A role for notochord in axial vascular development revealed by analysis of phenotype and the expression of VEGR-2 in zebrafish flh and ntl mutant embryos

Molecular Characterization of U6 Promoters from Orange-Spotted Grouper (Epinephelus coioides) and Its Application in DNA Vector-Based RNAi Technology

The U6 promoter, a typical RNA polymerase III promoter, is widely used to transcribe small RNAs in vector-based siRNA systems. The RNAi efficiency is mainly dependent on the transcriptional activity of the U6 promoter. However, studies have found that U6 promoters isolated from some fishes do not work well in distantly related species. To isolate a U6 promoter with high transcriptional efficiency from fish, in this study, we cloned five U6 promoters in orange-spotted grouper, of which only the grouper U6-1 (GU6-1) promoter contains the OCT element in the distant region. Functional studies revealed that the GU6-1 promoter has high transcriptional ability, which could efficiently transcribe shRNA and result in target gene knockdown in vitro and in vivo. Subsequently, the deletion or mutation of the OCT motif resulted in a significant decrease in promoter transcriptional activity, demonstrating that the OCT element plays an important role in enhancing the grouper U6 promoter transcription. Moreover, the transcriptional activity of the GU6-1 promoter showed little species specificity. It not only works in the grouper but also possesses high transcriptional activity in the zebrafish. Knockdown of the mstn gene in zebrafish and grouper through shRNA driven by the GU6-1 promoter could promote fish growth, suggesting that the GU6-1 promoter can be used as a potential molecular tool in aquaculture practice.

Oxamyl exerts developmental toxic effects in zebrafish by disrupting the mitochondrial electron transport chain and modulating PI3K/Akt and p38 Mapk signaling

Oxamyl, a carbamate insecticide, is mainly used to control nematodes in the agricultural field. Although oxamyl is a widely used insecticide that is associated with ecological concerns, limited studies have examined the toxic effects of oxamyl on the developmental stage and the underlying mechanisms. In this study, the developmental toxicity of oxamyl was demonstrated using zebrafish, which is a representative model as it is associated with rapid embryogenesis and a toxic response similar to that of other vertebrates. The morphological alteration of zebrafish larvae was analyzed to confirm the sub-lethal toxicity of oxamyl. Analysis of transgenic zebrafish (olig2:dsRED and flk1:eGFP line) and mRNA levels of genes associated with individual organ development revealed that oxamyl exerted toxic effects on the development of neuron, notochord, and vascular system. Next, the adverse effect of oxamyl on the mitochondrial electron transport chain was examined. Treatment with oxamyl altered the PI3K/Akt signaling and p38 Mapk signaling pathways in zebrafish. Thus, this study elucidated the mechanisms underlying the developmental toxicity of oxamyl and provided information on the parameters to assess the developmental toxicity of other environmental contaminants.

Glomerular Endothelial Cell-Derived miR-200c Impairs Glomerular Homeostasis by Targeting Podocyte VEGF-A

Deciphering the pathophysiological mechanisms of primary podocytopathies that can lead to end-stage renal disease and increased mortality is an unmet need. Studying how microRNAs (miRs) interfere with various signaling pathways enables identification of pathomechanisms, novel biomarkers and potential therapeutic options. We investigated the expression of miR-200c in urine from patients with different renal diseases as a potential candidate involved in podocytopathies. The role of miR-200c for the glomerulus and its potential targets were studied in cultured human podocytes, human glomerular endothelial cells and in the zebrafish model. miR-200c was upregulated in urine from patients with minimal change disease, membranous glomerulonephritis and focal segmental glomerulosclerosis and also in transforming growth factor beta (TGF-β) stressed glomerular endothelial cells, but not in podocytes. In zebrafish, miR-200c overexpression caused proteinuria, edema, podocyte foot process effacement and glomerular endotheliosis. Although zinc finger E-Box binding homeobox 1/2 (ZEB1/2), important in epithelial to mesenchymal transition (EMT), are prominent targets of miR-200c, their downregulation did not explain our zebrafish phenotype. We detected decreased vegfaa/bb in zebrafish overexpressing miR-200c and could further prove that miR-200c decreased VEGF-A expression and secretion in cultured human podocytes. We hypothesize that miR-200c is released from glomerular endothelial cells during cell stress and acts in a paracrine, autocrine, as well as context-dependent manner in the glomerulus. MiR-200c can cause glomerular damage most likely due to the reduction of podocyte VEGF-A. In contrast, miR-200c might also influence ZEB expression and therefore EMT, which might be important in other conditions. Therefore, we propose that miR-200c-mediated effects in the glomerulus are context-sensitive.

