Axis formation and patterning in zebrafish

Spatial-Omics Methods and Applications

Traditional tissue profiling approaches have evolved from bulk studies to single-cell analysis over the last decade; however, the spatial context in tissues and microenvironments has always been lost. Over the last 5 years, spatial technologies have emerged that enabled researchers to investigate tissues in situ for proteins and transcripts without losing anatomy and histology. The field of spatial-omics enables highly multiplexed analysis of biomolecules like RNAs and proteins in their native spatial context-and has matured from initial proof-of-concept studies to a thriving field with widespread applications from basic research to translational and clinical studies. While there has been wide adoption of spatial technologies, there remain challenges with the standardization of methodologies, sample compatibility, throughput, resolution, and ease of use. In this chapter, we discuss the current state of the field and highlight technological advances and limitations.

Spatiotemporal transcriptomic maps of whole mouse embryos at the onset of organogenesis

Spatiotemporal orchestration of gene expression is required for proper embryonic development. The use of single-cell technologies has begun to provide improved resolution of early regulatory dynamics, including detailed molecular definitions of most cell states during mouse embryogenesis. Here we used Slide-seq to build spatial transcriptomic maps of complete embryonic day (E) 8.5 and E9.0, and partial E9.5 embryos. To support their utility, we developed sc3D, a tool for reconstructing and exploring three-dimensional 'virtual embryos', which enables the quantitative investigation of regionalized gene expression patterns. Our measurements along the main embryonic axes of the developing neural tube revealed several previously unannotated genes with distinct spatial patterns. We also characterized the conflicting transcriptional identity of 'ectopic' neural tubes that emerge in Tbx6 mutant embryos. Taken together, we present an experimental and computational framework for the spatiotemporal investigation of whole embryonic structures and mutant phenotypes.

A spatiotemporal barrier formed by Follistatin is required for left-right patterning

How left-right (LR) asymmetry emerges in a patterning field along the anterior-posterior axis remains an unresolved problem in developmental biology. Left-biased Nodal emanating from the LR organizer propagates from posterior to anterior (PA) and establishes the LR pattern of the whole embryo. However, little is known about the regulatory mechanism of the PA spread of Nodal and its asymmetric activation in the forebrain. Here, we identify bilaterally expressed Follistatin (Fst) as a regulator blocking the propagation of the zebrafish Nodal ortholog Southpaw (Spaw) in the right lateral plate mesoderm (LPM), and restricting Spaw transmission in the left LPM to facilitate the establishment of a robust LR asymmetric Nodal patterning. In addition, Fst inhibits the Activin-Nodal signaling pathway in the forebrain thus preventing Nodal activation prior to the arrival, at a later time, of Spaw emanating from the left LPM. This contributes to the orderly propagation of asymmetric Nodal activation along the PA axis. The LR regulation function of Fst is further confirmed in chick and frog embryos. Overall, our results suggest that a robust LR patterning emerges by counteracting a Fst barrier formed along the PA axis.

Adhesion-Based Self-Organization in Tissue Patterning

Since the proposal of the differential adhesion hypothesis, scientists have been fascinated by how cell adhesion mediates cellular self-organization to form spatial patterns during development. The search for molecular tool kits with homophilic binding specificity resulted in a diverse repertoire of adhesion molecules. Recent understanding of the dominant role of cortical tension over adhesion binding redirects the focus of differential adhesion studies to the signaling function of adhesion proteins to regulate actomyosin contractility. The broader framework of differential interfacial tension encompasses both adhesion and nonadhesion molecules, sharing the common function of modulating interfacial tension during cell sorting to generate diverse tissue patterns. Robust adhesion-based patterning requires close coordination between morphogen signaling, cell fate decisions, and changes in adhesion. Current advances in bridging theoretical and experimental approaches present exciting opportunities to understand molecular, cellular, and tissue dynamics during adhesion-based tissue patterning across multiple time and length scales.

Ventx Family and Its Functional Similarities with Nanog: Involvement in Embryonic Development and Cancer Progression

The Ventx family is one of the subfamilies of the ANTP (antennapedia) superfamily and belongs to the NK-like (NKL) subclass. Ventx is a homeobox transcription factor and has a DNA-interacting domain that is evolutionarily conserved throughout vertebrates. It has been extensively studied in Xenopus, zebrafish, and humans. The Ventx family contains transcriptional repressors widely involved in embryonic development and tumorigenesis in vertebrates. Several studies have documented that the Ventx family inhibited dorsal mesodermal formation, neural induction, and head formation in Xenopus and zebrafish. Moreover, Ventx2.2 showed functional similarities to Nanog and Barx1, leading to pluripotency and neural-crest migration in vertebrates. Among them, Ventx protein is an orthologue of the Ventx family in humans. Studies have demonstrated that human Ventx was strongly associated with myeloid-cell differentiation and acute myeloid leukemia. The therapeutic potential of Ventx family inhibition in combating cancer progression in humans is discussed. Additionally, we briefly discuss genome evolution, gene duplication, pseudo-allotetraploidy, and the homeobox family in Xenopus.

The effect of Activin pathway modulation on the expression of both pluripotency and differentiation markers during early zebrafish development compared with other vertebrates

Activin-like factors control many developmental processes, including pluripotency maintenance and differentiation. Although Activin-like factors' action in mesendoderm induction has been demonstrated in zebrafish, their involvement in preserving the stemness remains unknown. To investigate the role of maternal Activin-like factors, their effects were promoted or blocked using synthetic human Activin A or SB-431542 treatments respectively until the maternal to zygotic transition. To study the role of zygotic Activin-like factors, SB-431542 treatment was also applied after the maternal to zygotic transition. The effect of the pharmacological modulations of the Activin/Smad pathway was then studied on the mRNA expressions of the ndr1, ndr2, tbxta (no tail/ntl) as the differentiation index, mych, nanog, and oct4 (pou5f3) as the pluripotency markers of the zebrafish embryonic cells as well as sox17 as a definitive endoderm marker. Expression of the target genes was measured at the 16-cell, 256-cell, 1K-cell, oblong, dome, and shield stages using the real-time quantitative polymerase chain reaction (RT-qPCR). Activation of the maternal Activin signaling pathway led to an increase in zygotic expression of the tbxta, particularly marked at the oblong stage. In other words, promotion of the maternal Activin/Smad pathway induced differentiation by advancing the major peaks of ndr1 and nanog, thereby eliciting tbxta expression. Whereas suppression of the maternal or zygotic Activin/Smad pathway sustained the pluripotency by preventing the major peaks of ndr1 and nanog as well as tbxta encoding.

