. First, we map the fate of undifferentiated progenitors to the different heart chambers. Then, we achieve single-cell recombination by titrating the dose of TAT-Cre, which allows clonal analysis of nascent mesoderm progenitors. Finally, injecting TAT-Cre to Mycnflox/flox embryos in the primitive heart tube revealed that Mycn plays a cell-autonomous role in maintaining cardiomyocyte proliferation. This tool will help researchers identify the cell progenitors and gene networks involved in organ development, helping to understand the origin of congenital defects.

Nutrient Transporter Gene Expression in the Early Conceptus—Implications From Two Mouse Models of Diabetic Pregnancy

Maternal diabetes in early pregnancy increases the risk for birth defects in the offspring, particularly heart, and neural tube defects. While elevated glucose levels are characteristic for diabetic pregnancies, these are also accompanied by hyperlipidemia, indicating altered nutrient availability. We therefore investigated whether changes in the expression of nutrient transporters at the conception site or in the early post-implantation embryo could account for increased birth defect incidence at later developmental stages. Focusing on glucose and fatty acid transporters, we measured their expression by RT-PCR in the spontaneously diabetic non-obese mouse strain NOD, and in pregnant FVB/N mouse strain dams with Streptozotocin-induced diabetes. Sites of expression in the deciduum, extra-embryonic, and embryonic tissues were determined by RNAscope in situ hybridization. While maternal diabetes had no apparent effects on levels or cellular profiles of expression, we detected striking cell-type specificity of particular nutrient transporters. For examples, Slc2a2/Glut2 expression was restricted to the endodermal cells of the visceral yolk sac, while Slc2a1/Glut1 expression was limited to the mesodermal compartment; Slc27a4/Fatp4 and Slc27a3/Fatp3 also exhibited reciprocally exclusive expression in the endodermal and mesodermal compartments of the yolk sac, respectively. These findings not only highlight the significance of nutrient transporters in the intrauterine environment, but also raise important implications for the etiology of birth defects in diabetic pregnancies, and for strategies aimed at reducing birth defects risk by nutrient supplementation.

Dissecting the Complexity of Early Heart Progenitor Cells

Early heart development depends on the coordinated participation of heterogeneous cell sources. As pioneer work from Adriana C. Gittenberger-de Groot demonstrated, characterizing these distinct cell sources helps us to understand congenital heart defects. Despite decades of research on the segregation of lineages that form the primitive heart tube, we are far from understanding its full complexity. Currently, single-cell approaches are providing an unprecedented level of detail on cellular heterogeneity, offering new opportunities to decipher its functional role. In this review, we will focus on three key aspects of early heart morphogenesis: First, the segregation of myocardial and endocardial lineages, which yields an early lineage diversification in cardiac development; second, the signaling cues driving differentiation in these progenitor cells; and third, the transcriptional heterogeneity of cardiomyocyte progenitors of the primitive heart tube. Finally, we discuss how single-cell transcriptomics and epigenomics, together with live imaging and functional analyses, will likely transform the way we delve into the complexity of cardiac development and its links with congenital defects.

A single-embryo, single-cell time-resolved model for mouse gastrulation

Mouse embryonic development is a canonical model system for studying mammalian cell fate acquisition. Recently, single-cell atlases comprehensively charted embryonic transcriptional landscapes, yet inference of the coordinated dynamics of cells over such atlases remains challenging. Here, we introduce a temporal model for mouse gastrulation, consisting of data from 153 individually sampled embryos spanning 36 h of molecular diversification. Using algorithms and precise timing, we infer differentiation flows and lineage specification dynamics over the embryonic transcriptional manifold. Rapid transcriptional bifurcations characterize the commitment of early specialized node and blood cells. However, for most lineages, we observe combinatorial multi-furcation dynamics rather than hierarchical transcriptional transitions. In the mesoderm, dozens of transcription factors combinatorially regulate multifurcations, as we exemplify using time-matched chimeric embryos of Foxc1/Foxc2 mutants. Our study rejects the notion of differentiation being governed by a series of binary choices, providing an alternative quantitative model for cell fate acquisition.

