The Xenopus T-box gene, Antipodean, encodes a vegetally localised maternal mRNA and can trigger mesoderm formation

Maternal and zygotic contributions to H3K4me1 chromatin marking during germ layer formation

An early step in triploblastic embryo differentiation is the formation of the three germ layers. Maternal pioneer transcription factors (TFs) bind to embryonic enhancers before zygotic genome activation, initiating germ layer specification. While maternal TFs' role in establishing epigenetic marks is known, how early pluripotent cells gain spatially restricted epigenetic identities remains unclear. We show that by the early gastrula stage, H3K4me1-marked regions become distinct in each germ layer, with certain chromatin regions forming high density H3K4me1 marked regions (HDRs). Genes associated with these HDRs are more robustly expressed compared to those associated with low density H3K4me1 marked regions (LDRs) in the genome. This process is driven by the sequential actions of maternal and zygotic factors. Knockdown of key maternal endodermal TFs (Otx1, Vegt and Foxh1) leads to a loss of endodermal H3K4me1 marks in endoderm, with a concurrent emergence of ectodermal and mesodermal marks, indicating a shift in chromatin state. This work highlights the importance of coordinated activities of maternal and zygotic TFs in defining the regionally-resolved and dynamic process of chromatin modification conferred by H3K4me1 in the early Xenopus embryo.

RNA localization during early development of the axolotl

The asymmetric localization of biomolecules is critical for body plan development. One of the most popular model organisms for early embryogenesis studies is Xenopus laevis but there is a lack of information in other animal species. Here, we compared the early development of two amphibian species-the frog X. laevis and the axolotl Ambystoma mexicanum. This study aimed to identify asymmetrically localized RNAs along the animal-vegetal axis during the early development of A. mexicanum. For that purpose, we performed spatial transcriptome-wide analysis at low resolution, which revealed dynamic changes along the animal-vegetal axis classified into the following categories: profile alteration, de novo synthesis and degradation. Surprisingly, our results showed that many of the vegetally localized genes, which are important for germ cell development, are degraded during early development. Furthermore, we assessed the motif presence in UTRs of degraded mRNAs and revealed the enrichment of several motifs in RNAs of germ cell markers. Our results suggest novel reorganization of the transcriptome during embryogenesis of A. mexicanum to converge to the similar developmental pattern as the X. laevis.

Normal Table of Xenopus development: a new graphical resource

Normal tables of development are essential for studies of embryogenesis, serving as an important resource for model organisms, including the frog Xenopus laevis. Xenopus has long been used to study developmental and cell biology, and is an increasingly important model for human birth defects and disease, genomics, proteomics and toxicology. Scientists utilize Nieuwkoop and Faber's classic 'Normal Table of Xenopus laevis (Daudin)' and accompanying illustrations to enable experimental reproducibility and reuse the illustrations in new publications and teaching. However, it is no longer possible to obtain permission for these copyrighted illustrations. We present 133 new, high-quality illustrations of X. laevis development from fertilization to metamorphosis, with additional views that were not available in the original collection. All the images are available on Xenbase, the Xenopus knowledgebase (http://www.xenbase.org/entry/zahn.do), for download and reuse under an attributable, non-commercial creative commons license. Additionally, we have compiled a 'Landmarks Table' of key morphological features and marker gene expression that can be used to distinguish stages quickly and reliably (https://www.xenbase.org/entry/landmarks-table.do). This new open-access resource will facilitate Xenopus research and teaching in the decades to come.

Characterization of convergent thickening, a major convergence force producing morphogenic movement in amphibians

The morphogenic process of convergent thickening (CT) was originally described as the mediolateral convergence and radial thickening of the explanted ventral involuting marginal zone (IMZ) of Xenopus gastrulae (Keller and Danilchik, 1988). Here, we show that CT is expressed in all sectors of the pre-involution IMZ, which transitions to expressing convergent extension (CE) after involution. CT occurs without CE and drives symmetric blastopore closure in ventralized embryos. Assays of tissue affinity and tissue surface tension measurements suggest CT is driven by increased interfacial tension between the deep IMZ and the overlying epithelium. The resulting minimization of deep IMZ surface area drives a tendency to shorten the mediolateral (circumblastoporal) aspect of the IMZ, thereby generating tensile force contributing to blastopore closure (Shook et al., 2018). These results establish CT as an independent force-generating process of evolutionary significance and provide the first clear example of an oriented, tensile force generated by an isotropic, Holtfreterian/Steinbergian tissue affinity change.

T-box transcription factors Dorsocross and optomotor-blind control Drosophila leg patterning in a functionally redundant manner

The T-box genes are essential transcription factors during limb development. In Drosophila, Dorsocross (Doc) and optomotor-blind (omb), members of the Tbx2 and Tbx6 families, are best studied in the Drosophila wing development. Despite prominently expressed in leg discs, the specific function of these genes in leg growth is still not revealed. Here we demonstrated that Doc and omb regulated the morphogenesis of leg intermediate regions in a functionally redundant manner. Loss of Doc or omb individually did not result in any developmental defects of the legs, but loss of both genes induced significant defects in femur and proximal tibia of the adult legs. These genes located in the dorsal domain, where the Doc region expanded and cross-overlapped with the omb region corresponding to the presumptive leg intermediate region. We detected that the normal epithelial folds in the leg discs were disrupted along with dorsal repression of cell proliferation and activation of cell apoptosis when Doc and omb were both reduced. Furthermore, the dorsal expression of dachshund (dac), a canonical leg developmental gene specifying the leg intermediate region, was maintained by Doc and omb. Meanwhile, the Notch pathway was compromised in the dorsal domain when these genes were reduced, which might contribute to the joint defect of the adult leg intermediate regions. Our study provides cytological and genetic evidence for understanding the redundant function of Doc and omb in leg morphogenesis.