Somite morphogenesis is required for axial blood vessel formation during zebrafish embryogenesis

Angioblasts that form the major axial blood vessels of the dorsal aorta and cardinal vein migrate toward the embryonic midline from distant lateral positions. Little is known about what controls the precise timing of angioblast migration and their final destination at the midline. Using zebrafish, we found that midline angioblast migration requires neighboring tissue rearrangements generated by somite morphogenesis. The somitic shape changes cause the adjacent notochord to separate from the underlying endoderm, creating a ventral midline cavity that provides a physical space for the angioblasts to migrate into. The anterior to posterior progression of midline angioblast migration is facilitated by retinoic acid-induced anterior to posterior somite maturation and the subsequent progressive opening of the ventral midline cavity. Our work demonstrates a critical role for somite morphogenesis in organizing surrounding tissues to facilitate notochord positioning and angioblast migration, which is ultimately responsible for creating a functional cardiovascular system.

Endothelial Cell Dynamics in Vascular Development: Insights From Live-Imaging in Zebrafish

The formation of the vertebrate vasculature involves the acquisition of endothelial cell identities, sprouting, migration, remodeling and maturation of functional vessel networks. To understand the cellular and molecular processes that drive vascular development, live-imaging of dynamic cellular events in the zebrafish embryo have proven highly informative. This review focusses on recent advances, new tools and new insights from imaging studies in vascular cell biology using zebrafish as a model system.

Developmental toxicity of fipronil in early development of zebrafish (Danio rerio) larvae: Disrupted vascular formation with angiogenic failure and inhibited neurogenesis

Fipronil has been widely used in agriculture to prevent aggressive insects from damaging agricultural products. Fipronil residues circulate in the environment and they have been detected in non-targeted organisms in aquatic environments. To study the effect of fipronil toxicity on environmental health, 6 h post fertilization (hpf) zebrafish embryos were treated with fipronil for 72 h. LC₅₀ value was obtained by applying varying concentrations of fipronil to zebrafish embryos for 72 h. As zebrafish embryos are useful vertebrate models for studying developmental and genetic findings in toxicology research, they were exposed to fipronil to study detailed elucidating mechanisms with hazardous end points of toxicity. Cell cycle arrest-related apoptosis supported pathological alterations, such as increased mortality, shortened body length, and reduced hatchability. Furthermore, observed heart defects, including edema and irregular heartbeat were caused due to abnormal blood circulation. In transgenic zebrafish models (fli1:eGFP and olig2:dsRED), disrupted blood vessel formations were indicated by eGFP⁺ endothelial cells. Moreover, neurogenic defects were observed by studying dsRED⁺ motor neurons and oligodendrocytes. This study demonstrates fipronil accumulation in aquatic environment and its ability to impair essential processes, such as angiogenesis and neurogenesis during early developmental stage of zebrafish, along with general developmental toxicity.

Quantitative assessment and clinical relevance of VEGFRs-positive tumor cells in refractory brain tumors

Vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR)1 and 2 signaling is a potent activator of tumor angiogenesis. Although the expressions of VEGFR1 and VEGFR2 were initially thought to be limited to the endothelial cells, it is now known that both the receptors are expressed in tumor cells. This is the first study wherein VEGFRs-positive tumor cells are quantitatively evaluated for brain tumors with upregulated VEGF/VEGFR signaling. The percentage of VEGFRs-positive tumor cells was quantitatively evaluated in various brain tumors (10 glioblastomas, 22 neurofibromatosis type 2 [NF2]-related schwannomas, 21 sporadic schwannomas, 27 chordomas, 36 meningiomas, 29 hemangioblastomas, 11 hemangiopericytoma, and 13 ependymomas) using immunohistochemistry. VEGF-A expression was also analyzed using quantitative real-time polymerase chain reaction. Double immunofluorescence staining using anti-PDGFR-β and anti-CD34 antibody, microvessel density, and vessel diameter were analyzed to evaluate the vascular characteristics. Chordomas demonstrated an extremely higher percentage of VEGFR1 and VEGFR2-positive tumor cells than other tumors. In contrast, meningiomas and hemangiopericytomas showed few VEGFRs-positive tumor cells. The percentage of positive tumor cells in chordomas, hemangioblastomas, and NF2 schwannomas was associated with clinical courses, such as shorter progression free survival, and growth speed. Glioblastomas and NF2 schwannomas showed larger tumor vessels without pericyte coverage. The present study is the first to quantitatively analyze VEGFR1- and VEGFR2- positive tumor cells in various types of refractory brain tumors. This novel parameter significantly correlated with the progressive clinical courses.