The Progresses of Spermatogonial Stem Cells Sorting Using Fluorescence-Activated Cell Sorting

In recent years, the research on stem cells has been more and more in-depth, and many achievements have been made in application. However, due to the small number of spermatogonial stem cells (SSCs) and deficiency of efficient markers, it is difficult to obtain very pure SSCs, which results in the research on them being hindered. In fact, many methods have been developed to isolate and purify SSCs, but these methods have their shortcomings. Fluorescence-activated cell sorting (FACS), as a method to enrich SSCs with the help of specific surface markers, has the characteristics of high efficiency and accuracy in enrichment of SSCs, thus it is widely accepted as an effective method for purification of SSCs. This review summarizes the recent studies on the application of FACS in SSCs, and introduces some commonly used markers of effective SSCs sorting, aiming to further optimize the FACS sorting method for SSCs, so as to promote the research of germline stem cells and provide new ideas for the research of reproductive biology.

Mouse gastrulation: Coordination of tissue patterning, specification and diversification of cell fate

During mouse embryonic development a mass of pluripotent epiblast tissue is transformed during gastrulation to generate the three definitive germ layers: endoderm, mesoderm, and ectoderm. During gastrulation, a spatiotemporally controlled sequence of events results in the generation of organ progenitors and positions them in a stereotypical fashion throughout the embryo. Key to the correct specification and differentiation of these cell fates is the establishment of an axial coordinate system along with the integration of multiple signals by individual epiblast cells to produce distinct outcomes. These signaling domains evolve as the anterior-posterior axis is established and the embryo grows in size. Gastrulation is initiated at the posteriorly positioned primitive streak, from which nascent mesoderm and endoderm progenitors ingress and begin to diversify. Advances in technology have facilitated the elaboration of landmark findings that originally described the epiblast fate map and signaling pathways required to execute those fates. Here we will discuss the current state of the field and reflect on how our understanding has shifted in recent years.

Follow Me! A Tale of Avian Heart Development with Comparisons to Mammal Heart Development

Avian embryos have been used for centuries to study development due to the ease of access. Because the embryos are sheltered inside the eggshell, a small window in the shell is ideal for visualizing the embryos and performing different interventions. The window can then be covered, and the embryo returned to the incubator for the desired amount of time, and observed during further development. Up to about 4 days of chicken development (out of 21 days of incubation), when the egg is opened the embryo is on top of the yolk, and its heart is on top of its body. This allows easy imaging of heart formation and heart development using non-invasive techniques, including regular optical microscopy. After day 4, the embryo starts sinking into the yolk, but still imaging technologies, such as ultrasound, can tomographically image the embryo and its heart in vivo. Importantly, because like the human heart the avian heart develops into a four-chambered heart with valves, heart malformations and pathologies that human babies suffer can be replicated in avian embryos, allowing a unique developmental window into human congenital heart disease. Here, we review avian heart formation and provide comparisons to the mammalian heart.

Convergent extension in mammalian morphogenesis

Convergent extension is a fundamental morphogenetic process that underlies not only the generation of the elongated vertebrate body plan from the initially radially symmetrical embryo, but also the specific shape changes characteristic of many individual tissues. These tissue shape changes are the result of specific cell behaviors, coordinated in time and space, and affected by the physical properties of the tissue. While mediolateral cell intercalation is the classic cellular mechanism for producing tissue convergence and extension, other cell behaviors can also provide similar tissue-scale distortions or can modulate the effects of mediolateral cell intercalation to sculpt a specific shape. Regulation of regional tissue morphogenesis through planar polarization of the variety of underlying cell behaviors is well-recognized, but as yet is not well understood at the molecular level. Here, we review recent advances in understanding the cellular basis for convergence and extension and its regulation.