Endodermal Maternal Transcription Factors Establish Super-Enhancers during Zygotic Genome Activation

Elucidation of the sequence of events underlying the dynamic interaction between transcription factors and chromatin states is essential. Maternal transcription factors function at the top of the regulatory hierarchy to specify the primary germ layers at the onset of zygotic genome activation (ZGA). We focus on the formation of endoderm progenitor cells and examine the interactions between maternal transcription factors and chromatin state changes underlying the cell specification process. Endoderm-specific factors Otx1 and Vegt together with Foxh1 orchestrate endoderm formation by coordinated binding to select regulatory regions. These interactions occur before the deposition of enhancer histone marks around the regulatory regions, and these TFs recruit RNA polymerase II, regulate enhancer activity, and establish super-enhancers associated with important endodermal genes. Therefore, maternal transcription factors Otx1, Vegt, and Foxh1 combinatorially regulate the activity of super-enhancers, which in turn activate key lineage-specifying genes during ZGA.

How do maternal transcription factors interact with chromatin regions to coordinate the endodermal gene regulatory program? Paraiso et al. demonstrate that combinatorial binding of maternal Otx1, Vegt, and Foxh1 to select enhancers and super-enhancers in the genome controls endodermal cell fate specification during zygotic gene activation.

Early Xenopus gene regulatory programs, chromatin states, and the role of maternal transcription factors

For decades, the early development of the Xenopus embryo has been an essential model system to study the gene regulatory mechanisms that govern cellular specification. At the top of the hierarchy of gene regulatory networks, maternally deposited transcription factors initiate this process and regulate the expression of zygotic genes that give rise to three distinctive germ layer cell types (ectoderm, mesoderm, and endoderm), and subsequent generation of organ precursors. The onset of germ layer specification is also closely coupled with changes associated with chromatin modifications. This review will examine the timing of maternal transcription factors initiating the zygotic genome activation, the epigenetic landscape of embryonic chromatin, and the network structure that governs the process.

Tbx2 is required for the suppression of mesendoderm during early Xenopus development

BACKGROUND

T-box family proteins are DNA-binding transcriptional regulators that play crucial roles during germ layer formation in the early vertebrate embryo. Well-characterized members of this family, including the transcriptional activators Brachyury and VegT, are essential for the proper formation of mesoderm and endoderm, respectively. To date, T-box proteins have not been shown to play a role in the promotion of the third primary germ layer, ectoderm.

RESULTS

Here, we report that the T-box factor Tbx2 is both sufficient and necessary for ectodermal differentiation in the frog Xenopus laevis. Tbx2 is expressed zygotically in the presumptive ectoderm, during blastula and gastrula stages. Ectopic expression of Tbx2 represses mesoderm and endoderm, while loss of Tbx2 leads to inappropriate expression of mesoderm- and endoderm-specific genes in the region fated to give rise to ectoderm. Misexpression of Tbx2 also promotes neural tissue in animal cap explants, suggesting that Tbx2 plays a role in both the establishment of ectodermal fate and its dorsoventral patterning.

CONCLUSIONS

Our studies demonstrate that Tbx2 functions as a transcriptional repressor during germ layer formation, and suggest that this activity is mediated in part through repression of target genes that are stimulated, in the mesendoderm, by transactivating T-box proteins. Taken together, our results point to a critical role for Tbx2 in limiting the potency of blastula-stage progenitor cells during vertebrate germ layer differentiation. Developmental Dynamics 247:903-913, 2018. © 2018 Wiley Periodicals, Inc.

A novel role for sox7 in Xenopus early primordial germ cell development: mining the PGC transcriptome

Xenopus primordial germ cells (PGCs) are determined by the presence of maternally derived germ plasm. Germ plasm components both protect PGCs from somatic differentiation and begin a unique gene expression program. Segregation of the germline from the endodermal lineage occurs during gastrulation, and PGCs subsequently initiate zygotic transcription. However, the gene network(s) that operate to both preserve and promote germline differentiation are poorly understood. Here, we utilized RNA-sequencing analysis to comprehensively interrogate PGC and neighboring endoderm cell mRNAs after lineage segregation. We identified 1865 transcripts enriched in PGCs compared with endoderm cells. We next compared the PGC-enriched transcripts with previously identified maternal, vegetally enriched transcripts and found that ∼38% of maternal transcripts were enriched in PGCs, including sox7 PGC-directed sox7 knockdown and overexpression studies revealed an early requirement for sox7 in germ plasm localization, zygotic transcription and PGC number. We identified pou5f3.3 as the most highly expressed and enriched POU5F1 homolog in PGCs. We compared the Xenopus PGC transcriptome with human PGC transcripts and showed that 80% of genes are conserved, underscoring the potential usefulness of Xenopus for understanding human germline specification.

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

Germ layer formation is among the earliest differentiation events in metazoan embryos. In triploblasts, three germ layers are formed, among which the endoderm gives rise to the epithelial lining of the gut tube and associated organs including the liver, pancreas and lungs. In frogs (Xenopus), where early germ layer formation has been studied extensively, the process of endoderm specification involves the interplay of dozens of transcription factors. Here, we review the interactions between these factors, summarized in a transcriptional gene regulatory network (GRN). We highlight regulatory connections conserved between frog, fish, mouse, and human endodermal lineages. Especially prominent is the conserved role and regulatory targets of the Nodal signaling pathway and the T-box transcription factors, Vegt and Eomes. Additionally, we highlight network topologies and motifs, and speculate on their possible roles in development.

