Mixl1is required for axial mesendoderm morphogenesis and patterning in the murine embryo

Signaling Control of Differentiation of Embryonic Stem Cells toward Mesendoderm

Mesendoderm (ME) refers to the primitive streak in mammalian embryos, which has the ability to further differentiate into mesoderm and endoderm. A better understanding on the regulatory networks of ME differentiation of embryonic stem (ES) cells would provide important insights on early embryo patterning and a possible guidance for ES applications in regenerative medicine. Studies on developmental biology and embryology have offered a great deal of knowledge about key signaling pathways involved in primitive streak formation. Recently, various chemically defined recipes have been formulated to induce differentiation of ES cells toward ME in vitro, which greatly facilitate the elucidation of the regulatory mechanisms of different signals involved in ME specification. Among the extrinsic signals, transforming growth factor-β/Activin signaling and Wnt signaling have been shown to be the most critical ones. On another side, intrinsic epigenetic regulation has been indicated to be important in ME determination. In this review, we summarize the current understanding on the extrinsic and intrinsic regulations of ES cells-to-ME differentiation and the crosstalk among them, aiming to get a general overview on ME specification and primitive streak formation.

Vaccination with cyclin-dependent kinase tick antigen confers protection against Ixodes infestation

Among arthropods, ticks lead as vectors of animal diseases and rank second to mosquitoes in transmitting human pathogens. Cyclin-dependent kinases (CDK) participate in cell cycle control in eukaryotes. CDKs are serine/threonine protein kinases and these catalytic subunits are activated or inactivated at specific stages of the cell cycle. To determine the potential of using CDKs as anti-tick vaccine antigens, hamsters were immunized with recombinant Ixodes persulcatus CDK10, followed by a homologous tick challenge. Though it was not exactly unexpected, IpCDK10 vaccination significantly impaired tick blood feeding and fecundity, which manifested as low engorgement weights, poor oviposition, and a reduction in 80% of hatching rates. These findings may underpin the development of more efficacious anti-tick vaccines based on the targeting of cell cycle control proteins.

Rotary suspension culture enhances mesendoderm differentiation of embryonic stem cells through modulation of Wnt/β-catenin pathway

Recently, physical factors in the local cellular microenvironment have been confirmed with strong influences on regulating stem cell fate. Despite the recent identification of the rotary cell culture system (RCCS) as a bioreactor for culturing stem cells, the underlying biological role provided by RCCS in the lineage differentiation of embryonic stem cells (ESCs) remains largely undefined. Here, we explored the embryoid body (EB) formation and subsequent differentiation of mouse ESCs in RCCS. We demonstrated that EBs formed in RCCS were more homogeneous and bigger in size compared with those in the static condition. Further, we determined that mesendoderm differentiation was prominently enhanced, while neuroectodermal differentiation was significantly suppressed in RCCS. Surprisingly, we found that Wnt/β-catenin signaling was greatly enhanced mainly due to the increased expression of Wnt3 during ESC differentiation in RCCS. Inhibition of Wnt/β-catenin signaling by DKK1 decreased the expression of Brachyury and attenuated mesendoderm differentiation in RCCS. Intriguingly, Wnt3a markedly increased Brachyury expression under static condition rather than in RCCS. Taken together, our findings uncover a new role of rotary suspension culture in initializing the early differentiation of ESCs.

Cyclosporin A induces cardiac differentiation but inhibits hemato-endothelial differentiation of P19 cells

Little is known about the mechanisms underlying the effects of Cyclosporin A (CsA) on the fate of stem cells, including cardiomyogenic differentiation. Therefore, we investigated the effects and the molecular mechanisms behind the actions of CsA on cell lineage determination of P19 cells. CsA induced cardiomyocyte-specific differentiation of P19 cells, with the highest efficiency at a concentration of 0.32 μM during embryoid body (EB) formation via activation of the Wnt signaling pathway molecules, Wnt3a, Wnt5a, and Wnt8a, and the cardiac mesoderm markers, Mixl1, Mesp1, and Mesp2. Interestingly, cotreatment of P19 cells with CsA plus dimethyl sulfoxide (DMSO) during EB formation significantly increases cardiac differentiation. In contrast, mRNA expression levels of hematopoietic and endothelial lineage markers, including Flk1 and Er71, were severely reduced in CsA-treated P19 cells. Furthermore, expression of Flk1 protein and the percentage of Flk1+ cells were severely reduced in 0.32 μM CsA-treated P19 cells compared to control cells. CsA significantly modulated mRNA expression levels of the cell cycle molecules, p53 and Cyclins D1, D2, and E2 in P19 cells during EB formation. Moreover, CsA significantly increased cell death and reduced cell number in P19 cells during EB formation. These results demonstrate that CsA induces cardiac differentiation but inhibits hemato-endothelial differentiation via activation of the Wnt signaling pathway, followed by modulation of cell lineage-determining genes in P19 cells during EB formation.

Early lineage restriction in temporally distinct populations of Mesp1 progenitors during mammalian heart development

Cardiac development arises from two sources of mesoderm progenitors, the first heart field (FHF) and the second (SHF). Mesp1 has been proposed to mark the most primitive multipotent cardiac progenitors common for both heart fields. Here, using clonal analysis of the earliest prospective cardiovascular progenitors in a temporally controlled manner during early gastrulation, we found that Mesp1 progenitors consist of two temporally distinct pools of progenitors restricted to either the FHF or the SHF. FHF progenitors were unipotent, whereas SHF progenitors were either unipotent or bipotent. Microarray and single-cell PCR with reverse transcription analysis of Mesp1 progenitors revealed the existence of molecularly distinct populations of Mesp1 progenitors, consistent with their lineage and regional contribution. Together, these results provide evidence that heart development arises from distinct populations of unipotent and bipotent cardiac progenitors that independently express Mesp1 at different time points during their specification, revealing that the regional segregation and lineage restriction of cardiac progenitors occur very early during gastrulation.