Lineage tracing of axial progenitors using Nkx1-2CreERT2 mice defines their trunk and tail contributions

The vertebrate body forms by continuous generation of new tissue from progenitors at the posterior end of the embryo. The study of these axial progenitors has proved to be challenging in vivo largely because of the lack of unique molecular markers to identify them. Here, we elucidate the expression pattern of the transcription factor Nkx1-2 in the mouse embryo and show that it identifies axial progenitors throughout body axis elongation, including neuromesodermal progenitors and early neural and mesodermal progenitors. We create a tamoxifen-inducible Nkx1-2CreER^T2 transgenic mouse and exploit the conditional nature of this line to uncover the lineage contributions of Nkx1-2-expressing cells at specific stages. We show that early Nkx1-2-expressing epiblast cells contribute to all three germ layers, mostly neuroectoderm and mesoderm, excluding notochord. Our data are consistent with the presence of some self-renewing axial progenitors that continue to generate neural and mesoderm tissues from the tail bud. This study identifies Nkx1-2-expressing cells as the source of most trunk and tail tissues in the mouse and provides a useful tool to genetically label and manipulate axial progenitors in vivo.

Summary: Changing lineage contributions of axial progenitors to the developing mouse embryo are revealed using a tamoxifen-inducible Cre line under the control of the endogenous Nkx1-2 promoter.

On the nature and function of organizers

Organizers, which comprise groups of cells with the ability to instruct adjacent cells into specific states, represent a key principle in developmental biology. The concept was first introduced by Spemann and Mangold, who showed that there is a cellular population in the newt embryo that elicits the development of a secondary axis from adjacent cells. Similar experiments in chicken and rabbit embryos subsequently revealed groups of cells with similar instructive potential. In birds and mammals, organizer activity is often associated with a structure known as the node, which has thus been considered a functional homologue of Spemann's organizer. Here, we take an in-depth look at the structure and function of organizers across species and note that, whereas the amphibian organizer is a contingent collection of elements, each performing a specific function, the elements of organizers in other species are dispersed in time and space. This observation urges us to reconsider the universality and meaning of the organizer concept.

Summary: This Review re-evaluates the notion of Spemann's organizer as identified in amphibians, highlighting the spatiotemporal dispersion of equivalent elements in mouse and the key influence of responsiveness to organizer signals.

Identification and fate mapping of the pancreatic mesenchyme

The murine pancreas buds from the ventral embryonic endoderm at approximately 8.75 dpc and a second pancreas bud emerges from the dorsal endoderm by 9.0 dpc. Although it is clear that secreted signals from adjacent mesoderm-derived sources are required for both the appropriate emergence and further refinement of the pancreatic endoderm, neither the exact signals nor the requisite tissue sources have been defined in mammalian systems. Herein we use DiI fate mapping of cultured murine embryos to identify the embryonic sources of both the early inductive and later condensed pancreatic mesenchyme. Despite being capable of supporting pancreas induction from dorsal endoderm in co-culture experiments, we find that in the context of the developing embryo, the dorsal aortae as well as the paraxial, intermediate, and lateral mesoderm derivatives only transiently associate with the dorsal pancreas bud, producing descendants that are decidedly anterior to the pancreas bud. Unlike these other mesoderm derivatives, the axial (notochord) descendants maintain association with the dorsal pre-pancreatic endoderm and early pancreas bud. This fate mapping data points to the notochord as the likely inductive source in vivo while also revealing dynamic morphogenetic movements displayed by individual mesodermal subtypes. Because none of the mesoderm examined above produced the pancreatic mesenchyme that condenses around the induced bud to support exocrine and endocrine differentiation, we also sought to identify the mesodermal origins of this mesenchyme. We identify a portion of the coelomic mesoderm that contributes to the condensed pancreatic mesenchyme. In conclusion, we identify a portion of the notochord as a likely source of the signals required to induce and maintain the early dorsal pancreas bud, demonstrate that the coelomic mesothelium contributes to the dorsal and ventral pancreatic mesenchyme, and provide insight into the dynamic morphological rearrangements of mesoderm-derived tissues during early organogenesis stages of mammalian development.

Cycling through developmental decisions: how cell cycle dynamics control pluripotency, differentiation and reprogramming

A strong connection exists between the cell cycle and mechanisms required for executing cell fate decisions in a wide-range of developmental contexts. Terminal differentiation is often associated with cell cycle exit, whereas cell fate switches are frequently linked to cell cycle transitions in dividing cells. These phenomena have been investigated in the context of reprogramming, differentiation and trans-differentiation but the underpinning molecular mechanisms remain unclear. Most progress to address the connection between cell fate and the cell cycle has been made in pluripotent stem cells, in which the transition through mitosis and G1 phase is crucial for establishing a window of opportunity for pluripotency exit and the initiation of differentiation. This Review will summarize recent developments in this area and place them in a broader context that has implications for a wide range of developmental scenarios.