Analyzing signaling activity and function in hematopoietic cells

Cells constantly sense their environment, allowing the adaption of cell behavior to changing needs. Fine-tuned responses to complex inputs are computed by signaling pathways, which are wired in complex connected networks. Their activity is highly context-dependent, dynamic, and heterogeneous even between closely related individual cells. Despite lots of progress, our understanding of the precise implementation, relevance, and possible manipulation of cellular signaling in health and disease therefore remains limited. Here, we discuss the requirements, potential, and limitations of the different current technologies for the analysis of hematopoietic stem and progenitor cell signaling and its effect on cell fates.

Characterizing the diverse cells that associate with the developing commissures of the zebrafish forebrain

During embryonic development of bilaterally symmetrical organisms, neurons send axons across the midline at specific points to connect the two halves of the nervous system with a commissure. Little is known about the cells at the midline that facilitate this tightly regulated process. We exploit the conserved process of vertebrate embryonic development in the zebrafish model system to elucidate the identity of cells at the midline that may facilitate postoptic (POC) and anterior commissure (AC) development. We have discovered that three different gfap+ astroglial cell morphologies persist in contact with pathfinding axons throughout commissure formation. Similarly, olig2+ progenitor cells occupy delineated portions of the postoptic and anterior commissures where they act as multipotent, neural progenitors. Moreover, we conclude that both gfap+ and olig2+ progenitor cells give rise to neuronal populations in both the telencephalon and diencephalon; however, these varied cell populations showed significant developmental timing differences between the telencephalon and diencephalon. Lastly, we also showed that fli1a+ mesenchymal cells migrate along the presumptive commissure regions before and during midline axon crossing. Furthermore, following commissure maturation, specific blood vessels formed at the midline of the POC and immediately ventral and parallel to the AC. This comprehensive account of the cellular populations that correlate with the timing and position of commissural axon pathfinding has supported the conceptual modeling and identification of the early forebrain architecture that may be necessary for proper commissure development.

Molecular insights into deuterostome evolution from hemichordate developmental biology

Hemichordates, along with echinoderms and chordates, belong to the lineage of bilaterians called the deuterostomes. Their phylogenetic position as an outgroup to chordates provides an opportunity to investigate the evolutionary origins of the chordate body plan and reconstruct ancestral deuterostome characters. The body plans of the hemichordates and chordates are organizationally divergent making anatomical comparisons very challenging. The developmental underpinnings of animal body plans are often more conservative than the body plans they regulate, and offer a novel data set for making comparisons between morphologically divergent body architectures. Here I review the hemichordate developmental data generated over the past 20 years that further test hypotheses of proposed morphological affinities between the two taxa, but also compare the conserved anteroposterior, dorsoventral axial patterning programs and germ layer specification programs. These data provide an opportunity to determine which developmental programs are ancestral deuterostome or bilaterian innovations, and which ones occurred in stem chordates or vertebrates representing developmental novelties of the chordate body plan.

Teratogenic, Oxidative Stress and Behavioural Outcomes of Three Fungicides of Natural Origin (Equisetum arvense, Mimosa tenuiflora, Thymol) on Zebrafish (Danio rerio)

The improper use of synthetic fungicides has raised public concerns related to environmental pollution and animal health. Over the years, plant-derived antifungals have been investigated as safer alternatives, although little scientific evidence of its neurodevelopmental effects exist. The main objective of this study was to explore the effects of three alternative natural extracts (Equisetum arvense, Mimosa tenuiflora, Thymol) with antifungal properties during the early development of zebrafish by evaluating different teratogenic, oxidative stress and behavioural outcomes. Following the determination of the 96 h-LC₅₀, exposure to sublethal concentrations showed the safety profile of both E. arvense and M. tenuiflora. However, following 96-h exposure to Thymol, increased lethality, pericardial oedema, yolk and eye deformations, and decreased body length were observed. The reduced and oxidized glutathione (GSH:GSSG) ratio was increased, and the glutathione-s-transferase activity in the group exposed to the highest Thymol concentration. Overall, these results support a more reducing environment associated with possible effects at the cellular proliferation level. In addition, the disruption of behavioural states (fear- and anxiety-like disorders) were noted, pointing to alterations in the c-Jun N-terminal kinase developmental signalling pathway, although further studies are required to explore this rationale. Notwithstanding, the results provide direct evidence of the teratogenic effects of Thymol, which might have consequences for non-target species.

Ottogi Inhibits Wnt/β-catenin Signaling by Regulating Cell Membrane Trafficking of Frizzled8

Wnt signaling controls critical developmental processes including tissue/body patterning. Here we report the identification of a novel regulator of Wnt signaling, OTTOGI (OTG), isolated from a large-scale expression screening of human cDNAs in zebrafish embryos. Overexpression of OTG in zebrafish embryos caused dorso-anteriorized phenotype, inhibited the expression of Wnt target genes, and prevented nuclear accumulation of β-catenin. Conversely, knockdown of zebrafish otg using specific antisense morpholino promoted nuclear accumulation of β-catenin and caused ventralization. However, OTG failed to rescue headless-like phenotype induced by inhibition of GSK-3β activity, suggesting that OTG acts upstream of GSK-3β. OTG bound specifically to Frizzled8 (Fz8) receptor and caused retention of Fz8 in the endoplasmic reticulum possibly by preventing N-linked glycosylation of Fz8. Taken together, our data indicate that OTG functions as a novel negative regulator of Wnt signaling during development by the modulation of cell surface expression of Fz receptor.