Maternal messages to live by: a personal historical perspective

In the 1980s, the study of localized maternal mRNAs was just emerging as a new research area. Classic embryological studies had linked the inheritance of cytoplasmic domains with specific cell lineages, but the underlying molecular nature of these putative determinants remained a mystery. The model system Xenopus would play a pivotal role in the progress of this new field. In fact, the first localized maternal mRNA to be identified and cloned from any organism was Xenopus vg1, a TGF-beta family member. This seminal finding opened the door to many subsequent studies focused on how RNAs are localized and what functions they had in development. As the field moves into the future, Xenopus remains the system of choice for studies identifying RNA/protein transport particles and maternal RNAs through RNA-sequencing.

A Maternal System Initiating the Zygotic Developmental Program through Combinatorial Repression in the Ascidian Embryo

Maternal factors initiate the zygotic developmental program in animal embryos. In embryos of the chordate, Ciona intestinalis, three maternal factors—Gata.a, β-catenin, and Zic-r.a—are required to establish three domains of gene expression at the 16-cell stage; the animal hemisphere, vegetal hemisphere, and posterior vegetal domains. Here, we show how the maternal factors establish these domains. First, only β-catenin and its effector transcription factor, Tcf7, are required to establish the vegetal hemisphere domain. Second, genes specifically expressed in the posterior vegetal domain have additional repressive cis-elements that antagonize the activity of β-catenin/Tcf7. This antagonizing activity is suppressed by Zic-r.a, which is specifically localized in the posterior vegetal domain and binds to DNA indirectly through the interaction with Tcf7. Third, Gata.a directs specific gene expression in the animal hemisphere domain, because β-catenin/Tcf7 weakens the Gata.a-binding activity for target sites through a physical interaction in the vegetal cells. Thus, repressive regulation through protein-protein interactions among the maternal transcription factors is essential to establish the first distinct domains of gene expression in the chordate embryo.

During animal development, transcription factors and signaling molecules transcriptionally regulate one another and constitute a gene regulatory network. This network is evoked by maternally provided factors. Many maternal factors are localized and thereby activate a set of genes in a specific region. In embryos of the chordate, Ciona intestinalis, three maternal factors with localized activities are known. The present study demonstrated that these localized maternal factors interact with one another through a fourth non-localized transcription factor, Tcf7, and negatively regulate one another. These repressive interactions are essential to establish the first distinct domains of gene expression and evoke the gene regulatory network properly. The findings indicate that not only activating target genes but also repressing activities of other transcription factors through protein-protein interactions are important to properly initiate the zygotic program. Intriguingly, in one repressive interaction, a transcription factor loses its binding activity for target sites through an interaction with another transcription factor. Thus, this study provides a description of the entire system in which maternal factors initiate the zygotic developmental program of the Ciona embryo.

Transcription factors Mix1 and VegT, relocalization of vegt mRNA, and conserved endoderm and dorsal specification in frogs

Protein expression of the transcription factor genes mix1 and vegt characterized the presumptive endoderm in embryos of the frogs Engystomops randi, Epipedobates machalilla, Gastrotheca riobambae, and Eleutherodactylus coqui, as in Xenopus laevis embryos. Protein VegT was detected in the animal hemisphere of the early blastula in all frogs, and only the animal pole was VegT-negative. This finding stimulated a vegt mRNA analysis in X. laevis eggs and embryos. vegt mRNA was detected in the animal region of X. laevis eggs and early embryos, in agreement with the VegT localization observed in the analyzed frogs. Moreover, a dorso-animal relocalization of vegt mRNA occurred in the egg at fertilization. Thus, the comparative analysis indicated that vegt may participate in dorsal development besides its known roles in endoderm development, and germ-layer specification. Zygotic vegt (zvegt) mRNA was detected as a minor isoform besides the major maternal (mvegt) isoform of the X. laevis egg. In addition, α-amanitin-insensitive vegt transcripts were detected around vegetal nuclei of the blastula. Thus, accumulation of vegt mRNA around vegetal nuclei was caused by relocalization rather than new mRNA synthesis. The localization of vegt mRNA around vegetal nuclei may contribute to the identity of vegetal blastomeres. These and previously reportedly localization features of vegt mRNA and protein derive from the master role of vegt in the development of frogs. The comparative analysis indicated that the strategies for endoderm, and dorsal specification, involving vegt and mix1, have been evolutionary conserved in frogs.

Multicellular Mathematical Modelling of Mesendoderm Formation in Amphibians

The earliest cell fate decisions in a developing embryo are those associated with establishing the germ layers. The specification of the mesoderm and endoderm is of particular interest as the mesoderm is induced from the endoderm, potentially from an underlying bipotential group of cells, the mesendoderm. Mesendoderm formation has been well studied in an amphibian model frog, Xenopus laevis, and its formation is driven by a gene regulatory network (GRN) induced by maternal factors deposited in the egg. We have recently demonstrated that the axolotl, a urodele amphibian, utilises a different topology in its GRN to specify the mesendoderm. In this paper, we develop spatially structured mathematical models of the GRNs governing mesendoderm formation in a line of cells. We explore several versions of the model of mesendoderm formation in both Xenopus and the axolotl, incorporating the key differences between these two systems. Model simulations are able to reproduce known experimental data, such as Nodal expression domains in Xenopus, and also make predictions about how the positional information derived from maternal factors may be interpreted to drive cell fate decisions. We find that whilst cell-cell signalling plays a minor role in Xenopus, it is crucial for correct patterning domains in axolotl.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

A novel role for Ascl1 in the regulation of mesendoderm formation via HDAC-dependent antagonism of VegT