Neuromesodermal progenitors and the making of the spinal cord

Neuromesodermal progenitors (NMps) contribute to both the elongating spinal cord and the adjacent paraxial mesoderm. It has been assumed that these cells arise as a result of patterning of the anterior neural plate. However, as the molecular mechanisms that specify NMps in vivo are uncovered, and as protocols for generating these bipotent cells from mouse and human pluripotent stem cells in vitro are established, the emerging data suggest that this view needs to be revised. Here, we review the characteristics, regulation, in vitro derivation and in vivo induction of NMps. We propose that these cells arise within primitive streak-associated epiblast via a mechanism that is separable from that which establishes neural fate in the anterior epiblast. We thus argue for the existence of two distinct routes for making central nervous system progenitors.

Determination phase at transition of gonocytes to spermatogonial stem cells improves establishment efficiency of spermatogonial stem cells in domestic cats

The development of germ cells has not been entirely documented in the cat especially the transition phase of the gonocyte to the spermatogonial stem cell (G/SSC). The aims of study were to examine testicular development and to identify the G/SSC transition in order to isolate and culture SSCs in vitro. Testes were divided into 3 groups according to donor age (I, < 4 months; II, 4–6 months; and III, > 6 months). In Exp. 1, we studied testicular development by histology, transmission electron microscopy and immunohistochemistry. In Exp. 2, we determined the expression of GFRα-1, DDX-4 and c-kit and performed flow cytometry. The SSCs isolated from groups II and III were characterized by RT-PCR and TEM (Exp. 3). Chronological changes in the G/SSC transition were demonstrated. The size, morphology and ultrastructure of SSCs were distinguishable from those of gonocytes. The results demonstrated that group II contained the highest numbers of SSCs per seminiferous cord/tubule (17.66 ± 2.20%) and GFRα-1⁺ cells (14.89 ± 5.66%) compared with the other groups. The findings coincided with an increased efficiency of SSC derivation in group II compared with group III (74.33 ± 2.64% vs. 23.33 ± 2.23%). The colonies expressed mRNA for GFRA1, ZBTB16, RET and POU5F1. Our study found that the G/SSC transition occurs at 4–6 months of age. This period is useful for isolation and improves the establishment efficiency of cat SSCs in vitro.

Wnt/ß-catenin signalling and the dynamics of fate decisions in early mouse embryos and embryonic stem (ES) cells

Wnt/ß-catenin signalling is a widespread cell signalling pathway with multiple roles during vertebrate development. In mouse embryonic stem (mES) cells, there is a dual role for ß-catenin: it promotes differentiation when activated as part of the Wnt/ß-catenin signalling pathway, and promotes stable pluripotency independently of signalling. Although mES cells resemble the preimplantation epiblast progenitors, the first requirement for Wnt/ß-catenin signalling during mouse development has been reported at implantation [1,2]. The relationship between ß-catenin and pluripotency and that of mES cells with epiblast progenitors suggests that ß-catenin might have a functional role during preimplantation development. Here we summarize the expression and function of Wnt/ß-catenin signalling elements during the early stages of mouse development and consider the reasons why the requirement in ES cells do not reflect the embryo.

The Dynamics of Morphogenesis in the Early Mouse Embryo

SUMMARYOver the past two decades, our understanding of mouse development from implantation to gastrulation has grown exponentially with an upsurge of genetic, molecular, cellular, and morphogenetic information. New discoveries have exalted the role of extraembryonic tissues in orchestrating embryonic patterning and axial specification. At the same time, the identification of unexpected morphogenetic processes occurring during mouse gastrulation has challenged established dogmas and brought new insights into the mechanisms driving germ layer formation. In this article, we summarize the key findings that have reinvigorated the contemporary view of early postimplantation mammalian development.