The maternal control in the embryonic development of zebrafish

The maternal control directing the very first hours of life is of pivotal importance for ensuring proper development to the growing embryo. Thanks to the finely regulated inheritance of maternal factors including mRNAs and proteins produced during oogenesis and stored into the mature oocyte, the embryo is sustained throughout the so-called maternal-to-zygotic transition, a period in development characterized by a species-specific length in time, during which critical biological changes regarding cell cycle and zygotic transcriptional activation occur. In order not to provoke any kind of persistent damage, the process must be delicately balanced. Surprisingly, our knowledge as to the possible effects of beneficial bacteria regarding the modulation of the quality and/or quantity of both maternally-supplied and zygotically-transcribed mRNAs, is very limited. To date, only one group has investigated the consequences of the parentally-supplied Lactobacillus rhamnosus on the storage of mRNAs into mature oocytes, leading to an altered maternal control process in the F1 generation. Particular attention was called on the monitoring of several biomarkers involved in autophagy, apoptosis and axis patterning, while data on miRNA generation and pluripotency maintenance are herein presented for the first time, and can assist in laying the ground for further investigations in this field. In this review, the reader is supplied with the current knowledge on the above-mentioned biological process, first by drawing the general background and then by emphasizing the most important findings that have highlighted their focal role in normal animal development.

Impacts of Petroleum-Derived Pollutants on Fish Development

The teleost fish embryo is particularly sensitive to petroleum hydrocarbons (polycyclic aromatic hydrocarbons, PAHs) at two distinct stages of development. The first is early during cleavage stages when PAHs alter normal signaling associated with establishment of the dorsal-ventral axis. This disruption involves the Wnt/β-catenin pathway and results in hyperdorsalized embryos that do not survive to hatching. The second, more sensitive period is during heart development, when oil and PAHs cause abnormal development of the heart as well as cardiac edema and arrhythmia. Even at extremely low levels (ng/L), PAHs cause subtle edema and altered contractility and heart rate, which impair swimming performance. Some PAHs are extremely phototoxic, such that exposures to trace concentrations result in severe membrane damage and mortality in sunlight. The developing fish embryo is a sensitive indicator of petroleum constituents in the environment, and healthy populations of fish likely require limited PAH exposure during development.

Temporally coordinated signals progressively pattern the anteroposterior and dorsoventral body axes

The vertebrate body plan is established through the precise spatiotemporal coordination of morphogen signaling pathways that pattern the anteroposterior (AP) and dorsoventral (DV) axes. Patterning along the AP axis is directed by posteriorizing signals Wnt, fibroblast growth factor (FGF), Nodal, and retinoic acid (RA), while patterning along the DV axis is directed by bone morphogenetic proteins (BMP) ventralizing signals. This review addresses the current understanding of how Wnt, FGF, RA and BMP pattern distinct AP and DV cell fates during early development and how their signaling mechanisms are coordinated to concomitantly pattern AP and DV tissues.

Two myostatin genes exhibit divergent and conserved functions in grass carp (Ctenopharyngodon idellus)

Myostatin (MSTN) is an important negative regulator of myogenesis, which inhibits myoblast proliferation and differentiation. Here, we report the isolation and characterization of two mstn genes in grass carp (Ctenopharyngodon idellus). Grass carp mstn-1 and mstn-2 cDNAs are highly divergent, sharing a relatively low amino acid sequence identity of 66%. In adult fish, both orthologs are expressed in numerous tissues and they are differentially regulated during a fasting/refeeding treatments. During embryogenesis, the mRNA levels of both mstn-1 and -2 were upregulated significantly at the beginning of somitogenesis, and maintained at high levels until hatching. Using in situ hybridization, grass carp mstn-1 mRNA was found to ubiquitously express at 12hpf, with strong signals in the notochord, and in the eyes, brain and tailbud at 24hpf, and in brain and notochord at 36hpf. In comparison, the mstn-2 mRNA can be detected in the eyes, brain and notochord at 24hpf, and in the notochord and hindbrain at 36hpf. Further overexpression of mstn-1 mRNA caused a strongly ventralized phenotype by inhibiting dorsal tissue development, while injection of mstn-2 mRNA resulted in obvious embryonic abnormalities in grass carp. These results provide some new insights into the functional conservation and divergence of mstn genes in teleost species.

Simplet/Fam53b is required for Wnt signal transduction by regulating β-catenin nuclear localization

Canonical β-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of β-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating β-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating β-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two β-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of β-catenin. Conversely, overexpression of smp increases β-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the β-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for β-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating β-catenin nuclear localization: the protein colocalizes with β-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces β-catenin nuclear localization and transcriptional activity; human SMP mediates β-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human β-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of β-catenin-dependent Wnt signal transduction.

Calcium signaling in extraembryonic domains during early teleost development

It is becoming recognized that the extraembryonic domains of developing vertebrates, that is, those that make no cellular contribution to the embryo proper, act as important signaling centers that induce and pattern the germ layers and help establish the key embryonic axes. In the embryos of teleost fish, in particular, significant progress has been made in understanding how signaling activity in extraembryonic domains, such as the enveloping layer, the yolk syncytial layer, and the yolk cell, might help regulate development via a combination of inductive interactions, cellular dynamics, and localized gene expression. Ca(2+) signaling in a variety of forms that include propagating waves and standing gradients is a feature found in all three teleostean extraembryonic domains. This leads us to propose that in addition to their other well-characterized signaling activities, extraembryonic domains are well suited (due to their relative stability and continuity) to act as Ca(2+) signaling centers and conduits.

A novel chemical screening strategy in zebrafish identifies common pathways in embryogenesis and rhabdomyosarcoma development

The zebrafish is a powerful genetic model that has only recently been used to dissect developmental pathways involved in oncogenesis. We hypothesized that operative pathways during embryogenesis would also be used for oncogenesis. In an effort to define RAS target genes during embryogenesis, gene expression was evaluated in Tg(hsp70-HRAS(G12V)) zebrafish embryos subjected to heat shock. dusp6 was activated by RAS, and this was used as the basis for a chemical genetic screen to identify small molecules that interfere with RAS signaling during embryogenesis. A KRAS(G12D)-induced zebrafish embryonal rhabdomyosarcoma was then used to assess the therapeutic effects of the small molecules. Two of these inhibitors, PD98059 and TPCK, had anti-tumor activity as single agents in both zebrafish embryonal rhabdomyosarcoma and a human cell line of rhabdomyosarcoma that harbored activated mutations in NRAS. PD98059 inhibited MEK1 whereas TPCK suppressed S6K1 activity; however, the combined treatment completely suppressed eIF4B phosphorylation and decreased translation initiation. Our work demonstrates that the activated pathways in RAS induction during embryogenesis are also important in oncogenesis and that inhibition of these pathways suppresses tumor growth.