Traffic jam in the primitive streak: the role of defective mesoderm migration in birth defects

Gastrulation is the process in which the three germ layers are formed that contribute to the formation of all major tissues in the developing embryo. We here review mouse genetic models in which defective gastrulation leads to mesoderm insufficiencies in the embryo. Depending on severity of the abnormalities, the outcomes range from incompatible with embryonic survival to structural birth defects, such as heart defects, spina bifida, or caudal dysgenesis. The combined evidence from the mutant models supports the notion that these congenital anomalies can originate from perturbations of mesoderm specification, epithelial-mesenchymal transition, and mesodermal cell migration. Knowledge about the molecular pathways involved may help to improve strategies for the prevention of major structural birth defects.

Geminin restrains mesendodermal fate acquisition of embryonic stem cells and is associated with antagonism of Wnt signaling and enhanced polycomb-mediated repression

Embryonic cells use both growth factor signaling and cell intrinsic transcriptional and epigenetic regulation to acquire early cell fates. Underlying mechanisms that integrate these cues are poorly understood. Here, we investigated the role of Geminin, a nucleoprotein that interacts with both transcription factors and epigenetic regulatory complexes, during fate acquisition of mouse embryonic stem cells. In order to determine Geminin's role in mesendoderm formation, a process which occurs during embryonic gastrulation, we selectively over-expressed or knocked down Geminin in an in vitro model of differentiating mouse embryonic stem cells. We found that Geminin antagonizes mesendodermal fate acquisition, while these cells instead maintain elevated expression of genes associated with pluripotency of embryonic stem cells. During mesendodermal fate acquisition, Geminin knockdown promotes Wnt signaling, while Bmp, Fgf, and Nodal signaling are not affected. Moreover, we showed that Geminin facilitates the repression of mesendodermal genes that are regulated by the Polycomb repressor complex. Geminin directly binds several of these genes, while Geminin knockdown in mesendodermal cells reduces Polycomb repressor complex occupancy at these loci and increases trimethylation of histone H3 lysine 4, which correlates with active gene expression. Together, these results indicate that Geminin is required to restrain mesendodermal fate acquisition of early embryonic cells and that this is associated with both decreased Wnt signaling and enhanced Polycomb repressor complex retention at mesendodermal genes.

Recreating kidney progenitors from pluripotent cells

Access to human pluripotent cells theoretically provides a renewable source of cells that can give rise to any required cell type for use in cellular therapy or bioengineering. However, successfully directing this differentiation remains challenging for most desired endpoints cell type, including renal cells. This challenge is compounded by the difficulty in identifying the required cell type in vitro and the multitude of renal cell types required to build a kidney. Here we review our understanding of how the embryo goes about specifying the cells of the kidney and the progress to date in adapting this knowledge for the recreation of nephron progenitors and their mature derivatives from pluripotent cells.

A conditional mutant allele for analysis of Mixl1 function in the mouse

Mixl1 is the only member of the Mix/Bix homeobox gene family identified in mammals. During mouse embryogenesis, Mixl1 is first expressed at embryonic day (E)5.5 in cells of the visceral endoderm (VE). At the time of gastrulation, Mixl1 expression is detected in the vicinity of the primitive streak. Mixl1 is expressed in cells located within the primitive streak, in nascent mesoderm cells exiting the primitive streak, and in posterior VE overlying the primitive streak. Genetic ablation of Mixl1 in mice has revealed its crucial role in mesoderm and endoderm cell specification and tissue morphogenesis during early embryonic development. However, the early lethality of the constitutive Mixl1(-/-) mutant precludes the study of its role at later stages of embryogenesis and in adult mice. To circumvent this limitation, we have generated a conditional Mixl1 allele (Mixl1(cKO) that permits temporal as well as spatial control of gene ablation. Animals homozygous for the Mixl1(cKO) conditional allele were viable and fertile. Mixl1(KO/KO) embryos generated by crossing of Mixl1(cKO/cKO) mice with Sox2-Cre or EIIa-Cre transgenic mice were embryonic lethal at early somite stages. By contrast to wild-type embryos, Mixl1(KO/KO) embryos contained no detectable Mixl1, validating the Mixl1(cKO) as a protein null after Cre-mediated excision. Mixl1(KO/KO) embryos resembled the previously reported Mixl1(-/-) mutant phenotype. Therefore, the Mixl1 cKO allele provides a tool for investigating the temporal and tissue-specific requirements for Mixl1 in the mouse.

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

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

Regulation of pluripotent cell differentiation by a small molecule, staurosporine

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

The formation of proximal and distal definitive endoderm populations in culture requires p38 MAPK activity

Endoderm formation in the mammal is a complex process with two lineages forming during the first weeks of development, the primitive (or extraembryonic) endoderm, which is specified in the blastocyst, and the definitive endoderm that forms later, at gastrulation, as one of the germ layers of the embryo proper. Fate mapping evidence suggests that the definitive endoderm arises as two waves, which potentially reflect two distinct cell populations. Early primitive ectoderm-like (EPL) cell differentiation has been used successfully to identify and characterise mechanisms regulating molecular gastrulation and lineage choice during differentiation. The roles of the p38 MAPK family in the formation of definitive endoderm were investigated using EPL cells and chemical inhibitors of p38 MAPK activity. These approaches define a role for p38 MAPK activity in the formation of the primitive streak and a second role in the formation of the definitive endoderm. Characterisation of the definitive endoderm populations formed from EPL cells demonstrates the formation of two distinct populations, defined by gene expression and ontogeny, that were analogous to the proximal and distal definitive endoderm populations of the embryo. Formation of the proximal definitive endoderm was found to require p38 MAPK activity and is correlated with molecular gastrulation, defined by the expression of brachyury (T). Distal definitive endoderm formation also requires p38 MAPK activity but can form when T expression is inhibited. Understanding lineage complexity will be a prerequisite for the generation of endoderm derivatives for commercial and clinical use.