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

The developmental dismantling of pluripotency is reversed by ectopic Oct4 expression

The transcription factors Nanog and Oct4 regulate pluripotency in the pre-implantation epiblast and in derivative embryonic stem cells. During post-implantation development, the precise timing and mechanism of the loss of pluripotency is unknown. Here, we show that in the mouse, pluripotency is extinguished at the onset of somitogenesis, coincident with reduced expression and chromatin accessibility of Oct4 and Nanog regulatory regions. Prior to somitogenesis expression of both Nanog and Oct4 is regionalized. We show that pluripotency tracks the in vivo level of Oct4 and not Nanog by assessing the ability to reactivate or maintain Nanog expression in cell culture. Enforced Oct4 expression in somitogenesis-stage tissue provokes rapid reopening of Oct4 and Nanog chromatin, Nanog re-expression and resuscitates moribund pluripotency. Our data suggest that decreasing Oct4 expression is converted to a sudden drop in competence to maintain pluripotency gene regulatory network activity that is subsequently stabilized by epigenetic locks.

A late requirement for Wnt and FGF signaling during activin-induced formation of foregut endoderm from mouse embryonic stem cells

Here we examine how BMP, Wnt, and FGF signaling modulate activin-induced mesendodermal differentiation of mouse ES cells grown under defined conditions in adherent monoculture. We monitor ES cells containing reporter genes for markers of primitive streak (PS) and its progeny and extend previous findings on the ability of increasing concentrations of activin to progressively induce more ES cell progeny to anterior PS and endodermal fates. We find that the number of Sox17- and Gsc-expressing cells increases with increasing activin concentration while the highest number of T-expressing cells is found at the lowest activin concentration. The expression of Gsc and other anterior markers induced by activin is prevented by treatment with BMP4, which induces T expression and subsequent mesodermal development. We show that canonical Wnt signaling is required only during late stages of activin-induced development of Sox17-expressing endodermal cells. Furthermore, Dkk1 treatment is less effective in reducing development of Sox17(+) endodermal cells in adherent culture than in aggregate culture and appears to inhibit nodal-mediated induction of Sox17(+) cells more effectively than activin-mediated induction. Notably, activin induction of Gsc-GFP(+) cells appears refractory to inhibition of canonical Wnt signaling but shows a dependence on early as well as late FGF signaling. Additionally, we find a late dependence on FGF signaling during induction of Sox17(+) cells by activin while BMP4-induced T expression requires FGF signaling in adherent but not aggregate culture. Lastly, we demonstrate that activin-induced definitive endoderm derived from mouse ES cells can incorporate into the developing foregut endoderm in vivo and adopt a mostly anterior foregut character after further culture in vitro.

Making a commitment: cell lineage allocation and axis patterning in the early mouse embryo

Genetic studies have identified the key signalling pathways and developmentally regulated transcription factors that govern cell lineage allocation and axis patterning in the early mammalian embryo. Recent advances have uncovered details of the molecular circuits that tightly control cell growth and differentiation in the mammalian embryo from the blastocyst stage, through the establishment of initial anterior-posterior polarity, to gastrulation, when the germ cells are set aside and the three primary germ layers are specified. Relevant studies in lower vertebrates indicate the conservation and divergence of regulatory mechanisms for cell lineage allocation and axis patterning.

Rest-mediated regulation of extracellular matrix is crucial for neural development

Neural development from blastocysts is strictly controlled by intricate transcriptional programmes that initiate the down-regulation of pluripotent genes, Oct4, Nanog and Rex1 in blastocysts followed by up-regulation of lineage-specific genes as neural development proceeds. Here, we demonstrate that the expression pattern of the transcription factor Rest mirrors those of pluripotent genes during neural development from embryonic stem (ES) cells and an early abrogation of Rest in ES cells using a combination of gene targeting and RNAi approaches causes defects in this process. Specifically, Rest ablation does not alter ES cell pluripotency, but impedes the production of Nestin⁺ neural stem cells, neural progenitor cells and neurons, and results in defective adhesion, decrease in cell proliferation, increase in cell death and neuronal phenotypic defects typified by a reduction in migration and neurite elaboration. We also show that these Rest-null phenotypes are due to the dysregulation of its direct or indirect target genes, Lama1, Lamb1, Lamc1 and Lama2 and that these aberrant phenotypes can be rescued by laminins.