Maternally expressed proteins function in vertebrates to establish the major body axes of the embryo and to establish a pre-pattern that sets the stage for later-acting zygotic signals. This pre-patterning drives the propensity of Xenopus animal cap cells to adopt neural fates under various experimental conditions. Previous studies found that the maternally expressed transcription factor, encoded by the Xenopus achaete scute-like gene ascl1, is enriched at the animal pole. Asc1l is a bHLH protein involved in neural development, but its maternal function has not been studied. Here, we performed a series of gain- and loss-of-function experiments on maternal ascl1, and present three novel findings. First, Ascl1 is a repressor of mesendoderm induced by VegT, but not of Nodal-induced mesendoderm. Second, a previously uncharacterized N-terminal domain of Ascl1 interacts with HDAC1 to inhibit mesendoderm gene expression. This N-terminal domain is dispensable for its neurogenic function, indicating that Ascl1 acts by different mechanisms at different times. Ascl1-mediated repression of mesendoderm genes was dependent on HDAC activity and accompanied by histone deacetylation in the promoter regions of VegT targets. Finally, maternal Ascl1 is required for animal cap cells to retain their competence to adopt neural fates. These results establish maternal Asc1l as a key factor in establishing pre-patterning of the early embryo, acting in opposition to VegT and biasing the animal pole to adopt neural fates. The data presented here significantly extend our understanding of early embryonic pattern formation.

Summary: The proneural factor ASCL1 recruits HDAC1 to repress VegT-induced, but not Nodal-induced, mesendoderm formation via a previously uncharacterized N-terminal domain.

Novel animal pole-enriched maternal mRNAs are preferentially expressed in neural ectoderm

BACKGROUND

Many animals utilize maternal mRNAs to pre-pattern the embryo before the onset of zygotic transcription. In Xenopus laevis, vegetal factors specify the germ line, endoderm, and dorsal axis, but there are few studies demonstrating roles for animal-enriched maternal mRNAs. Therefore, we carried out a microarray analysis to identify novel maternal transcripts enriched in 8-cell-stage animal blastomeres.

RESULTS

We identified 39 mRNAs isolated from 8-cell animal blastomeres that are >4-fold enriched compared to vegetal pole mRNAs. We characterized 14 of these that are of unknown function. We validated the microarray results for 8/14 genes by qRT-PCR and for 14/14 genes by in situ hybridization assays. Because no developmental functions are reported yet, we provide the expression patterns for each of the 14 genes. Each is expressed in the animal hemisphere of unfertilized eggs, 8-cell animal blastomeres, and diffusely in blastula animal cap ectoderm, gastrula ectoderm and neural ectoderm, neural crest (and derivatives) and cranial placodes (and derivatives). They have varying later expression in some mesodermal and endodermal tissues in tail bud through larval stages.

CONCLUSIONS

Novel animal-enriched maternal mRNAs are preferentially expressed in ectodermal derivatives, particularly neural ectoderm. However, they are later expressed in derivatives of other germ layers.

Commitment to nutritional endoderm in Eleutherodactylus coqui involves altered nodal signaling and global transcriptional repression

The vegetal cells of a Xenopus laevis embryo commit to mesendoderm via the Nodal-signaling pathway. In the direct developing frog Eleutherodactylus coqui, mesendoderm is specified at the marginal zone of the early gastrula, and vegetal core cells transform into nutritional endoderm. Nutritional endoderm, a novel tissue, consists of transient, yolky cells that provide nutrition but remain undifferentiated. We report a dual regulation for the generation of nutritional endoderm. First, differential expressions of the Nodal-signal transducers Smad2 and Smad4 were observed during early gastrulation between the marginal zone and the vegetal core cells. Although EcSmad2 RNA as well as total and activated Smad2 protein were detected in the vegetal core, Smad4 protein was expressed less in vegetal core during early gastrulation. Only 12% and 50% of vegetal core cells were positive for nuclear Smad2 and Smad4 signals respectively compared to 100% of marginal zone cells. These results suggest a signaling disruption in the vegetal core. Second, vegetal core cells were transcriptionally repressed. At the blastula stage, both marginal zone and vegetal core cells were transcriptionally silent, but during early gastrulation, only marginal zone cells became transcriptionally active. This indicates the occurrence of a mid-blastula transition in the marginal zone by early gastrulation, but global transcriptional repression persisted in the vegetal core and its derivative, nutritional endoderm, throughout development. We have described a novel mechanism, which prevents differentiation of the vegetal core through differential Nodal-signaling and global transcriptional repression.

Characterization of the nutritional endoderm in the direct developing frog Eleutherodactylus coqui

Unlike Xenopus laevis, Eleutherodactylus coqui develops without a tadpole. The yolk-rich vegetal region of the embryo forms a transient nutritive tissue, the nutritional endoderm (NE). The definitive endoderm (DE) in E. coqui comes from cells closer to the animal pole in contrast to its vegetal origin in X. laevis. RNA important for initiating the endoderm specification network is absent in presumptive NE cells, raising the question whether signaling occurs in them. We explored the nature of NE and asked how differences between NE and DE cells arise. We identified differences between NE and DE that first become evident at gastrula, when NE cells become multinucleated. Nuclear β-catenin, an essential cofactor of sox 17, important for endoderm formation in X. laevis, is present in NE and DE at gastrula but remains in NE long after it is not seen in DE. We cloned E. coqui homologs of TGFβs activin b and derriere and provide evidence for their maternal expression. We also detected activin b and derriere RNAs in NE at gastrula and show that NE possesses some mesoderm-inducing activity, but it is delayed with respect to DE. Our findings indicate that altered development of NE begins at gastrula. RNAs important for mesendoderm induction and some mesoderm-inducing activity are present in NE.