Location of transient ectodermal progenitor potential in mouse development

Ectoderm is one of the three classic germ layers in the early mouse embryo, with the capacity to develop into both the central nervous system and epidermis. Because it is a transient phase of development with few molecular markers, the early ectoderm is the least understood germ layer in mouse embryonic development. In this work, we studied the differentiation potential of isolated ectoderm tissue in response to BMP signaling at various developmental stages (E6.5, E7.0 and E7.5), and identified a transient region in the anterior-proximal side of the embryo at E7.0 that possesses the ability to become neural or epidermal ectoderm in response to the absence or presence of BMP4, respectively. Furthermore, we demonstrated that inhibition of Nodal signaling could direct the pluripotent E6.5 epiblast cells towards ectoderm lineages during differentiation in explants in vitro. Our work not only improves our understanding of ectodermal layer development in early embryos, but also provides a framework for regenerative differentiation towards ectodermal tissues.

Initiating head development in mouse embryos: integrating signalling and transcriptional activity

The generation of an embryonic body plan is the outcome of inductive interactions between the progenitor tissues that underpin their specification, regionalization and morphogenesis. The intercellular signalling activity driving these processes is deployed in a time- and site-specific manner, and the signal strength must be precisely controlled. Receptor and ligand functions are modulated by secreted antagonists to impose a dynamic pattern of globally controlled and locally graded signals onto the tissues of early post-implantation mouse embryo. In response to the WNT, Nodal and Bone Morphogenetic Protein (BMP) signalling cascades, the embryo acquires its body plan, which manifests as differences in the developmental fate of cells located at different positions in the anterior–posterior body axis. The initial formation of the anterior (head) structures in the mouse embryo is critically dependent on the morphogenetic activity emanating from two signalling centres that are juxtaposed with the progenitor tissues of the head. A common property of these centres is that they are the source of antagonistic factors and the hub of transcriptional activities that negatively modulate the function of WNT, Nodal and BMP signalling cascades. These events generate the scaffold of the embryonic head by the early-somite stage of development. Beyond this, additional tissue interactions continue to support the growth, regionalization, differentiation and morphogenesis required for the elaboration of the structure recognizable as the embryonic head.

Cripto is required for mesoderm and endoderm cell allocation during mouse gastrulation

During mouse gastrulation, cells in the primitive streak undergo epithelial-mesenchymal transformation and the resulting mesenchymal cells migrate out laterally to form mesoderm and definitive endoderm across the entire embryonic cylinder. The mechanisms underlying mesoderm and endoderm specification, migration, and allocation are poorly understood. In this study, we focused on the function of mouse Cripto, a member of the EGF-CFC gene family that is highly expressed in the primitive streak and migrating mesoderm cells on embryonic day 6.5. Conditional inactivation of Cripto during gastrulation leads to varied defects in mesoderm and endoderm development. Mutant embryos display accumulation of mesenchymal cells around the shortened primitive streak indicating a functional requirement of Cripto during the formation of mesoderm layer in gastrulation. In addition, some mutant embryos showed poor formation and abnormal allocation of definitive endoderm cells on embryonic day 7.5. Consistently, many mutant embryos that survived to embryonic day 8.5 displayed defects in ventral closure of the gut endoderm causing cardia bifida. Detailed analyses revealed that both the Fgf8-Fgfr1 pathway and p38 MAP kinase activation are partially affected by the loss of Cripto function. These results demonstrate a critical role for Cripto during mouse gastrulation, especially in mesoderm and endoderm formation and allocation.

Definitive Endoderm Formation from Plucked Human Hair-Derived Induced Pluripotent Stem Cells and SK Channel Regulation

Pluripotent stem cells present an extraordinary powerful tool to investigate embryonic development in humans. Essentially, they provide a unique platform for dissecting the distinct mechanisms underlying pluripotency and subsequent lineage commitment. Modest information currently exists about the expression and the role of ion channels during human embryogenesis, organ development, and cell fate determination. Of note, small and intermediate conductance, calcium-activated potassium channels have been reported to modify stem cell behaviour and differentiation. These channels are broadly expressed throughout human tissues and are involved in various cellular processes, such as the after-hyperpolarization in excitable cells, and also in differentiation processes. To this end, human induced pluripotent stem cells (hiPSCs) generated from plucked human hair keratinocytes have been exploited in vitro to recapitulate endoderm formation and, concomitantly, used to map the expression of the SK channel (SKCa) subtypes over time. Thus, we report the successful generation of definitive endoderm from hiPSCs of ectodermal origin using a highly reproducible and robust differentiation system. Furthermore, we provide the first evidence that SKCas subtypes are dynamically regulated in the transition from a pluripotent stem cell to a more lineage restricted, endodermal progeny.

In vitro modeling of paraxial mesodermal progenitors derived from induced pluripotent stem cells

Induced pluripotent stem (iPS) cells are generated from adult somatic cells by transduction of defined factors. Given their unlimited proliferation and differentiation potential, iPS cells represent promising sources for cell therapy and tools for research and drug discovery. However, systems for the directional differentiation of iPS cells toward paraxial mesodermal lineages have not been reported. In the present study, we established a protocol for the differentiation of mouse iPS cells into paraxial mesodermal lineages in serum-free culture. The protocol was dependent on Activin signaling in addition to BMP and Wnt signaling which were previously shown to be effective for mouse ES cell differentiation. Independently of the cell origin, the number of transgenes, or the type of vectors used to generate iPS cells, the use of serum-free monolayer culture stimulated with a combination of BMP4, Activin A, and LiCl enabled preferential promotion of mouse iPS cells to a PDGFR-α⁺/Flk-1⁻ population, which represents a paraxial mesodermal lineage. The mouse iPS cell-derived paraxial mesodermal cells exhibited differentiation potential into osteogenic, chondrogenic, and myogenic cells both in vitro and in vivo and contributed to muscle regeneration. Moreover, purification of the PDGFR-α⁺/KDR⁻ population after differentiation allowed enrichment of human iPS cell populations with paraxial mesodermal characteristics. The resultant PDGFR-α⁺/KDR⁻ population derived from human iPS cells specifically exhibited osteogenic, chondrogenic, and myogenic differentiation potential in vitro, implying generation of paraxial mesodermal progenitors similar to mouse iPS cell-derived progenitors. These findings highlight the potential of protocols based on the serum-free, stepwise induction and purification of paraxial mesodermal cell lineages for use in stem cell therapies to treat diseased bone, cartilage, and muscle.