Embryonic stem cells contribute to mouse chimeras in the absence of detectable cell fusion

Embryonic stem (ES) cells are capable of differentiating into all embryonic and adult cell types following mouse chimera production. Although injection of diploid ES cells into tetraploid blastocysts suggests that tetraploid cells have a selective disadvantage in the developing embryo, tetraploid hybrid cells, formed by cell fusion between ES cells and somatic cells, have been reported to contribute to mouse chimeras. In addition, other examples of apparent stem cell plasticity have recently been shown to be the result of cell fusion. Here we investigate whether ES cells contribute to mouse chimeras through a cell fusion mechanism. Fluorescence in situ hybridization (FISH) analysis for X and Y chromosomes was performed on dissociated tissues from embryonic, neonatal, and adult wild-type, and chimeric mice to follow the ploidy distributions of cells from various tissues. FISH analysis showed that the ploidy distributions in dissociated tissues, notably the tetraploid cell number, did not differ between chimeric and wild-type tissues. To address the possibility that early cell fusion events are hidden by subsequent reductive divisions or other changes in cell ploidy, we injected Z/EG (lacZ/EGFP) ES cells into ACTB-cre blastocysts. Recombination can only occur as the result of cell fusion, and the recombined allele should persist through any subsequent changes in cell ploidy. We did not detect evidence of fusion in embryonic chimeras either by direct fluorescence microscopy for GFP or by PCR amplification of the recombined Z/EG locus on genomic DNA from ACTB-cre::Z/EG chimeric embryos. Our results argue strongly against cell fusion as a mechanism by which ES cells contribute to chimeras.

A systematic screen for genes expressed in definitive endoderm by Serial Analysis of Gene Expression (SAGE)

Background

The embryonic definitive endoderm (DE) gives rise to organs of the gastrointestinal and respiratory tract including the liver, pancreas and epithelia of the lung and colon. Understanding how DE progenitor cells generate these tissues is critical to understanding the cause of visceral organ disorders and cancers, and will ultimately lead to novel therapies including tissue and organ regeneration. However, investigation into the molecular mechanisms of DE differentiation has been hindered by the lack of early DE-specific markers.

Results

We describe the identification of novel as well as known genes that are expressed in DE using Serial Analysis of Gene Expression (SAGE). We generated and analyzed three longSAGE libraries from early DE of murine embryos: early whole definitive endoderm (0–6 somite stage), foregut (8–12 somite stage), and hindgut (8–12 somite stage). A list of candidate genes enriched for expression in endoderm was compiled through comparisons within these three endoderm libraries and against 133 mouse longSAGE libraries generated by the Mouse Atlas of Gene Expression Project encompassing multiple embryonic tissues and stages. Using whole mount in situ hybridization, we confirmed that 22/32 (69%) genes showed previously uncharacterized expression in the DE. Importantly, two genes identified, Pyy and 5730521E12Rik, showed exclusive DE expression at early stages of endoderm patterning.

Conclusion

The high efficiency of this endoderm screen indicates that our approach can be successfully used to analyze and validate the vast amount of data obtained by the Mouse Atlas of Gene Expression Project. Importantly, these novel early endoderm-expressing genes will be valuable for further investigation into the molecular mechanisms that regulate endoderm development.

Wnt and TGF-beta signaling are required for the induction of an in vitro model of primitive streak formation using embryonic stem cells

The establishment of the primitive streak and its derivative germ layers, mesoderm and endoderm, are prerequisite steps in the formation of many tissues. To model these developmental stages in vitro, an ES cell line was established that expresses CD4 from the foxa2 locus in addition to GFP from the brachyury locus. A GFP-Bry(+) population expressing variable levels of CD4-Foxa2 developed upon differentiation of this ES cell line. Analysis of gene-expression patterns and developmental potential revealed that the CD4-Foxa2(hi)GFP-Bry(+) population displays characteristics of the anterior primitive streak, whereas the CD4-Foxa2(lo)GFP-Bry(+) cells resemble the posterior streak. Using this model, we were able to demonstrate that Wnt and TGF-beta/nodal/activin signaling simultaneously were required for the generation of the CD4-Foxa2(+)GFP-Bry(+) population. Wnt or low levels of activin-induced a posterior primitive streak population, whereas high levels of activin resulted in an anterior streak fate. Finally, sustained activin signaling was found to stimulate endoderm commitment from the CD4-Foxa2(+)GFP-Bry(+) ES cell population. These findings demonstrate that the early developmental events involved in germ-layer induction in the embryo are recapitulated in the ES cell model and uncover insights into the signaling pathways involved in the establishment of mesoderm and endoderm.