In vivo T-box transcription factor profiling reveals joint regulation of embryonic neuromesodermal bipotency

The design of effective cell replacement therapies requires detailed knowledge of how embryonic stem cells form primary tissues, such as mesoderm or neurectoderm that later become skeletal muscle or nervous system. Members of the T-box transcription factor family are key in the formation of these primary tissues, but their underlying molecular activities are poorly understood. Here, we define in vivo genome-wide regulatory inputs of the T-box proteins Brachyury, Eomesodermin, and VegT, which together maintain neuromesodermal stem cells and determine their bipotential fates in frog embryos. These T-box proteins are all recruited to the same genomic recognition sites, from where they activate genes involved in stem cell maintenance and mesoderm formation while repressing neurogenic genes. Consequently, their loss causes embryos to form an oversized neural tube with no mesodermal derivatives. This collaboration between T-box family members thus ensures the continuous formation of correctly proportioned neural and mesodermal tissues in vertebrate embryos during axial elongation.

Regulation of cell polarity and RNA localization in vertebrate oocytes

It has long been appreciated that the inheritance of maternal cytoplasmic determinants from different regions of the egg can lead to differential specification of blastomeres during cleavage. Localized RNAs are important determinants of cell fate in eggs and embryos but are also recognized as fundamental regulators of cell structure and function. This chapter summarizes recent molecular and genetic experiments regarding: (1) mechanisms that regulate polarity during different stages of vertebrate oogenesis, (2) pathways that localize presumptive protein and RNA determinants within the polarized oocyte and egg, and (3) how these determinants act in the embryo to determine the ultimate cell fates. Emphasis is placed on studies done in Xenopus, where extensive work has been done in these areas, and comparisons are drawn with fish and mammals. The prospects for future work using in vivo genome manipulation and other postgenomic approaches are also discussed.

Foxi2 is an animally localized maternal mRNA in Xenopus, and an activator of the zygotic ectoderm activator Foxi1e

Foxi1e is a zygotic transcription factor that is essential for the expression of early ectodermal genes. It is expressed in a highly specific pattern, only in the deep cell layers of the animal hemisphere, and in a mosaic pattern in which expressing cells are interspersed with non-expressing cells. Previous work has shown that several signals in the blastula control this expression pattern, including nodals, the TGFβ family member Vg1, and Notch. However, these are all inhibitory, which raises the question of what activates Foxi1e. In this work, we show that a related Forkhead family protein, Foxi2, is a maternal activator of Foxi1e. Foxi2 mRNA is maternally encoded, and highly enriched in animal hemisphere cells of the blastula. ChIP assays show that it acts directly on upstream regulatory elements of Foxi1e. Its effect is specific, since animal cells depleted of Foxi2 are able to respond normally to mesoderm inducing signals from vegetal cells. Foxi2 thus acts as a link between the oocyte and the early pathway to ectoderm, in a similar fashion to the vegetally localized VegT acts to initiate endoderm and mesoderm formation.

Xenopus staufen2 is required for anterior endodermal organ formation

Defining the regulatory molecular networks involved in patterning the developing anterior endoderm is essential to understand how the pancreas, liver, stomach, and duodenum are discretely specified from each other. In this study, we analyzed the expression and function of the double-stranded RNA-binding protein Staufen2 in Xenopus laevis endoderm. We found that staufen2 was broadly expressed within the developing endoderm beginning at gastrulation becoming localized to the anterior endoderm at later stages. Through morpholino-mediated knockdown, we demonstrate that Staufen2 function is required for proper formation of the stomach, liver, and pancreas. We define that its function is required during gastrulation for proper patterning of the dorsal-ventral axis and that it acts to regulate expression of BMP signaling components.

The RNA-binding protein XSeb4R regulates maternal Sox3 at the posttranscriptional level during maternal-zygotic transition in Xenopus

The maternal-zygotic transition (MZT) is an embryonic event that overlaps with and plays key roles in primary germ layer specification in vertebrates. During MZT, maternally supplied mRNAs are degraded while zygotic transcripts are synthesized to either reinforce the already specified cell fate or to trigger new cell identity. Here, we show that forced expression of the RNA-binding protein, XSeb4R, in animal pole blastomeres of Xenopus embryos, inappropriately stabilizes transcripts there, including maternal Sox3. This leads to the impaired ability of the ectodermal progenitors to respond to factors regulating brain patterning and their eventual loss by apoptosis. XSeb4R protein binds specifically to the 3'UTR of Sox3 mRNA. XSeb4R gain-of-function in ectodermal explants reveals increased stability of the maternal Sox3 transcripts, associated with a robust Sox3 protein production. Conversely, whereas XSeb4R depletion abolishes VegT expression, the amount of the maternal Sox3 mRNA is rather increased but without augmentation in the amount of Sox3 protein. Moreover, XSeb4R protein knockdown leads to the modification of the ectoderm-mesoderm boundary, marked by expanded/shifted expression of the mesodermal marker genes such as Xbra and Apod, followed by an expression inhibition of Epi. K., an ectodermal marker. Overall, our data suggest XSeb4R as a novel player in gene expression regulation, acting at the posttranscriptional level during ectoderm specification in Xenopus.

Putting RNAs in the right place at the right time: RNA localization in the frog oocyte

Localization of maternal mRNAs in many developing organisms provides the basis for both initial polarity during oogenesis and patterning during embryogenesis. Prominent examples of this phenomenon are found in Xenopus laevis, where localized maternal mRNAs generate developmental polarity along the animal/vegetal axis. Targeting of mRNA molecules to specific subcellular regions is a fundamental mechanism for spatial regulation of gene expression, and considerable progress has been made in defining the underlying molecular pathways.

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.