Polycyclic aromatic hydrocarbons and dibutyl phthalate disrupt dorsal-ventral axis determination via the Wnt/β-catenin signaling pathway in zebrafish embryos

The canonical Wnt/β-catenin signaling pathway is critical during early teleost development for establishing the dorsal-ventral axis. Within this pathway, GSK-3β, a key regulatory kinase in the Wnt pathway, regulates β-catenin degradation and thus the ability of β-catenin to enter nuclei, where it can activate expression of genes that have been linked to the specification of the dorsal-ventral axis. In this study, we describe the morphological abnormalities that resulted in zebrafish embryos when axis determination was disrupted by environmental contaminants. These abnormalities were linked to abnormal nuclear accumulation of β-catenin. Furthermore, we demonstrated that the developmental abnormalities and altered nuclear β-catenin accumulation occurred when embryos were exposed to commercial GSK-3β inhibitors. Zebrafish embryos were exposed to commercially available GSK-3 inhibitors (GSK-3 Inhibitor IX and 1-azakenpaullone), or common environmental contaminants (dibutyl phthalate or the polycyclic aromatic hydrocarbons phenanthrene and fluorene) from the 2 to 8-cell stage through the mid-blastula transition (MBT). These embryos displayed morphological abnormalities at 12.5 h post-fertilization (hpf) that were comparable to embryos exposed to lithium chloride (LiCl) (300 mM LiCl for 10 min, prior to the MBT), a classic disruptor of embryonic axis determination. Whole-mount immunolabeling and laser scanning confocal microscopy were used to localize β-catenin. The commercial GSK-3 Inhibitors as well as LiCl, dibutyl phthalate, fluorene and phenanthrene all induced an increase in the levels of nuclear β-catenin throughout the embryo, indicating that the morphological abnormalities were a result of disruption of Wnt/β-catenin signaling during dorsal-ventral axis specification. The ability of environmental chemicals to directly or indirectly target GSK-3β was assessed. Using Western blot analysis, the ability of these chemicals to affect enzymatic inhibitory phosphorylation at serine 9 on GSK-3β was examined, but no change in the serine phosphorylation state of GSK-3β was detected in exposed embryos. Furthermore, polycyclic aromatic hydrocarbons and dibutyl phthalate had no direct effect on the in vitro kinase activity of GSK-3β. While developmental abnormalities resulting from these axis-disrupting contaminants were linked to β-catenin accumulation in nuclei, the details of the disruption of this signaling pathway remain unknown. Since phenanthrene and fluorene as well as other hydrocarbons have been shown to disrupt axial development in sea urchin embryos, and since axis determination and the Wnt/β-catenin signaling pathway are highly conserved, we propose that these environmental contaminants may impact embryo development through a similar mechanism across phyla.

Retinoic acid signaling and the initiation of mammary gland development

Retinoic acid receptors (RARs), which are involved in retinoic acid signal transduction, are essential for maintaining the differentiated state of epithelial tissues. Mammary glands are skin appendages whose development is initiated through continuous cell-cell interactions between the ectoderm and the adjacent mesenchyme. Considerable progress has been made in elucidating the molecular basis of these interactions in mammary gland formation in mouse embryos, including the network of initiating signals comprising Fgfs, Wnts and Bmps involved in gland positioning and the transcription factors, Tbx3 and Lef1, essential for mammary gland development. Here, we provide evidence that retinoic acid signaling may also be involved in mammary gland development. We documented the expression of gene-encoding enzymes that produce retinoic acid (Raldh2) and enzymes that degrade it (Cyp26a1, Cyp26b1). We also analyzed the expression of RAR-β, a direct transcriptional target of retinoic acid signaling. Raldh2 and RAR-β were expressed in E10-E10.5 mouse embryos in somites adjacent to the flank region where mammary buds 2, 3 and 4 develop. These expression patterns overlapped with that of Fgf10, which is known to be required for mammary gland formation. RAR-β was also expressed in the mammary mesenchyme in E12 mouse embryos; RAR-β protein was expressed in the mammary epithelium and developing fat pad. Retinoic acid levels in organ cultures of E10.5 mouse embryo flanks were manipulated by adding either retinoic acid or citral, a retinoic acid synthesis inhibitor. Reduced retinoic acid synthesis altered the expression of genes involved in retinoic acid homeostasis and also demonstrated that retinoic acid signaling is required for Tbx3 expression, whereas high levels of retinoic acid signaling inhibited Bmp4 expression and repressed Wnt signaling. The results of the experiments using RNAi against Tbx3 and Wnt10b suggested feedback interactions that regulate retinoic acid homeostasis in mammary gland-forming regions. We produced a molecular model for mammary gland initiation that incorporated retinoic acid signaling.

Neural crest specification: tissues, signals, and transcription factors

The neural crest is a transient population of multipotent and migratory cells unique to vertebrate embryos. Initially derived from the borders of the neural plate, these cells undergo an epithelial to mesenchymal transition to leave the central nervous system, migrate extensively in the periphery, and differentiate into numerous diverse derivatives. These include but are not limited to craniofacial cartilage, pigment cells, and peripheral neurons and glia. Attractive for their similarities to stem cells and metastatic cancer cells, neural crest cells are a popular model system for studying cell/tissue interactions and signaling factors that influence cell fate decisions and lineage transitions. In this review, we discuss the mechanisms required for neural crest formation in various vertebrate species, focusing on the importance of signaling factors from adjacent tissues and conserved gene regulatory interactions, which are required for induction and specification of the ectodermal tissue that will become neural crest.

Ptenb mediates gastrulation cell movements via Cdc42/AKT1 in zebrafish

Phosphatidylinositol 3-kinase (PI3 kinase) mediates gastrulation cell migration in zebrafish via its regulation of PIP₂/PIP₃ balance. Although PI3 kinase counter enzyme PTEN has also been reported to be essential for gastrulation, its role in zebrafish gastrulation has been controversial due to the lack of gastrulation defects in pten-null mutants. To clarify this issue, we knocked down a pten isoform, ptenb by using anti-sense morpholino oligos (MOs) in zebrafish embryos and found that ptenb MOs inhibit convergent extension by affecting cell motility and protrusion during gastrulation. The ptenb MO-induced convergence defect could be rescued by a PI3-kinase inhibitor, LY294002 and by overexpressing dominant negative Cdc42. Overexpression of human constitutively active akt1 showed similar convergent extension defects in zebrafish embryos. We also observed a clear enhancement of actin polymerization in ptenb morphants under cofocal microscopy and in actin polymerization assay. These results suggest that Ptenb by antagonizing PI3 kinase and its downstream Akt1 and Cdc42 to regulate actin polymerization that is critical for proper cell motility and migration control during gastrulation in zebrafish.