Growth of the normal skull vault and its alteration in craniosynostosis: insights from human genetics and experimental studies

The mammalian skull vault is constructed principally from five bones: the paired frontals and parietals, and the unpaired interparietal. These bones abut at sutures, where most growth of the skull vault takes place. Sutural growth involves maintenance of a population of proliferating osteoprogenitor cells which differentiate into bone matrix-secreting osteoblasts. Sustained function of the sutures as growth centres is essential for continuous expansion of the skull vault to accommodate the growing brain. Craniosynostosis, the premature fusion of the cranial sutures, occurs in 1 in 2500 children and often presents challenging clinical problems. Until a dozen years ago, little was known about the causes of craniosynostosis but the discovery of mutations in the MSX2, FGFR1, FGFR2, FGFR3, TWIST1 and EFNB1 genes in both syndromic and non-syndromic cases has led to considerable insights into the aetiology, classification and developmental pathology of these disorders. Investigations of the biological roles of these genes in cranial development and growth have been carried out in normal and mutant mice, elucidating their individual and interdependent roles in normal sutures and in sutures undergoing synostosis. Mouse studies have also revealed a significant correspondence between the neural crest-mesoderm boundary in the early embryonic head and the position of cranial sutures, suggesting roles for tissue interaction in suture formation, including initiation of the signalling system that characterizes the functionally active suture.

The mouse homeobox gene Not is required for caudal notochord development and affected by the truncate mutation

The floating head (flh) gene in zebrafish encodes a homeodomain protein, which is essential for notochord formation along the entire body axis. flh orthologs, termed Not genes, have been isolated from chick and Xenopus, but no mammalian ortholog has yet been identified. Truncate (tc) is an autosomal recessive mutation in mouse that specifically disrupts the development of the caudal notochord. Here, we demonstrate that truncate arose by a mutation in the mouse Not gene. The truncate allele (Nottc) contains a point mutation in the homeobox of Not that changes a conserved Phenylalanine residue in helix 1 to a Cysteine (F20C), and significantly destabilizes the homeodomain. Reversion of F20C in one allele of homozygous tc embryonic stem (ES) cells is sufficient to restore normal notochord formation in completely ES cell-derived embryos. We have generated a targeted mutation of Not by replacing most of the Not coding sequence, including the homeobox with the eGFP gene. The phenotype of NoteGFP/eGFP, NoteGFP/tc, and Nottc/tc embryos is very similar but slightly more severe in NoteGFP/eGFP than in Nottc/tc embryos. This confirms allelism of truncate and Not, and indicates that tc is not a complete null allele. Not expression is abolished in Foxa2 and T mutant embryos, suggesting that Not acts downstream of both genes during notochord development. This is in contrast to zebrafish embryos, in which flh interacts with ntl (zebrafish T) in a regulatory loop and is essential for development of the entire notochord, and suggests that different genetic control circuits act in different vertebrate species during notochord formation.

Neuroectodermal specification and regionalization of the Spemann organizer in Xenopus

During gastrulation in Xenopus convergence and extension movements, mediated by mediolateral intercalations, are the driving force for early neural plate morphogenesis. Here we show that the winged helix transcriptional regulator, Xfd-12' is dynamically expressed in medial neural plate precursors that undergo convergence and extension movements. These medial neuraxial progenitors are specified in and beyond the Spemann organizer prior to specification of the basal anlage of the neural plate. The initiation of Xfd-12' expression coincides with the induction of mesendoderm by Nodal-related growth factors at the late blastula stage. Comparative expression analysis suggests that cellular rearrangements at the pre-gastrulation stage account for regionalization of the Spemann organizer into head and trunk organizer compartments, the latter in which medial neural plate progenitors reside. While the maintenance of Xfd-12' expression in the dorsal non-involuting marginal zone requires FGF signalling, its subsequent positioning along the medial aspect of the neuraxis depends on signalling by Wnt and Nodal-related family members. Based on these findings we propose that XFD-12' is a trunk organizer component that might control convergence and extension movements of medial neural plate precursors during gastrulation.