Morphogen gradients in development: from form to function

Morphogens are substances that establish a graded distribution and elicit distinct cellular responses in a dose-dependent manner. They function to provide individual cells within a field with positional information, which is interpreted to give rise to spatial patterns. Morphogens can consist of intracellular factors that set up a concentration gradient by diffusion in the cytoplasm. More commonly, morphogens comprise secreted proteins that form an extracellular gradient across a field of cells. Experimental studies and computational analyses have provided support for a number of diverse strategies by which extracellular morphogen gradients are formed. These include free diffusion in the extracellular space, restricted diffusion aided by interactions with heparan sulfate proteoglycans, transport on lipid-containing carriers or transport aided by soluble binding partners. More specialized modes of transport have also been postulated such as transcytosis, in which repeated rounds of secretion, endocytosis, and intracellular trafficking move morphogens through cells rather than around them, or cytonemes, which consist of filopodial extensions from signal-receiving cells that are hypothesized to reach out to morphogen-sending cells. Once the gradient has formed, cells must distinguish small differences in morphogen concentration and store this information even after the gradient has dissipated. This is often achieved by translating ligand concentration into a proportional increase in numbers of activated cell surface receptors that are internalized and continue to signal from endosomal compartments. Ultimately, this leads to activation of one or a few transcription factors that transduce this information into qualitatively distinct gene responses inside the nucleus.

Polarizing animal cells via mRNA localization in oogenesis and early development

The localization of mRNAs in developing animal cells is essential for establishing cellular polarity and setting up the body plan for subsequent development. Cellular and molecular mechanisms by which maternal mRNAs are localized during oogenesis have been extensively studied in Drosophila and Xenopus. In contrast, evidence for mechanisms used in the localization of mRNAs encoded by developmentally important genes has also been accumulating in several other organisms. This offers the opportunity to unravel the fundamental mechanisms of mRNA localization shared among many species, as well as unique mechanisms specifically acquired or retained by animals based on their developmental needs. In addition to maternal mRNAs, the localization of zygotically expressed mRNAs in the cells of cleaving embryos is also important for early development. In this review, mRNA localization dynamics in the oocytes/eggs of Drosophila and Xenopus are first summarized, and evidence for localized mRNAs in the oocytes/eggs and cleaving embryos of other organisms is then presented.

Snail2 controls mesodermal BMP/Wnt induction of neural crest

The neural crest is an induced tissue that is unique to vertebrates. In the clawed frog Xenopus laevis, neural crest induction depends on signals secreted from the prospective dorsolateral mesodermal zone during gastrulation. The transcription factors Snail2 (Slug), Snail1 and Twist1 are expressed in this region. It is known that Snail2 and Twist1 are required for both mesoderm formation and neural crest induction. Using targeted blastomere injection, morpholino-based loss of function and explant studies, we show that: (1) Snail1 is also required for mesoderm and neural crest formation; (2) loss of snail1, snail2 or twist1 function in the C2/C3 lineage of 32-cell embryos blocks mesoderm formation, but neural crest is lost only in the case of snail2 loss of function; (3) snail2 mutant loss of neural crest involves mesoderm-derived secreted factors and can be rescued synergistically by bmp4 and wnt8 RNAs; and (4) loss of snail2 activity leads to changes in the RNA levels of a number of BMP and Wnt agonists and antagonists. Taken together, these results identify Snail2 as a key regulator of the signals involved in mesodermal induction of neural crest.

Completion of the epithelial to mesenchymal transition in zebrafish mesoderm requires Spadetail

The process of gastrulation is highly conserved across vertebrates on both the genetic and morphological levels, despite great variety in embryonic shape and speed of development. This mechanism spatially separates the germ layers and establishes the organizational foundation for future development. Mesodermal identity is specified in a superficial layer of cells, the epiblast, where cells maintain an epithelioid morphology. These cells involute to join the deeper hypoblast layer where they adopt a migratory, mesenchymal morphology. Expression of a cascade of related transcription factors orchestrates the parallel genetic transition from primitive to mature mesoderm. Although the early and late stages of this process are increasingly well understood, the transition between them has remained largely mysterious. We present here the first high resolution in vivo observations of the blebby transitional morphology of involuting mesodermal cells in a vertebrate embryo. We further demonstrate that the zebrafish spadetail mutation creates a reversible block in the maturation program, stalling cells in the transition state. This mutation creates an ideal system for dissecting the specific properties of cells undergoing the morphological transition of maturing mesoderm, as we demonstrate with a direct measurement of cell-cell adhesion.

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been is critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular, and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play, and equally importantly, where disparities in experimental results suggest areas of profitable study.

Identification of vegetal RNA-localization elements in Xenopus oocytes

Localized mRNAs have been identified in a large variety of cell types where they contribute to the establishment of cell asymmetries and can function as cell fate determinants. In Xenopus, RNAs that localize to the vegetal cortex during oogenesis function in early embryonic patterning as well as in the development of primordial germ cells. Based on their temporal and spatial localization patterns, vegetally localizing RNAs are referred to as either early-pathway RNAs which transiently localize in the mitochondrial cloud, or as late-pathway RNAs. Vegetal RNA-localization is driven by cis-acting signal sequences that, in most cases, were found to reside in the 3'-UTRs and which are recognized by trans-acting localization factors. Here we describe the methods of how vegetal RNA-localization elements can be identified by injection of fluorescently-labeled or tagged RNAs.

The use of antisense oligonucleotides in Xenopus oocytes

The ability to manipulate gene expression in Xenopus oocytes and then generate fertilized embryos by transfer into host females has made it possible to rapidly characterize maternal signaling pathways in vertebrate development. Maternal mRNAs in particular can be efficiently depleted using antisense deoxyoligonucleotides (oligos), mediated by endogenous RNase-H activity. Since the microinjection of antisense reagents or mRNAs into eggs after fertilization often fails to affect maternal signaling pathways, mRNA depletion in the Xenopus oocyte is uniquely suited to assessing maternal functions. In this review, we highlight the advantages of using antisense in Xenopus oocytes and describe basic methods for designing and choosing effective oligos. We also summarize the procedures for fertilizing cultured oocytes by host-transfer and interpreting the specificity of antisense effects. Although these methods can be technically demanding, the use of antisense in oocytes can be used to address biological questions that are intractable in other experimental settings.

Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

The mammalian definitive endoderm arises as a simple epithelial sheet. This sheet of cells will eventually produce the innermost tube that comprises the entire digestive tract from the esophagus to the colon as well as the epithelial component of the digestive and respiratory organs including the thymus, thyroid, lung, liver, gallbladder, and pancreas. Thus a wide array of tissue types are derived from the early endodermal sheet, and understanding the morphological and molecular mechanisms used to produce this tissue is integral to understanding the development of all these organs. The goal of this chapter is to summarize what is known about the morphological and molecular mechanisms used to produce this embryonic germ layer. Although this chapter mainly focuses on the mechanisms used to generate the murine endoderm, supportive or suggestive data from other species, including chick, frog (Xenopus laevis), and the Zebrafish (Danio rerio) are also examined.

Participation of Xenopus Elr-type proteins in vegetal mRNA localization during oogenesis

Directional transport of specific mRNAs is of primary biological relevance. In Xenopus oocytes, mRNA localization to the vegetal pole is important for germ layer formation and germ cell development. Using a biochemical approach, we identified Xenopus Elr-type proteins, homologs of the Hu/ELAV proteins, as novel components of the vegetal mRNA localization machinery. They bind specifically to the localization elements of several different vegetally localizing Xenopus mRNAs, and they are part of one RNP together with other localization proteins, such as Vg1RBP and XStaufen 1. Blocking Elr-type protein binding by either localization element mutagenesis or antisense morpholino oligonucleotide-mediated masking of their target RNA structures, as well as overexpression of wild type and mutant ElrB proteins, interferes with vegetal localization in Xenopus oocytes.

The RNA-binding protein XSeb4R: a positive regulator of VegT mRNA stability and translation that is required for germ layer formation in Xenopus

VegT represents a localized maternal determinant essentially required for endoderm formation in Xenopus. Here, we report on the identification of the RNA-binding protein XSeb4R as a positive regulator of VegT. XSeb4R interacts directly with the 3'-untranslated region of VegT mRNA, stabilizes it, and stimulates translation. Ablation of XSeb4R activity results in impairment of endoderm and mesoderm formation, while ectopic expression of XSeb4R in ectodermal cells induces endodermal and mesodermal gene expression. These observations unravel a novel mode of VegT regulation at the post-transcriptional level that is essential for germ layer formation in Xenopus.

The role of FGF signaling in the establishment and maintenance of mesodermal gene expression in Xenopus

FGF signaling is important for the formation of mesoderm in vertebrates, and when it is perturbed in Xenopus, most trunk and tail mesoderm fails to form. Here we have further dissected the activities of FGF in patterning the embryo by addressing its inductive and maintenance roles. We show that FGF signaling is necessary for the establishment of xbra expression in addition to its well-characterized role in maintaining xbra expression. The role of FGF signaling in organizer formation is not clear in Xenopus. We find that FGF signaling is essential for the initial specification of paraxial mesoderm but not for activation of several pan-mesodermal and most organizer genes; however, early FGF signaling is necessary for the maintenance of organizer gene expression into the neurula stage. Inhibition of FGF signaling prevents VegT activation of specific mesodermal transcripts. These findings illuminate how FGF signaling contributes to the establishment of distinct types of mesoderm.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

Identification of asymmetrically localized transcripts along the animal-vegetal axis of the Xenopus egg

In many organisms, proper embryo development depends on the asymmetrical distribution of mRNA in the cytoplasm of the egg. Here we report comprehensive screening of RNA localized in the animal or vegetal hemisphere of the Xenopus egg. Macroarrays including over 40,000 independent embryonic cDNA clones, representing at least 17,000 unigenes, were differentially hybridized with labeled probes synthesized from the mRNA of animal or vegetal blastomeres. After two rounds of screening, we identified 33 clones of transcripts that may be preferentially distributed in the vegetal region of the early stage embryo, but transcripts localized in the animal region were not found. To assess the array results, we performed northern blot and quantitative real-time reverse transcription-polymerase chain reaction analysis. As a result, 21 transcripts of the 33 were confirmed to be localized in the vegetal region of the early stage embryo. Whole-mount in situ hybridization analysis revealed that 11 transcripts, including 7 previously reported genes, were localized in the vegetal hemisphere of the egg. These 11 transcripts were categorized into three groups according to their expression patterns in the egg. The first group, which contained four transcripts, showed uniform expression in the vegetal hemisphere, similar to VegT. The second group, which contained three transcripts, showed gradual expression from the vegetal pole to the equator, similar to Vg1. The last group, which contained three transcripts, was expressed at the germ plasm, similar to Xdazl. One transcript, Xwnt11, showed both the second and the third expression patterns.

The Sox axis, Nodal signaling, and germ layer specification

Asymmetries in the egg, established during oogenesis, set the stage for a cascade of intercellular signaling events leading to differential gene expression and subsequent tissue and organ formation. Maternally supplied Sox-type transcription factors have recently emerged as key components in the patterning of the early embryo and the regulation of embryonic stem cell differentiation. In deuterostomes, B1-type Soxs are asymmetrically localized to the future animal/ectodermal region where they act to suppress mesendodermal, and favor neuroectodermal differentiation, while vegetally localized F-type Soxs are involved in mesendodermal differentiation. Here, we review past observations and present new data from studies on the clawed frog Xenopus laevis. Animally localized Sox3 acts to inhibit Nodal (Xnr5 and Xnr6) expression, and induces the expression of genes (Ectodermin, Xema, and Coco) whose products repress Nodal signaling. Vegetally localized Sox7 positively regulates Nodal (Xnr4, Xnr5, and Xnr6) expression, as well as the expression of genes involved in mesodermal (Xmenf, Slug, and Snail) and endodermal (Endodermin and Sox17beta) differentiation. Given the evolutionary strategy of using common regulatory networks, it seems likely that a homologous Sox-Axis is active during embryonic development in many metazoans.