Ancestral regulatory circuits governing ectoderm patterning downstream of Nodal and BMP2/4 revealed by gene regulatory network analysis in an echinoderm

Echinoderms, which are phylogenetically related to vertebrates and produce large numbers of transparent embryos that can be experimentally manipulated, offer many advantages for the analysis of the gene regulatory networks (GRN) regulating germ layer formation. During development of the sea urchin embryo, the ectoderm is the source of signals that pattern all three germ layers along the dorsal-ventral axis. How this signaling center controls patterning and morphogenesis of the embryo is not understood. Here, we report a large-scale analysis of the GRN deployed in response to the activity of this signaling center in the embryos of the Mediterranean sea urchin Paracentrotus lividus, in which studies with high spatial resolution are possible. By using a combination of in situ hybridization screening, overexpression of mRNA, recombinant ligand treatments, and morpholino-based loss-of-function studies, we identified a cohort of transcription factors and signaling molecules expressed in the ventral ectoderm, dorsal ectoderm, and interposed neurogenic ("ciliary band") region in response to the known key signaling molecules Nodal and BMP2/4 and defined the epistatic relationships between the most important genes. The resultant GRN showed a number of striking features. First, Nodal was found to be essential for the expression of all ventral and dorsal marker genes, and BMP2/4 for all dorsal genes. Second, goosecoid was identified as a central player in a regulatory sub-circuit controlling mouth formation, while tbx2/3 emerged as a critical factor for differentiation of the dorsal ectoderm. Finally, and unexpectedly, a neurogenic ectoderm regulatory circuit characterized by expression of "ciliary band" genes was triggered in the absence of TGF beta signaling. We propose a novel model for ectoderm regionalization, in which neural ectoderm is the default fate in the absence of TGF beta signaling, and suggest that the stomodeal and neural subcircuits that we uncovered may represent ancient regulatory pathways controlling embryonic patterning.

Nodal cis-regulatory elements reveal epiblast and primitive endoderm heterogeneity in the peri-implantation mouse embryo

Nodal, a secreted factor known for its conserved functions in cell-fate specification and the establishment of embryonic axes, is also required in mammals to maintain the pluripotency of the epiblast, the tissue that gives rise to all fetal lineages. Although Nodal is expressed as early as E3.5 in the mouse embryo, its regulation and functions at pre- and peri-implantation stages are currently unknown. Sensitive reporter transgenes for two Nodal cis-regulatory regions, the PEE and the ASE, exhibit specific expression profiles before implantation. Mutant and inhibitor studies find them respectively regulated by Wnt/β-catenin signaling and Activin/Nodal signaling, and provide evidence for localized and heterogeneous activities of these pathways in the inner cell mass, the epiblast and the primitive endoderm. These studies also show that Nodal and its prime effector, FoxH1, are not essential to preimplantation Activin/Nodal signaling. Finally, a strong upregulation of the ASE reporter in implanting blastocysts correlates with a downregulation of the pluripotency factor Nanog in the maturing epiblast. This study uncovers conservation in the mouse blastocyst of Wnt/β-catenin and Activin/Nodal-dependent activities known to govern Nodal expression and the establishment of polarity in the blastula of other deuterostomes. Our results indicate that these pathways act early on to initiate distinct cell-specification processes in the ICM derivatives. Our data also suggest that the activity of the Activin/Nodal pathway is dampened by interactions with the molecular machinery of pluripotency until just before implantation, possibly delaying cell-fate decisions in the mouse embryo.

Neural induction and factors that stabilize a neural fate

The neural ectoderm of vertebrates forms when the bone morphogenetic protein (BMP) signaling pathway is suppressed. Herein, we review the molecules that directly antagonize extracellular BMP and the signaling pathways that further contribute to reduce BMP activity in the neural ectoderm. Downstream of neural induction, a large number of "neural fate stabilizing" (NFS) transcription factors are expressed in the presumptive neural ectoderm, developing neural tube and ultimately in neural stem cells. Herein, we review what is known about their activities during normal development to maintain a neural fate and regulate neural differentiation. Further elucidation of how the NFS genes interact to regulate neural specification and differentiation should ultimately prove useful for regulating the expansion and differentiation of neural stem and progenitor cells.

Germ layer patterning in bichir and lamprey; an insight into its evolution in vertebrates

Amphibian holoblastic cleavage in which all blastomeres contribute to any one of the three primary germ layers has been widely thought to be a developmental pattern in the stem lineage of vertebrates, and meroblastic cleavage to have evolved independently in each vertebrate lineage. In extant primitive vertebrates, agnathan lamprey and basal bony fishes also undergo holoblastic cleavage, and their vegetal blastomeres have been generally thought to contribute to embryonic endoderm. However, the present marker analyses in basal ray-finned fish bichir and agnathan lamprey embryos indicated that their mesoderm and endoderm develop in the equatorial marginal zone, and their vegetal cell mass is extraembryonic nutritive yolk cells, having non-cell autonomous meso-endoderm inducing activity. Eomesodermin (eomes), but not VegT, orthologs are expressed maternally in these animals, suggesting that VegT is a maternal factor for endoderm differentiation only in amphibian. The study raises the viewpoint that the lamprey/bichir type holoblastic development would have been ancestral to extant vertebrates and retained in their stem lineage; amphibian-type holoblastic development would have been acquired secondarily, accompanied by the exploitation of new molecular machinery such as maternal VegT.

A maternal-zygotic effect gene, Zfp57, maintains both maternal and paternal imprints

The mechanisms responsible for maintaining genomic methylation imprints in mouse embryos are not understood. We generated a knockout mouse in the Zfp57 locus encoding a KRAB zinc finger protein. Loss of just the zygotic function of Zfp57 causes partial neonatal lethality, whereas eliminating both the maternal and zygotic functions of Zfp57 results in a highly penetrant embryonic lethality. In oocytes, absence of Zfp57 results in failure to establish maternal methylation imprints at the Snrpn imprinted region. Intriguingly, methylation imprints are reacquired specifically at the maternally derived Snrpn imprinted region when the zygotic Zfp57 is present in embryos. This suggests that there may be DNA methylation-independent memory for genomic imprints. Zfp57 is also required for the postfertilization maintenance of maternal and paternal methylation imprints at multiple imprinted domains. The effects on genomic imprinting are consistent with the maternal-zygotic lethality of Zfp57 mutants.