Regionalization of the prosencephalic neural plate

Recent embryological studies are beginning to establish that the underlying organization of the forebrain may be reduced to relatively simple elements that are common to all vertebrates. We begin this chapter by reviewing studies that describe the similarities in prospective fate and molecular organization of the developing neural plate in fish, frogs, chickens, and mice. The chapter next addresses mechanisms that regulate regional specification in the anterior central nervous system. There is now evidence that the axial mesendoderm anterior to the notochord (the prechordal plate) has a central role in induction of the floor and basal plate primordia (hypothalamus) of the forebrain. Patterning of the anterolateral neural plate (telencephalon) may be regulated by FGF8 produced in the anterior neural ridge. Thus, the synthesis of information from fate mapping and experimental embryological and genetic studies is illuminating the mechanisms that generate the different components of the forebrain.

The type I activin receptor ActRIB is required for egg cylinder organization and gastrulation in the mouse

ActRIB is a type I transmembrane serine/threonine kinase receptor that has been shown to form heteromeric complexes with the type II activin receptors to mediate activin signal. To investigate the function of ActRIB in mammalian development, we generated ActRIB-deficient ES cell lines and mice by gene targeting. Analysis of the ActRIB-/- embryos showed that the epiblast and the extraembryonic ectoderm were disorganized, resulting in disruption and developmental arrest of the egg cylinder before gastrulation. To assess the function of ActRIB in mesoderm formation and gastrulation, chimera analysis was conducted. We found that ActRIB-/- ES cells injected into wild-type blastocysts were able to contribute to the mesoderm in chimeric embryos, suggesting that ActRIB is not required for mesoderm formation. Primitive streak formation, however, was impaired in chimeras when ActRIB-/- cells contributed highly to the epiblast. Further, chimeras generated by injection of wild-type ES cells into ActRIB-/- blastocysts formed relatively normal extraembryonic tissues, but the embryo proper developed poorly probably resulting from severe gastrulation defect. These results provide genetic evidence that ActRIB functions in both epiblast and extraembryonic cells to mediate signals that are required for egg cylinder organization and gastrulation.

Mouse gastrulation: the formation of a mammalian body plan

The process of gastrulation is a pivotal step in the formation of the vertebrate body plan. The primary function of gastrulation is the correct placement of precursor tissues for subsequent morphogenesis. There is now mounting evidence that the body plan is established through inductive interactions between germ layer tissues and by the global patterning activity emanating from embryonic organizers. An increasing number of mouse mutants have been described that have gastrulation defects, providing important insights into the molecular mechanisms that regulate this complex process. In this review, we explore the mouse embryo before and during gastrulation, highlighting its similarities with other vertebrate embryos and its unique characteristics.

A murine homolog of the yeast RNA1 gene is required for postimplantation development

A gene has been characterized that is required for postimplantation mouse development. The gene, designated fug1, was disrupted in embryonic stem cells by the U3Neo gene trap retrovirus, and the disrupted allele was introduced into the germ line. Homozygous mutant embryos arrest at the egg cylinder stage at about embryonic day 6 and are mostly resorbed by day 8.5. The appearance of the proamniotic cavity is delayed, and epiblast cells that surround the cavity are disorganized. fug1 transcripts are undetectable at E6 but are induced throughout the embryo after E6.5. The gene is expressed at low levels in all adult tissues examined, maps to chromosome 15, and is conserved among mammals. The cDNA sequence encodes a protein of 589 amino acids, the first 400 of which are 38% identical to the Saccaromyces cerevisiae RNA1 gene. Regions of greatest similarity include a long acidic domain and 11 leucine-rich motifs, thought to mediate high affinity protein-protein interactions. These similarities suggest that Fug1 may be required for developmental changes in RNA processing or chromatin structure prior to gastrulation.