TGF-β control of stem cell differentiation genes

The canonical TGF-β/Smad signaling pathway was delineated in the mid 90s and enriched over the past decade with many findings about its specificity, regulation, networking, and malfunctions in disease. However, a growing understanding of the chromatin status of a critical class of TGF-β target genes - the genes controlling differentiation of embryonic stem cells - recently prompted a reexamination of this pathway and its critical role in the regulation of stem cell differentiation. The new findings reveal master regulators of the pluripotent state set the stage for Smad-mediated activation of master regulators of the next differentiation stage. Furthermore, a novel branch of the TGF-β/Smad pathway has been identified in which a chromatin-reading Smad complex makes the master differentiation genes accessible to canonical Smad complexes for transcriptional activation. These findings provide exciting new insights into the global role of TGF-β signaling in the regulators of stem cell fate.

Human chondrogenic paraxial mesoderm, directed specification and prospective isolation from pluripotent stem cells

Directed specification and prospective isolation of chondrogenic paraxial mesoderm progeny from human pluripotent stem (PS) cells have not yet been achieved. Here we report the successful generation of KDR⁻PDGFRα⁺ progeny expressing paraxial mesoderm genes and the mesendoderm reporter MIXL1-GFP in a chemically defined medium containing the canonical WNT signaling activator, BMP-inhibitor, and the Nodal/Activin/TGFβ signaling controller. Isolated (GFP⁺)KDR⁻PDGFRα⁺ mesoderm cells were sensitive to sequential addition of the three chondrogenic factors PDGF, TGFβ and BMP. Under these conditions, the cells showed robust chondrogenic activity in micromass culture, and generated a hyaline-like translucent cartilage particle in serum-free medium. In contrast, both STRO1⁺ mesenchymal stem/stromal cells from adult human marrow and mesenchymal cells spontaneously arising from hPS cells showed a relatively weaker chondrogenic response in vitro, and formed more of the fibrotic cartilage particles. Thus, hPS cell-derived KDR⁻PDGFRα⁺ paraxial mesoderm-like cells have potential in engineered cartilage formation and cartilage repair.

Distinct gene expression signatures in human embryonic stem cells differentiated towards definitive endoderm at single-cell level

Characterization of directed differentiation of pluripotent stem cells towards therapeutically relevant cell types, including pancreatic beta-cells and hepatocytes, depends on molecular markers and assays that resolve the signature of individual cells. Pancreas and liver both have a common origin of anterior definitive endoderm (DE). Here, we differentiated human embryonic stem cells towards DE using three different activin A based treatments. Differentiation efficiencies were evaluated by gene expression profiling over time at cell population level. A panel of key markers was used to study DE formation. Final DE differentiation was also analyzed with immunocytochemistry and single-cell gene expression profiling. We found that cells treated with activin A in combination with sodium butyrate and B27 serum-free supplement medium generated the most mature DE cells. Cell population studies were useful to monitor the temporal expression of genes involved in primitive streak formation and endoderm formation, while single-cell analysis allowed us to study cell culture heterogeneity and fingerprint individual cells. In addition, single-cell analysis revealed distinct gene expression patterns for the three activin A based protocols applied. Our data provide novel insights in DE gene expression at the cellular level of in vitro differentiated human embryonic stem cells, and illustrate the power of using single-cell gene expression profiling to study differentiation heterogeneity and to characterize cell types and subpopulations.

The hypoblast (visceral endoderm): an evo-devo perspective

When amniotes appeared during evolution, embryos freed themselves from intracellular nutrition; development slowed, the mid-blastula transition was lost and maternal components became less important for polarity. Extra-embryonic tissues emerged to provide nutrition and other innovations. One such tissue, the hypoblast (visceral endoderm in mouse), acquired a role in fixing the body plan: it controls epiblast cell movements leading to primitive streak formation, generating bilateral symmetry. It also transiently induces expression of pre-neural markers in the epiblast, which also contributes to delay streak formation. After gastrulation, the hypoblast might protect prospective forebrain cells from caudalizing signals. These functions separate mesendodermal and neuroectodermal domains by protecting cells against being caught up in the movements of gastrulation.

A molecular signature for purified definitive endoderm guides differentiation and isolation of endoderm from mouse and human embryonic stem cells

Embryonic definitive endoderm (DE) generates the epithelial compartment of vital organs such as liver, pancreas, and intestine. However, purification of DE in mammals has not been achieved, limiting the molecular "definition" of endoderm, and hindering our understanding of DE development and attempts to produce endoderm from sources such as embryonic stem (ES) cells. Here, we describe purification of mouse DE using fluorescence-activated cell sorting (FACS) and mice harboring a transgene encoding enhanced green fluorescent protein (eGFP) inserted into the Sox17 locus, which is expressed in the embryonic endoderm. Comparison of patterns of signaling pathway activation in native mouse DE and endoderm-like cells generated from ES cells produced novel culture modifications that generated Sox17-eGFP⁺ progeny whose gene expression resembled DE more closely than achieved with standard methods. These studies also produced new FACS methods for purifying DE from nontransgenic mice and mouse ES cell cultures. Parallel studies of a new human SOX17-eGFP ES cell line allowed analysis of endoderm differentiation in vitro, leading to culture modifications that enhanced expression of an endoderm-like signature. This work should accelerate our understanding of mechanisms regulating DE development in mice and humans, and guide further use of ES cells for tissue replacement.