Two highly related regulatory subunits of PP2A exert opposite effects on TGF-beta/Activin/Nodal signalling

We identify Balpha (PPP2R2A) and Bdelta (PPP2R2D), two highly related members of the B family of regulatory subunits of the protein phosphatase PP2A, as important modulators of TGF-beta/Activin/Nodal signalling that affect the pathway in opposite ways. Knockdown of Balpha in Xenopus embryos or mammalian tissue culture cells suppresses TGF-beta/Activin/Nodal-dependent responses, whereas knockdown of Bdelta enhances these responses. Moreover, in Drosophila, overexpression of Smad2 rescues a severe wing phenotype caused by overexpression of the single Drosophila PP2A B subunit Twins. We show that, in vertebrates, Balpha enhances TGF-beta/Activin/Nodal signalling by stabilising the basal levels of type I receptor, whereas Bdelta negatively modulates these pathways by restricting receptor activity. Thus, these highly related members of the same subfamily of PP2A regulatory subunits differentially regulate TGF-beta/Activin/Nodal signalling to elicit opposing biological outcomes.

Diaphanous-related formin 2 and profilin I are required for gastrulation cell movements

Intensive cellular movements occur during gastrulation. These cellular movements rely heavily on dynamic actin assembly. Rho with its associated proteins, including the Rho-activated formin, Diaphanous, are key regulators of actin assembly in cellular protrusion and migration. However, the function of Diaphanous in gastrulation cell movements remains unclear. To study the role of Diaphanous in gastrulation, we isolated a partial zebrafish diaphanous-related formin 2 (zdia2) clone with its N-terminal regulatory domains. The GTPase binding domain of zDia2 is highly conserved compared to its mammalian homologues. Using a yeast two-hybrid assay, we showed that zDia2 interacts with constitutively-active RhoA and Cdc42. The zdia2 mRNAs were ubiquitously expressed during early embryonic development in zebrafish as determined by RT-PCR and whole-mount in situ hybridization analyses. Knockdown of zdia2 by antisense morpholino oligonucleotides (MOs) blocked epiboly formation and convergent extension in a dose-dependent manner, whereas ectopic expression of a human mdia gene partially rescued these defects. Time-lapse recording further showed that bleb-like cellular processes of blastoderm marginal deep marginal cells and pseudopod-/filopod-like processes of prechordal plate cells and lateral cells were abolished in the zdia2 morphants. Furthermore, zDia2 acts cell-autonomously since transplanted zdia2-knockdown cells exhibited low protrusive activity with aberrant migration in wild type host embryos. Lastly, co-injection of antisense MOs of zdia2 and zebrafish profilin I (zpfn 1), but not zebrafish profilin II, resulted in a synergistic inhibition of gastrulation cell movements. These results suggest that zDia2 in conjunction with zPfn 1 are required for gastrulation cell movements in zebrafish.

A beta-catenin-independent dorsalization pathway activated by Axin/JNK signaling and antagonized by aida

Axin is a scaffold protein that controls multiple important pathways, including the canonical Wnt pathway and JNK signaling. Here we have identified an Axin-interacting protein, Aida, which blocks Axin-mediated JNK activation by disrupting Axin homodimerization. During investigation of in vivo functions of Axin/JNK signaling and aida in development, it was found that Axin, besides ventralizing activity by facilitating beta-catenin degradation, possesses a dorsalizing activity that is mediated by Axin-induced JNK activation. This dorsalizing activity is repressed when aida is overexpressed in zebrafish embryos. Whereas Aida-MO injection leads to dorsalized embryos, JNK-MO and MKK4-MO can ventralize embryos. The anti-dorsalization activity of aida is conferred by its ability to block Axin-mediated JNK activity. We further demonstrate that dorsoventral patterning regulated by Axin/JNK signaling is independent of maternal or zygotic Wnt signaling. We have thus identified a dorsalization pathway that is exerted by Axin/JNK signaling and its inhibitor Aida during vertebrate embryogenesis.

Signaling pathways and preimplantation development of mammalian embryos

The mammalian preimplantation embryo is a critical and unique stage in embryonic development. This stage includes a series of crucial events: the transition from oocyte to embryo, the first cell divisions, and the establishment of cellular contacts. These events are regulated by multiple signal-transduction pathways. In this article we describe patterns of stage-specific expression in several signal-transduction pathways and try to give a profile of the signaling transduction network in preimplantation development of mammalian embryo.

Wnt signals provide a timing mechanism for the FGF-retinoid differentiation switch during vertebrate body axis extension

Differentiation onset in the vertebrate body axis is controlled by a conserved switch from fibroblast growth factor (FGF) to retinoid signalling,which is also apparent in the extending limb and aberrant in many cancer cell lines. FGF protects tail-end stem zone cells from precocious differentiation by inhibiting retinoid synthesis, whereas later-produced retinoic acid (RA)attenuates FGF signalling and drives differentiation. The timing of RA production is therefore crucial for the preservation of stem zone cells and the continued extension of the body axis. Here we show that canonical Wnt signalling mediates the transition from FGF to retinoid signalling in the newly generated chick body axis. FGF promotes Wnt8c expression, which persists in the neuroepithelium as FGF signalling declines. Wnt signals then act here to repress neuronal differentiation. Furthermore, although FGF inhibition of neuronal differentiation involves repression of the RA-responsive gene,retinoic acid receptor β (RARβ), Wnt signals are weaker repressors of neuron production and do not interfere with RA signal transduction. Strikingly, as FGF signals decline in the extending axis, Wnt signals now elicit RA synthesis in neighbouring presomitic mesoderm. This study identifies a directional signalling relay that leads from FGF to retinoid signalling and demonstrates that Wnt signals serve, as cells leave the stem zone, to permit and promote RA activity, providing a mechanism to control the timing of the FGF-RA differentiation switch.