The competence of Xenopus blastomeres to produce neural and retinal progeny is repressed by two endo-mesoderm promoting pathways

Only a subset of cleavage stage blastomeres in the Xenopus embryo is competent to contribute cells to the retina; ventral vegetal blastomeres do not form retina even when provided with neuralizing factors or transplanted to the most retinogenic position of the embryo. These results suggest that endogenous maternal factors in the vegetal region repress the ability of blastomeres to form retina. Herein we provide three lines of evidence that two vegetal-enriched maternal factors (VegT, Vg1), which are known to promote endo-mesodermal fates, negatively regulate which cells are competent to express anterior neural and retinal fates. First, both molecules can repress the ability of dorsal-animal retinogenic blastomeres to form retina, converting the lineage from neural/retinal to non-neural ectodermal and endo-mesodermal fates. Second, reducing the endogenous levels of either factor in dorsal-animal retinogenic blastomeres expands expression of neural/retinal genes and enlarges the retina. The dorsal-animal repression of neural/retinal fates by VegT and Vg1 is likely mediated by Sox17alpha and Derriere but not by XNr1. VegT and Vg1 likely exert their effects on neural/retinal fates through at least partially independent pathways because Notch1 can reverse the effects of VegT and Derriere but not those of Vg1 or XNr1. Third, reduction of endogenous VegT and/or Vg1 in ventral vegetal blastomeres can induce a neural fate, but only allows expression of a retinal fate when both BMP and Wnt signaling pathways are concomitantly repressed.

Ribonucleoprotein remodeling during RNA localization

Cytoplasmic RNA localization is a means to create polarity by restricting protein expression to a discrete subcellular location. RNA localization is a multistep process that begins with the recognition of cis-acting sequences within the RNA by specific trans-factors, and RNAs are localized in ribonucleoprotein (RNP) complexes that contain both the RNA and numerous protein components. Components of the localization machinery transport the RNP complex, usually in a translationally repressed state, to a distinct subcellular region, resulting in spatially restricted gene expression. Recent efforts to identify both the cis- and trans-factors required for RNA localization have elucidated RNA-protein interactions that are remodeled during localization.

The maternally localized RNA fatvg is required for cortical rotation and germ cell formation

Fatvg is a localized maternal transcript that translocates to the vegetal cortex of Xenopus laevis oocytes through both the METRO and Late RNA localization pathways. It is a member of a gene family that functions in vesicular trafficking. Depletion of the maternal store of fatvg mRNA results in a dual phenotype in which embryos are ventralized and also lack primordial germ cells. This complex fatvg loss of function phenotype is the result of stabilization of the dorsalizing factor beta-catenin at the vegetal pole and the inability of the germ cell determinants to move to their proper locations. This is coincident with the inhibition of cortical rotation and the abnormal aggregation of the germ plasm. Fatvg protein is located at the periphery of vesicles in the oocyte and embryo, supporting its proposed role in vesicular trafficking in the embryo. These results point to a common fundamental mechanism that is regulated by fatvg through which germ cell determinants and dorsalizing factors segregate during early development.

Maternal control of pattern formation inXenopus laevis

We review the essential role of maternal factors in pattern formation for Xenopus laevis, focusing on VegT, Vg1, and Wnt11. Results from loss of function experiments demonstrate a clear requirement for these genes in germ layer specification, dorsal-ventral axis formation, and convergence extension. We also discuss these genes in the broader context of metazoan development, exploring whether and how their functions in the X. laevis model organism may or may not be conserved in other species. Wnt11 signaling in particular provides a classic example where understanding context in development is crucial to understanding function. Genomic sequencing, gene expression, and functional screening data that are becoming available in more species are providing invaluable aid to decoding and modeling signaling pathways. More work is needed to develop a comprehensive catalog of the Wnt signaling, T-box, and TGF-beta genes in metazoans both near and far in evolutionary distance. We finally discuss some specific experimental and modeling efforts that will be needed to understand the behavior of these signaling networks in vivo so that we can interpret these critical pathways in an evolutionary framework.

Molecular Basis of Vertebrate Endoderm Development

The embryonic endoderm gives rise to the epithelial lining of the digestive and respiratory systems and organs such as the thyroid, lungs, liver, gallbladder, and pancreas. Studies in Xenopus, zebrafish, and mice have revealed a conserved molecular pathway controlling vertebrate endoderm development. The TGFbeta/Nodal signaling pathway is at the top of this molecular hierarchy and controls the expression of a number of key transcription factors including Mix-like homeodomain proteins, Gata zinc finger factors, Sox HMG domain proteins, and Fox forkhead factors. Here we review the function of these molecules comparing and contrasting their roles in each model organism. Finally, we will describe how our understanding of the molecular pathway governing endoderm development in embryos is being used to differentiate embryonic stem cells in vitro along endodermal lineages, with the ultimate goal of making therapeutically useful tissue.

Ascidian embryonic development: An emerging model system for the study of cell fate specification in chordates

The ascidian tadpole larva represents the basic body plan of all chordates in a relatively small number of cells and tissue types. Although it had been considered that ascidians develop largely in a determinative way, whereas vertebrates develop in an inductive way, recent studies at the molecular and cellular levels have uncovered several similarities in the way developmental fates are specified. In this review, we describe ascidian embryogenesis and its cell lineages, introduce several characteristics of ascidian embryos, describe recent advances in understanding of the mechanisms of cell fate specification, and discuss them in the context of what is known in vertebrates and other organisms.