A poised chromatin platform for TGF-β access to master regulators

Specific chromatin marks keep master regulators of differentiation silent yet poised for activation by extracellular signals. We report that nodal TGF-β signals use the poised histone mark H3K9me3 to trigger differentiation of mammalian embryonic stem cells. Nodal receptors induce the formation of companion Smad4-Smad2/3 and TRIM33-Smad2/3 complexes. The PHD-Bromo cassette of TRIM33 facilitates binding of TRIM33-Smad2/3 to H3K9me3 and H3K18ac on the promoters of mesendoderm regulators Gsc and Mixl1. The crystal structure of this cassette, bound to histone H3 peptides, illustrates that PHD recognizes K9me3, and Bromo binds an adjacent K18ac. The interaction between TRIM33-Smad2/3 and H3K9me3 displaces the chromatin-compacting factor HP1γ, making nodal response elements accessible to Smad4-Smad2/3 for Pol II recruitment. In turn, Smad4 increases K18 acetylation to augment TRIM33-Smad2/3 binding. Thus, nodal effectors use the H3K9me3 mark as a platform to switch master regulators of stem cell differentiation from the poised to the active state.

Differentiation of definitive endoderm from mouse embryonic stem cells

Efficient production of definitive endoderm from embryonic stem (ES) cells opens doors to the possibilities of differentiation of endoderm-derived tissues such as the intestines, pancreas, and liver that could address the needs of people with chronic diseases involving these organs. The lessons learned from developmental biology have contributed significantly to in vitro differentiation of definitive endoderm. Gastrulation, a process that results in the production of all three embryonic germ cell layers, definitive endoderm, mesoderm, and ectoderm, is an important step in embryonic development. Gastrulation occurs as a result of the events that are orchestrated by the signaling pathways involving Nodal, FGF, Wnt, and BMP. Understanding these signaling pathways has led to the introduction of key ingredients such as Activin A, FGF, Wnt, and BMP to the differentiation protocols that have been able to produce definitive endoderm from ES cells. Efficient production of definitive endoderm needs to meet the specific criteria that include (a) increase in the production of markers of definitive endoderm such as Sox 17, FOXA2, GSC, and Mixl1; (b) decrease in the production of markers of primitive/visceral/parietal endoderm, Sox 7 and OCT4; and (c) decrease in the mesoderm markers (Brachyury, MEOX) and ectoderm markers (Sox1 and ZIC1).

Brachyury and related Tbx proteins interact with the Mixl1 homeodomain protein and negatively regulate Mixl1 transcriptional activity

Mixl1 is a homeodomain transcription factor required for mesoderm and endoderm patterning during mammalian embryogenesis. Despite its crucial function in development, co-factors that modulate the activity of Mixl1 remain poorly defined. Here we report that Mixl1 interacts physically and functionally with the T-box protein Brachyury and related members of the T-box family of transcription factors. Transcriptional and protein analyses demonstrated overlapping expression of Mixl1 and Brachyury during embryonic stem cell differentiation. In vitro protein interaction studies showed that the Mixl1 with Brachyury associated via their DNA-binding domains and gel shift assays revealed that the Brachyury T-box domain bound to Mixl1-DNA complexes. Furthermore, luciferase reporter experiments indicated that association of Mixl1 with Brachyury and related T-box factors inhibited the transactivating potential of Mixl1 on the Gsc and Pdgfrα promoters. Our results indicate that the activity of Mixl1 can be modulated by protein-protein interactions and that T-box factors can function as negative regulators of Mixl1 activity.

Pdgfrα and Flk1 are direct target genes of Mixl1 in differentiating embryonic stem cells

The Mixl1 homeodomain protein plays a key role in mesendoderm patterning during embryogenesis, but its target genes remain to be identified. We compared gene expression in differentiating heterozygous Mixl1(GFP/w) and homozygous null Mixl1(GFP/Hygro) mouse embryonic stem cells to identify potential downstream transcriptional targets of Mixl1. Candidate Mixl1 regulated genes whose expression was reduced in GFP+ cells isolated from differentiating Mixl1(GFP/Hygro) embryoid bodies included Pdgfrα and Flk1. Mixl1 bound to ATTA sequences located in the Pdgfrα and Flk1 promoters and chromatin immunoprecipitation assays confirmed Mixl1 occupancy of these promoters in vivo. Furthermore, Mixl1 transactivated the Pdgfrα and Flk1 promoters through ATTA sequences in a DNA binding dependent manner. These data support the hypothesis that Mixl1 directly regulates Pdgfrα and Flk1 gene expression and strengthens the position of Mixl1 as a key regulator of mesendoderm development during mammalian gastrulation.

Cell type-specific target selection by combinatorial binding of Smad2/3 proteins and hepatocyte nuclear factor 4alpha in HepG2 cells

Specific regulation of target genes by transforming growth factor-β (TGF-β) in a given cellular context is determined in part by transcription factors and cofactors that interact with the Smad complex. In this study, we determined Smad2 and Smad3 (Smad2/3) binding regions in the promoters of known genes in HepG2 hepatoblastoma cells, and we compared them with those in HaCaT epidermal keratinocytes to elucidate the mechanisms of cell type- and context-dependent regulation of transcription induced by TGF-β. Our results show that 81% of the Smad2/3 binding regions in HepG2 cells were not shared with those found in HaCaT cells. Hepatocyte nuclear factor 4α (HNF4α) is expressed in HepG2 cells but not in HaCaT cells, and the HNF4α-binding motif was identified as an enriched motif in the HepG2-specific Smad2/3 binding regions. Chromatin immunoprecipitation sequencing analysis of HNF4α binding regions under TGF-β stimulation revealed that 32.5% of the Smad2/3 binding regions overlapped HNF4α bindings. MIXL1 was identified as a new combinatorial target of HNF4α and Smad2/3, and both the HNF4α protein and its binding motif were required for the induction of MIXL1 by TGF-β in HepG2 cells. These findings generalize the importance of binding of HNF4α on Smad2/3 binding genomic regions for HepG2-specific regulation of transcription by TGF-β and suggest that certain transcription factors expressed in a cell type-specific manner play important roles in the transcription regulated by the TGF-β-Smad signaling pathway.