Localization of Shh expression by Wnt and Eda affects axial polarity and shape of hairs

Axial patterning is a recurrent theme during embryonic development. To elucidate its fundamental principles, the hair follicle is an attractive model due to its easy accessibility and dispensability. Hair follicle asymmetry is evident from its angling and the localization of associated structures. However, axial patterning is not restricted to the follicle itself but also generates rotational hair shaft asymmetry which, for zigzag hairs, generates 3-4 bends that alternately point into opposite directions. Here we show by analyzing mutant and transgenic mice that WNT and ectodysplasin signaling are involved in the control of the molecular and morphological asymmetry of the follicle and the associated hair shaft, respectively. Asymmetry is affected by polarized WNT and ectodysplasin signaling in mature hair follicles. When endogenous signaling is impaired, molecular asymmetry is lost and mice no longer form zigzag hairs. Both signaling pathways affect the polarized expression of Shh which likely functions as a directional reference for hair shaft production in all follicles. We propose that this regulatory pathway also establishes follicular asymmetry during morphogenesis. Moreover, the identified molecular hierarchy offers a model for the periodic patterning of zigzag hairs mechanistically similar to mesodermal segmentation.

Hedgehog Signaling Pathway Database: a repository of current annotation efforts and resources for the Hh research community

The Hedgehog Signaling Pathway Database is a curated repository of information pertaining to the Hedgehog developmental pathway. It was designed to provide centralized access to a wide range of relevant information in an organism-agnostic manner. Data are provided for all genes and gene targets known to be involved in the Hh pathway across various organisms. The data provided include DNA and protein sequences as well as domain structure motifs. All known human diseases associated with the Hh pathway are indexed including experimental data on therapeutic agents and their molecular targets. Hh researchers will find useful information on relevant protocols, tissue cell lines and reagents used in current Hh research projects. Curated content is also provided for publications, grants and patents relating to the Hh pathway. The database can be accessed at .

Symmetry breaking and convergent extension in early chordate development

The initiation of axis, polarity, cell differentiation, and gastrulation in the very early chordate development is due to the breaking of radial symmetry. It is believed that this occurs by an external signal. We suggest instead spontaneous symmetry breaking through the agency of the Turing-Child field. Increased size or decreased diffusivity, both brought about by mitotic activity, cause the spontaneous loss of stability of the homogeneous state and the evolution of the metabolic pattern during development. The polar metabolic pattern is the cause of polar gene expression, polar morphogenesis (gastrulation), and polar mitotic activity. The Turing-Child theory explains not only the spontaneous formation of the invagination in gastrulation but also the coherent cell movement observed in convergence and extension during gastrulation and neurulation. The theory is demonstrated with respect to experimental observations on the early development of fish, amphibian, and the chick. The theory can explain a multitude of experimental details. For example, it explains the splayed polar progression of reduction in the fish blastoderm. Reduction starts on that side of the blastoderm margin, which will initiate invagination several hours later. It progresses toward the blastoderm center and somewhat laterally from this future "dorsal lip". This is precisely as predicted by a Turing-Child system in a circle. And for a fish like zebrafish with a blastoderm that is slightly oval, reduction is observed to progress along the long axis of the ellipse, which is what Turing-Child theory predicts. In general the shape and the chemical nature of the experimental patterns are the same as predicted by the Turing couple (cAMP, ATP). Embryological polarity and convergent extension are based on polar eigenfunction and saddle-shaped eigenfunction, respectively.

mED2--a novel gene involved in mouse embryonic development

Dissection of new genes underlying embryonic development is important for our understanding of the molecular mechanism of vertebrate embryonic development. In this study, the expression pattern and functional analysis of a new gene, called mED2, originally cloned from mouse embryos using subtractive hybridization was reported. mED2 expression patterns were characterized by RT-PCR-Southern hybridization and in situ hybridization. The results showed that mED2 was mainly expressed in the embryonic nervous system and mesoderm-derived tissues and its expression varied depending on the embryonic developmental stages. The knockdown of mED2 activity by antisense RNA injection inhibited zygote cleavage and blastocyst formation during pre-implantation in mice. Subcellular localization of mED2-eGFP fusion protein revealed a pattern of nuclear membrane and juxta-/perinuclear location such as in the rough endoplasmic reticulum and Golgi apparatus. This finding was supported by bioinformatics analysis, which indicated mED2 protein to be a transmembrane protein with partial homology to the thioredoxin family of proteins. It is inferred that mED2 gene can probably take part in early embryonic development in mouse and may be involved in target protein posttranslational modification, turnover, folding, and stability at the endoplasmic reticulum and/or the Golgi apparatus.

Maternal control of vertebrate dorsoventral axis formation and epiboly by the POU domain protein Spg/Pou2/Oct4

Dorsoventral (DV) axis formation of the vertebrate embryo is controlled by the maternal genome and is subsequently refined zygotically. In the zygote, repression of ventralizing Bmp activity on the dorsal side through chordin and noggin is crucial for establishment of a dorsally located organizer. This interplay generates a zygotic Bmp activity gradient that defines distinct positional values along the DV axis. The maternal processes that control expression of the zygotic genes implicated in DV patterning are largely unknown. spiel-ohne-grenzen (spg/pou2) is a maternally and zygotically expressed zebrafish gene that encodes the POU domain transcription factor Pou2, an ortholog of mammalian Oct4/Pou5f1. We show that embryos that are genetically depleted of both maternal and zygotic pou2 function (MZspg) exhibit extreme DV patterning defects and, independently, a blastoderm-specific arrest of epiboly. Dorsal tissues expand to the ventral side at the expense of ventrolateral tissue in MZspg embryos. Dorsally expressed Bmp-antagonists, such as Chd and Nog1, and Gsc are ectopically activated at ventral levels in MZspg. Lack of ventral specification is apparent very early, suggesting that maternal processes are affected in MZspg. Indeed, maternal pou2 function is necessary to initiate zygotic expression of ventrally expressed genes such as bmp2b and bmp4, and for proper activation of bmp7, vox, vent and eve1. A constitutively active Alk8-TGFbeta-receptor can ectopically induce bmp2b and bmp4 and rescues the dorsalization of MZspg. This indicates that pou2 acts upstream of Alk8, a maternally provided receptor implicated in the activation of zygotic bmp2b and bmp4 transcription. Consistent with this possibility, Bmp gene misexpression can rescue MZspg embryos, indicating that TGFbeta-mediated signal transduction itself is intact in absence of Pou2. Inhibition of Fgf signaling, another pathway with early dorsalizing activity, can also restore and even ventralize MZspg embryos. The requirement for pou2 to initiate bmp2b expression can therefore be bypassed by releasing the repressive function of Fgf signaling upon bmp2b transcription. In transplantation experiments, we find that dorsalized cells from prospective ventrolateral regions of MZspg embryos are non cell-autonomously respecified to a ventral fate within wild-type host embryos. Analysis of pou2 mRNA injected MZspg embryos shows that pou2 is required on the ventral side of cleavage stage embryos. Based on the maternal requirement for pou2 in ventral specification, we propose that ventral specification employs an active, pou2-dependent maternal induction step, rather than a default ventralizing program.