Prognostic significance of VEGF receptors expression on the tumor cells in skull base chordoma

BACKGROUND

Chordoma is a rare refractory neoplasm that arises from the embryological remnants of the notochord, which is incurable using any multimodality therapy. Vascular endothelial growth factor (VEGF) is a potent activator of angiogenesis that is strongly associated with the tumor-immune microenvironment. These factors have not been elucidated for chordomas.

METHODS

To evaluate the characteristics of vascular and tumor cells in chordoma, we first analyzed the expression of VEGF receptor (VEGFR) 1, VEGFR2, CD34, and Brachyury in a cell line and 54 tumor tissues. Patients with primary skull base chordomas were divided into the following two groups as per the tumor growth rate: patients with slow progression (SP: < 3 mm/year) and those with rapid progression (RP: ≥ 3 mm/year). Thus, the expressions of VEGF-A, VEGFR 1, and VEGFR2 on tumor cells; tumor infiltrative immune cells, including regulatory T cells (Tregs) and tumor-associated macrophages (TAMs); and immune-checkpoint molecules (PD-1/PD-L1) were analyzed with the clinical courses, especially in a comparison between the two groups.

RESULTS

In chordomas, both VEGFR1 and VEGFR2 were strongly expressed not only on vascular endothelial cells, but also on tumor cells. The recurrent cases showed significantly higher VEGFR1 expressions on tumor cells than the primary cases. The expression of VEGF-A was significantly higher in RP than that in SP group. The numbers of CD163+ TAMs and Foxp3+ Tregs were higher in RP than that in SP group.

CONCLUSIONS

Expression of VEGFR1 and VEGFR2 on tumor cells and immunosuppressive tumor-microenvironment were related to tumor growth in patients with chordomas.

Gfi1aa and Gfi1b set the pace for primitive erythroblast differentiation from hemangioblasts in the zebrafish embryo

The transcriptional repressors Gfi1(a) and Gfi1b are epigenetic regulators with unique and overlapping roles in hematopoiesis. In different contexts, Gfi1 and Gfi1b restrict or promote cell proliferation, prevent apoptosis, influence cell fate decisions, and are essential for terminal differentiation. Here, we show in primitive red blood cells (prRBCs) that they can also set the pace for cellular differentiation. In zebrafish, prRBCs express 2 of 3 zebrafish Gfi1/1b paralogs, Gfi1aa and Gfi1b. The recently identified zebrafish gfi1aa gene trap allele qmc551 drives erythroid green fluorescent protein (GFP) instead of Gfi1aa expression, yet homozygous carriers have normal prRBCs. prRBCs display a maturation defect only after splice morpholino-mediated knockdown of Gfi1b in gfi1aa qmc551 homozygous embryos. To study the transcriptome of the Gfi1aa/1b double-depleted cells, we performed an RNA-Seq experiment on GFP-positive prRBCs sorted from 20-hour-old embryos that were heterozygous or homozygous for gfi1aa qmc551 , as well as wt or morphant for gfi1b We subsequently confirmed and extended these data in whole-mount in situ hybridization experiments on newly generated single- and double-mutant embryos. Combined, the data showed that in the absence of Gfi1aa, the synchronously developing prRBCs were delayed in activating late erythroid differentiation, as they struggled to suppress early erythroid and endothelial transcription programs. The latter highlighted the bipotent nature of the progenitors from which prRBCs arise. In the absence of Gfi1aa, Gfi1b promoted erythroid differentiation as stepwise loss of wt gfi1b copies progressively delayed Gfi1aa-depleted prRBCs even further, showing that Gfi1aa and Gfi1b together set the pace for prRBC differentiation from hemangioblasts.