A synthetic substrate to support early mesodermal differentiation of human embryonic stem cells

Our ability to guide differentiation of human pluripotent stem cells (hPSCs) toward desired lineages efficiently and reproducibly in xeno-free conditions is the key to advancing hPSC technology from the laboratory to clinical use. Here we report an engineered biomimetic substrate functionalized with both peptide ligands for α5β1 and α6β1 integrins to support efficient early mesodermal differentiation of human embryonic stem cells (hESCs) when cultured in a differentiation medium containing BMP4. In contrast, mesodermal differentiation is not induced on substrates functionalized with either ligand alone even though the culture medium is identical. Mesodermal differentiation was characterized by immunofluorescent staining, flow cytometric analysis, and RT-PCR analysis of early mesodermal markers Brachyury, Mixl1, and Wnt3. The early mesodermal progenitors derived on the substrate functionalized with both integrin ligands have the normal developmental potential to further differentiate along the hemato-endothelial and cardiac lineages. Immobilized ligands for α5β1 and α6β1 integrins both are permissive, necessary, and sufficient insoluble ligands in this engineered system to support early mesodermal differentiation of hESCs. This synthetic substrate, in conjunction with defined soluble factors, constructs a well-controlled and xeno-free early mesodermal differentiation niche that offers advantages over the previously reported niche constructed with the Matrigel-coated substrate.

Graded Nodal/Activin signaling titrates conversion of quantitative phospho-Smad2 levels into qualitative embryonic stem cell fate decisions

Nodal and Activin are morphogens of the TGFbeta superfamily of signaling molecules that direct differential cell fate decisions in a dose- and distance-dependent manner. During early embryonic development the Nodal/Activin pathway is responsible for the specification of mesoderm, endoderm, node, and mesendoderm. In contradiction to this drive towards cellular differentiation, the pathway also plays important roles in the maintenance of self-renewal and pluripotency in embryonic and epiblast stem cells. The molecular basis behind stem cell interpretation of Nodal/Activin signaling gradients and the undertaking of disparate cell fate decisions remains poorly understood. Here, we show that any perturbation of endogenous signaling levels in mouse embryonic stem cells leads to their exit from self-renewal towards divergent differentiation programs. Increasing Nodal signals above basal levels by direct stimulation with Activin promotes differentiation towards the mesendodermal lineages while repression of signaling with the specific Nodal/Activin receptor inhibitor SB431542 induces trophectodermal differentiation. To address how quantitative Nodal/Activin signals are translated qualitatively into distinct cell fates decisions, we performed chromatin immunoprecipitation of phospho-Smad2, the primary downstream transcriptional factor of the Nodal/Activin pathway, followed by massively parallel sequencing, and show that phospho-Smad2 binds to and regulates distinct subsets of target genes in a dose-dependent manner. Crucially, Nodal/Activin signaling directly controls the Oct4 master regulator of pluripotency by graded phospho-Smad2 binding in the promoter region. Hence stem cells interpret and carry out differential Nodal/Activin signaling instructions via a corresponding gradient of Smad2 phosphorylation that selectively titrates self-renewal against alternative differentiation programs by direct regulation of distinct target gene subsets and Oct4 expression.

Vertebrate intestinal endoderm development

The endoderm gives rise to the lining of the esophagus, stomach and intestines, as well as associated organs. To generate a functional intestine, a series of highly orchestrated developmental processes must occur. In this review, we attempt to cover major events during intestinal development from gastrulation to birth, including endoderm formation, gut tube growth and patterning, intestinal morphogenesis, epithelial reorganization, villus emergence, as well as proliferation and cytodifferentiation. Our discussion includes morphological and anatomical changes during intestinal development as well as molecular mechanisms regulating these processes.

Pluripotency factors regulate definitive endoderm specification through eomesodermin

Understanding the molecular mechanisms controlling early cell fate decisions in mammals is a major objective toward the development of robust methods for the differentiation of human pluripotent stem cells into clinically relevant cell types. Here, we used human embryonic stem cells and mouse epiblast stem cells to study specification of definitive endoderm in vitro. Using a combination of whole-genome expression and chromatin immunoprecipitation (ChIP) deep sequencing (ChIP-seq) analyses, we established an hierarchy of transcription factors regulating endoderm specification. Importantly, the pluripotency factors NANOG, OCT4, and SOX2 have an essential function in this network by actively directing differentiation. Indeed, these transcription factors control the expression of EOMESODERMIN (EOMES), which marks the onset of endoderm specification. In turn, EOMES interacts with SMAD2/3 to initiate the transcriptional network governing endoderm formation. Together, these results provide for the first time a comprehensive molecular model connecting the transition from pluripotency to endoderm specification during mammalian development.