Identification of regulators of germ layer morphogenesis using proteomics in zebrafish

During vertebrate gastrulation, a well-orchestrated series of morphogenetic changes leads to the formation of the three germ layers: the ectoderm, mesoderm and endoderm. The analysis of gene expression patterns during gastrulation has been central to the identification of genes involved in germ layer formation. However, many proteins are regulated on a translational or post-translational level and are thus undetectable by gene expression analysis. Therefore, we developed a 2D-gel-based comparative proteomic approach to target proteins involved in germ layer morphogenesis during zebrafish gastrulation. Proteomes of ectodermal and mesendodermal progenitor cells were compared and 35 significantly regulated proteins were identified by mass spectrometry, including several proteins with predicted functions in cytoskeletal organization. A comparison of our proteomic results with data obtained in an accompanying microarray-based gene expression analysis revealed no significant overlap, confirming the complementary nature of proteomics and transcriptomics. The regulation of ezrin2, which was identified based on a reduction in spot intensity in mesendodermal cells, was independently validated. Furthermore, we show that ezrin2 is activated by phosphorylation in mesendodermal cells and is required for proper germ layer morphogenesis. We demonstrate the feasibility of proteomics in zebrafish, concluding that proteomics is a valuable tool for analysis of early development.

Specification of the primitive myeloid precursor pool requires signaling through Alk8 in zebrafish

In the zebrafish embryo, primitive hematopoiesis initiates in two spatially distinct regions. Rostrally, the cells of the anterior lateral plate mesoderm (ALPM) give rise exclusively to cells of the myeloid lineage in a pu.1-dependent manner. Caudally, in the posterior lateral plate mesoderm (PLPM), the expression of gata1 defines a precursor pool that gives rise predominantly to the embryonic erythrocytes. The transcription factor scl acts upstream of both gata1 and pu.1 in these precursor pools, activating a series of conserved transcription factors that cell-autonomously specify either myeloid or erythroid fates. However, the mechanisms underlying the spatial separation of the hematopoietic precursor pools and the induction of differential gene expression within these pools are not well understood. We show here that the Bmp receptor lost-a-fin/alk8 is required for rostral pu.1 expression and myelopoiesis, identifying an early genetic event that distinguishes between the induction of anterior and posterior hematopoiesis. Introducing a constitutively active version of the Alk8 receptor led to increased pu.1 expression, but the role of alk8 was independent of the scl-dependent cell-fate pathway. Furthermore, the role of Alk8 in myelopoiesis was genetically separable from its earlier role in dorsal-ventral embryonic patterning.

Phylogenomic analysis of vertebrate thrombospondins reveals fish-specific paralogues, ancestral gene relationships and a tetrapod innovation

BACKGROUND

Thrombospondins (TSPs) are evolutionarily-conserved, extracellular, calcium-binding glycoproteins with important roles in cell-extracellular matrix interactions, angiogenesis, synaptogenesis and connective tissue organisation. Five TSPs, designated TSP-1 through TSP-5, are encoded in the human genome. All but one have known roles in acquired or inherited human diseases. To further understand the roles of TSPs in human physiology and pathology, it would be advantageous to extend the repertoire of relevant vertebrate models. In general the zebrafish is proving an excellent model organism for vertebrate biology, therefore we set out to evaluate the status of TSPs in zebrafish and two species of pufferfish.

RESULTS

We identified by bioinformatics that three fish species encode larger numbers of TSPs than vertebrates, yet all these sequences group as homologues of TSP-1 to -4. By phylogenomic analysis of neighboring genes, we uncovered that, in fish, a TSP-4-like sequence is encoded from the gene corresponding to the tetrapod TSP-5 gene. Thus, all TSP genes show conservation of synteny between fish and tetrapods. In the human genome, the TSP-1, TSP-3, TSP-4 and TSP-5 genes lie within paralogous regions that provide insight into the ancestral genomic context of vertebrate TSPs.

CONCLUSION

A new model for TSP evolution in vertebrates is presented. The TSP-5 protein sequence has evolved rapidly from a TSP-4-like sequence as an innovation in the tetrapod lineage. TSP biology in fish is complicated by the presence of additional lineage- and species-specific TSP paralogues. These novel results give deeper insight into the evolution of TSPs in vertebrates and open new directions for understanding the physiological and pathological roles of TSP-4 and TSP-5 in humans.

Knockdown of zebrafish crim1 results in a bent tail phenotype with defects in somite and vascular development

The Crim1 gene encodes a transmembrane protein containing six cysteine-rich repeats similar to those found in the BMP antagonist, chordin (chd). To investigate its physiological role, zebrafish crim1 was cloned and shown to be both maternally and zygotically expressed during zebrafish development in sites including the vasculature, intermediate cell mass, notochord, and otic vesicle. Bent or hooked tails with U-shaped somites were observed in 85% of morphants from 12 hpf. This was accompanied by a loss of muscle pioneer cells. While morpholino knockdown of crim1 showed some evidence of ventralisation, including expansion of the intermediate cell mass (ICM), reduction in head size bent tails and disruption to the somites and notochord, this did not mimic the classically ventralised phenotype, as assessed by the pattern of expression of the dorsal markers chordin, otx2 and the ventral markers eve1, pax2.1, tal1 and gata1 between 75% epiboly and six-somites. From 24 hpf, morphants displayed an expansion of the ventral mesoderm-derived ICM, as evidenced by expansion of tal1, lmo2 and crim1 itself. Analysis of the crim1 morphant phenotype in Tg(fli:EGFP) fish showed a clear reduction in the endothelial cells forming the intersegmental vessels and a loss of the dorsal longitudinal anastomotic vessel (DLAV). Hence, the primary role of zebrafish crim1 is likely to be the regulation of somitic and vascular development.