The zebrafish: A fintastic model for hematopoietic development and disease

Hematopoiesis is a complex process with a variety of different signaling pathways influencing every step of blood cell formation from the earliest precursors to final differentiated blood cell types. Formation of blood cells is crucial for survival. Blood cells carry oxygen, promote organ development and protect organs in different pathological conditions. Hematopoietic stem and progenitor cells (HSPCs) are responsible for generating all adult differentiated blood cells. Defects in HSPCs or their downstream lineages can lead to anemia and other hematological disorders including leukemia. The zebrafish has recently emerged as a powerful vertebrate model system to study hematopoiesis. The developmental processes and molecular mechanisms involved in zebrafish hematopoiesis are conserved with higher vertebrates, and the genetic and experimental accessibility of the fish and the optical transparency of its embryos and larvae make it ideal for in vivo analysis of hematopoietic development. Defects in zebrafish hematopoiesis reliably phenocopy human blood disorders, making it a highly attractive model system to screen small molecules to design therapeutic strategies. In this review, we summarize the key developmental processes and molecular mechanisms of zebrafish hematopoiesis. We also discuss recent findings highlighting the strengths of zebrafish as a model system for drug discovery against hematopoietic disorders. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Vertebrate Organogenesis > Musculoskeletal and Vascular Nervous System Development > Vertebrates: Regional Development Comparative Development and Evolution > Organ System Comparisons Between Species.

Utilizing Zebrafish to Identify Anti-(Lymph)Angiogenic Compounds for Cancer Treatment: Promise and Future Challenges

Cancer metastasis which predominantly occurs through blood and lymphatic vessels, is the leading cause of death in cancer patients. Consequently, several anti-angiogenic agents have been approved as therapeutic agents for human cancers such as metastatic renal cell carcinoma. Also, anti-lymphangiogenic drugs such as monoclonal antibodies VGX-100 and IMC-3C5 have undergone phase I clinical trials for advanced and metastatic solid tumors. Although anti-tumor-associated angiogenesis has proven to be a promising therapeutic strategy for human cancers, this approach is fraught with toxicities and development of drug resistance. This emphasizes the need for alternative anti-(lymph)angiogenic drugs. The use of zebrafish has become accepted as an established model for high-throughput screening, vascular biology, and cancer research. Importantly, various zebrafish transgenic lines have now been generated that can readily discriminate different vascular compartments. This now enables detailed in vivo studies that are relevant to both human physiological and tumor (lymph)angiogenesis to be conducted in zebrafish. This review highlights recent advancements in the zebrafish anti-vascular screening platform and showcases promising new anti-(lymph)angiogenic compounds that have been derived from this model. In addition, this review discusses the promises and challenges of the zebrafish model in the context of anti-(lymph)angiogenic compound discovery for cancer treatment.

Genome editing of factor X in zebrafish reveals unexpected tolerance of severe defects in the common pathway

Deficiency of factor X (F10) in humans is a rare bleeding disorder with a heterogeneous phenotype and limited therapeutic options. Targeted disruption of F10 and other common pathway factors in mice results in embryonic/neonatal lethality with rapid resorption of homozygous mutants, hampering additional studies. Several of these mutants also display yolk sac vascular defects, suggesting a role for thrombin signaling in vessel development. The zebrafish is a vertebrate model that demonstrates conservation of the mammalian hemostatic and vascular systems. We have leveraged these advantages for in-depth study of the role of the coagulation cascade in the developmental regulation of hemostasis and vasculogenesis. In this article, we show that ablation of zebrafish f10 by using genome editing with transcription activator-like effector nucleases results in a major embryonic hemostatic defect. However, widespread hemorrhage and subsequent lethality does not occur until later stages, with absence of any detectable defect in vascular development. We also use f10^-/- zebrafish to confirm 5 novel human F10 variants as causative mutations in affected patients, providing a rapid and reliable in vivo model for testing the severity of F10 variants. These findings as well as the prolonged survival of f10^-/- mutants will enable us to expand our understanding of the molecular mechanisms of hemostasis, including a platform for screening variants of uncertain significance in patients with F10 deficiency and other coagulation disorders. Further study as to how fish tolerate what is an early lethal mutation in mammals could facilitate improvement of diagnostics and therapeutics for affected patients with bleeding disorders.

Bipotent progenitors as embryonic origin of retinal stem cells

In lower vertebrates, retinal stem cells (RSCs) capable of producing all retinal cell types are a resource for retinal tissue growth throughout life. However, the embryonic origin of RSCs remains largely elusive. Using a Zebrabow-based clonal analysis, we characterized the RSC niche in the ciliary marginal zone of zebrafish retina and illustrate that blood vessels associated with RSCs are required for the maintenance of actively proliferating RSCs. Full lineage analysis of RSC progenitors reveals lineage patterns of RSC production. Moreover, in vivo lineage analysis demonstrates that these RSC progenitors are the direct descendants of a set of bipotent progenitors in the medial epithelial layer of developing optic vesicles, suggesting the involvement of the mixed-lineage states in the RSC lineage specification.