Uni-directional differentiation of mouse embryonic stem cells into neurons by the neural stem sphere method

We previously showed that our neural stem sphere (NSS) method promotes the neuronal differentiation of mouse, monkey and human embryonic stem (ES) cells. Here we analyzed changes in expression of marker genes and proteins during neuronal differentiation. When cultured in astrocyte-conditioned medium (ACM) under free-floating conditions, colonies of ES cells formed floating cell spheres, which, within 4 days, gave rise to NSSs. In the spheres, the expression of ES cell marker genes was consistently down-regulated, while expression of an epiblast marker was transiently up-regulated, beginning on day 2, and the expression of neuroectoderm, neural stem cell and neuron markers was up-regulated, beginning on days 3, 4 and 6, respectively. The expression of the marker genes was consistent with that of marker proteins. The time course of expression of these markers in the spheres resembled that of neuronal differentiation from the inner cell mass (ICM) cells of blastula. In contrast, the expression of endoderm, mesoderm, epidermis, astrocyte and oligodendrocyte markers was low and not up-regulated during differentiation. Only a small number of apoptotic cells were present in the spheres. These results suggest that mouse ES cells uni-directionally differentiate into neurons via epiblast cells, neuroectodermal cells and neural stem cells.

miR-124a is important for migratory cell fate transition during gastrulation of human embryonic stem cells

Precise control of gene expression is of paramount importance for proper embryonic development. Although a number of microRNAs (miRNAs) has been implicated in fine-tuning mRNA translation during development, their exact roles for gastrulation, particularly in connection with functional targets, have yet to be clarified, with regard to stage-specific cell migration to form three embryonic germ layers. We found that miR-124a is expressed in human embryonic stem cells (hESC), but is gradually downregulated during embryoid body (EB) formation in vitro. We also provide evidence that SLUG and IQGAP1, which modulates rearrangement of the migratory cytoskeleton, are specific targets for miR-124a during EB formation. Furthermore, we show that the beginning of cell migration, a hallmark event in gastrulation, is tightly coupled with downregulation of miR-124a during EB formation and induction of SLUG and IQGAP1. Overexpressed miR-124a in hESC reduced expression of SLUG and IQGAP1 and blocked migratory cell behavior in EB. An expression level of MIXL1, associated with gastulation process, was also inversely correlated with expression of miR-124a. Taken together, our results strongly suggest that miR-124a may play an active role in inhibiting hESCs from differentiation into EB by downregulating expression of SLUG and IQGAP1, thereby maintaining stemness.

Retinoic acid synthesis promotes development of neural progenitors from mouse embryonic stem cells by suppressing endogenous, Wnt-dependent nodal signaling

Embryonic stem (ES) cells differentiate spontaneously toward a neuroectodermal fate in serum-free, adherent monocultures. Here, we show that this spontaneous neural fate requires retinoic acid (RA) synthesis. We monitor ES cells containing reporter genes for markers of the early neural plate as well as the primitive streak and its progeny to determine the cell fates induced when RA signaling is perturbed. We demonstrate that the spontaneous neural commitment of mouse ES cells requires endogenous RA production from vitamin A (vitA) in the medium. Formation of neural progenitors is inhibited by removing vitA from the medium, by inhibiting the enzymes that catalyze the synthesis of RA, or by inhibiting RA receptors. We show that subnanomolar concentrations of RA restore neuroectodermal differentiation when RA synthesis is blocked. We demonstrate that a neural to mesodermal fate change occurring when RA signaling is inhibited is dependent on Nodal-, Wnt-, and fibroblast growth factor-signaling. We show that Nodal suppresses neural development in a Wnt-dependent manner and that Wnt-mediated inhibition of neural development is reversed by inhibition of Nodal signaling. Together, our results show that neural induction in ES cells requires RA at subnanomolar levels to suppress Nodal signaling and suggest that the mechanism by which Wnt signaling suppresses neural development is through facilitation of Nodal signaling.

Transitions between epithelial and mesenchymal states and the morphogenesis of the early mouse embryo

Multicellular organisms arise from the generation of different cell types and the organization of cells into tissues and organs. Cells of metazoa display two main phenotypes, the ancestral epithelial state and the recent mesenchymal derivative. Epithelial cells are usually stationary and reside in two-dimensional sheets. By contrast mesenchymal cells are loosely packed and can move to new positions, thereby providing a vehicle for cell rearrangement, dispersal and novel cell-cell interactions. Transitions between epithelial and mesenchymal states drive key morphogenetic events in the early vertebrate embryo, including gastrulation, germ layer formation and somitogenesis. The cell behaviors and molecular mechanisms promoting transitions between these two states in the early mouse embryo are discussed in this review.

Cellular dynamics in the early mouse embryo: from axis formation to gastrulation

Coordinated cell movements and reciprocal tissue interactions direct the formation of the definitive germ layers and the elaboration of the major axes of the mouse embryo. Genetic and embryological studies have defined the major molecular pathways that mediate these morphogenetic processes and provided 'snapshots' of the morphogenetic program. However, it is increasingly clear that this foundation needs to be validated, and can be significantly refined and extended using live imaging approaches. In situ visualization of these processes in living specimens is a major goal, as it provides unprecedented detail of the individual cellular behaviors, which translate into the large-scale tissue rearrangements that shape the embryo.

Differentiating embryonic stem cells pass through 'temporal windows' that mark responsiveness to exogenous and paracrine mesendoderm inducing signals

Background

Mesendoderm induction during embryonic stem cell (ESC) differentiation in vitro is stimulated by the Transforming Growth Factor and Wingless (Wnt) families of growth factors.

Principal Findings

We identified the periods during which Bone Morphogenetic Protein (BMP) 4, Wnt3a or Activin A were able to induce expression of the mesendoderm marker, Mixl1, in differentiating mouse ESCs. BMP4 and Wnt3a were required between differentiation day (d) 1.5 and 3 to most effectively induce Mixl1, whilst Activin A induced Mixl1 expression in ESC when added between d2 and d4, indicating a subtle difference in the requirement for Activin receptor signalling in this process. Stimulation of ESCs with these factors at earlier or later times resulted in little Mixl1 induction, suggesting that the differentiating ESCs passed through ‘temporal windows’ in which they sequentially gained and lost competence to respond to each growth factor. Inhibition of either Activin or Wnt signalling blocked Mixl1 induction by any of the three mesendoderm-inducing factors. Mixing experiments in which chimeric EBs were formed between growth factor-treated and untreated ESCs revealed that BMP, Activin and Wnt signalling all contributed to the propagation of paracrine mesendoderm inducing signals between adjacent cells. Finally, we demonstrated that the differentiating cells passed through ‘exit gates’ after which point they were no longer dependent on signalling from inducing molecules for Mixl1 expression.

Conclusions

These studies suggest that differentiating ESCs are directed by an interconnected network of growth factors similar to those present in early embryos and that the timing of growth factor activity is critical for mesendoderm induction.

A conserved mechanism for vertebrate mesoderm specification in urodele amphibians and mammals

Understanding how mesoderm is specified during development is a fundamental issue in biology, and it has been studied intensively in embryos from Xenopus. The gene regulatory network (GRN) for Xenopus is surprisingly complex and is not conserved in vertebrates, including mammals, which have single copies of the key genes Nodal and Mix. Why the Xenopus GRN should express multiple copies of Nodal and Mix genes is not known. To understand how these expanded gene families evolved, we investigated mesoderm specification in embryos from axolotls, representing urodele amphibians, since urodele embryology is basal to amphibians and was conserved during the evolution of amniotes, including mammals. We show that single copies of Nodal and Mix are required for mesoderm specification in axolotl embryos, suggesting the ancestral vertebrate state. Furthermore, we uncovered a novel genetic interaction in which Mix induces Brachyury expression, standing in contrast to the relationship of these molecules in Xenopus. However, we demonstrate that this functional relationship is conserved in mammals by showing that it is involved in the production of mesoderm from mouse embryonic stem cells. From our results, we produced an ancestral mesoderm (m)GRN, which we suggest is conserved in vertebrates. The results are discussed within the context of a theory in which the evolution of mechanisms governing early somatic development is constrained by the ancestral germ line-soma relationship, in which germ cells are produced by epigenesis.

A novel role for gamma-secretase in the formation of primitive streak-like intermediates from ES cells in culture

gamma-Secretase is a membrane-associated protease with multiple intracellular targets, a number of which have been shown to influence embryonic development and embryonic stem (ES) cell differentiation. This paper describes the use of the gamma-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT) to evaluate the role of gamma-secretase in the differentiation of pluripotent stem cells to the germ lineages. The addition of DAPT did not prevent the formation of primitive ectoderm-like cells from ES cells in culture. In contrast, the addition of DAPT during primitive ectoderm-like cell differentiation interfered with the ability of both serum and BMP4 to induce a primitive streak-like intermediate and resulted in the preferential formation of neurectoderm. Similarly, DAPT reduced the formation of primitive streak-like intermediates from differentiating human ES cells; the culture conditions used resulted in a population enriched in human surface ectoderm. These data suggest that gamma-secretase may form part of the general pathway by which mesoderm is specified within the primitive streak. The addition of an E-cadherin neutralizing antibody was able to partially reverse the effect of DAPT, suggesting that DAPT may be preventing the formation of primitive streak-like intermediates and promoting neurectoderm differentiation by stabilizing E-cadherin and preventing its proteolysis.

Transcriptional activation by the Mixl1 homeodomain protein in differentiating mouse embryonic stem cells

Members of the Mix/Bix family of paired class homeobox genes play important roles in the development of vertebrate mesoderm and endoderm. The single Mix/Bix family member identified in the mouse, Mix-like 1 (Mixl1), is required for mesendoderm patterning during gastrulation and promotes mesoderm formation and hematopoiesis in embryonic stem cell (ESC)-derived embryoid bodies. Despite its crucial functions the transcriptional activity and targets of Mixl1 have not been well described. To investigate the molecular mechanisms of Mixl1-mediated transcriptional regulation, we have characterized the DNA-binding specificity and transcriptional properties of this homeodomain protein in differentiating ESCs. Mixl1 binds preferentially as a dimer to an 11-base pair (bp) Mixl1 binding sequence (MBS) that contains two inverted repeats separated by a 3-bp spacer. The MBS mediates transcriptional activation by Mixl1 in both NIH 3T3 cells and in a new application of an inducible ESC differentiation system. Consistent with our previous observation that early induction of Mixl1 expression in ESCs results in premature activation of Goosecoid (Gsc), we have found that Mixl1 occupies two variant MBSs within and activates transcription from the Gsc promoter in vitro and in vivo. These results strongly suggest that Gsc is a direct target gene of Mixl1 during embryogenesis. STEM CELLS 2009;27:2884-2895.

Differentiation of mesodermal cells from pluripotent stem cells

The pluripotency of embryonic stem cells has been well demonstrated by a vast variety of studies showing the induction of differentiation into desired cell types that have the potential to be used not only in basic studies but also in medical applications. The induction of mesodermal cells, especially blood cells, from embryonic stem cells is notable from the point of view of transplantation, and the methods for this induction have improved over the last few years, with more defined culture conditions in place. Concurrently, the generation of induced pluripotent stem cells from somatic cells opens the possibility of autologous transplantation. In fact, there are a growing number of reports demonstrating that several mesodermal cells can be differentiated from induced pluripotent stem cells using the same methods used for embryonic stem cells. This review summarizes recent advances in the differentiation of mesodermal cells from embryonic stem cells and induced pluripotent stem cells.

Vertebrate endoderm development and organ formation

The endoderm germ layer contributes to the respiratory and gastrointestinal tracts and to all of their associated organs. Over the past decade, studies in vertebrate model organisms, including frog, fish, chick, and mouse, have greatly enhanced our understanding of the molecular basis of endoderm organ development. We review this progress with a focus on early stages of endoderm organogenesis including endoderm formation, gut tube morphogenesis and patterning, and organ specification. Lastly, we discuss how developmental mechanisms that regulate endoderm organogenesis are used to direct differentiation of embryonic stem cells into specific adult cell types, which function to alleviate disease symptoms in animal models.

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.