The notochord: structure and functions

The notochord is an embryonic midline structure common to all members of the phylum Chordata, providing both mechanical and signaling cues to the developing embryo. In vertebrates, the notochord arises from the dorsal organizer and it is critical for proper vertebrate development. This evolutionary conserved structure located at the developing midline defines the primitive axis of embryos and represents the structural element essential for locomotion. Besides its primary structural function, the notochord is also a source of developmental signals that patterns surrounding tissues. Among the signals secreted by the notochord, Hedgehog proteins play key roles during embryogenesis. The Hedgehog signaling pathway is a central regulator of embryonic development, controlling the patterning and proliferation of a wide variety of organs. In this review, we summarize the current knowledge on notochord structure and functions, with a particular emphasis on the key developmental events that take place in vertebrates. Moreover, we discuss some genetic studies highlighting the phenotypic consequences of impaired notochord development, which enabled to understand the molecular basis of different human congenital defects and diseases.

The hormonal peptide Elabela guides angioblasts to the midline during vasculogenesis

A key step in the de novo formation of the embryonic vasculature is the migration of endothelial precursors, the angioblasts, to the position of the future vessels. To form the first axial vessels, angioblasts migrate towards the midline and coalesce underneath the notochord. Vascular endothelial growth factor has been proposed to serve as a chemoattractant for the angioblasts and to regulate this medial migration. Here we challenge this model and instead demonstrate that angioblasts rely on their intrinsic expression of Apelin receptors (Aplr, APJ) for their migration to the midline. We further show that during this angioblast migration Apelin receptor signaling is mainly triggered by the recently discovered ligand Elabela (Ela). As neither of the ligands Ela or Apelin (Apln) nor their receptors have previously been implicated in regulating angioblast migration, we hereby provide a novel mechanism for regulating vasculogenesis, with direct relevance to physiological and pathological angiogenesis.

DOI:http://dx.doi.org/10.7554/eLife.06726.001

The circulatory system enables blood to move around the body and deliver substances including nutrients and oxygen to the cells that need them. In the embryos of animals with a backbone, blood flows from the heart through the aorta into branching smaller vessels to the cells. The blood then gets collected by progressively bigger vessels and flows back to the heart via the cardinal vein. The cells that make up these blood vessels develop from cells called angioblasts—but first, during development these angioblasts must move to the place where the vessels will form.

A protein called Vascular endothelial growth factor (VEGF) had been suggested to help guide and align the angioblasts as the embryo develops. Now, Helker, Schuermann et al. have examined developing zebrafish embryos using new technologies. This revealed that VEGF is in fact not essential for the dorsal aorta and cardinal vein to develop. Instead, the angioblasts only move to the correct part of the embryo if they can produce the Apelin receptor protein, which forms part of a signaling pathway.

There are two hormones—called Apelin and Elabela—that can bind to and activate the Apelin receptor. Helker, Schuermann et al. show that Elabela alone is needed to guide the angioblasts to the right part of the embryo during blood vessel development. However, in embryos where there is not enough Elabela, the Apelin hormone can compensate for this deficiency and the first blood vessels will later develop correctly. Future research will address whether this signaling pathway not only guides angioblasts to establish a circulatory system, but also guides blood vessel growth. As blood vessel growth is very relevant to human disease, identifying the mechanisms that regulate it will have an impact on biomedical research.

DOI:http://dx.doi.org/10.7554/eLife.06726.002

Shape and position of the node and notochord along the bilateral plane of symmetry are regulated by cell-extracellular matrix interactions

The node and notochord (and their equivalents in other species) are essential signaling centers, positioned along the plane of bilateral symmetry in developing vertebrate embryos. However, genes and mechanisms regulating morphogenesis of these structures and their placement along the embryonic midline are not well understood. In this work, we provide the first evidence that the position of the node and the notochord along the bilateral plane of symmetry are under genetic control and are regulated by integrin α5β1 and fibronectin in mice. We found that the shape of the node is often inverted in integrin α5-null and fibronectin-null mutants, and that the positioning of node and the notochord is often skewed away from the perceived plane of embryonic bilateral of symmetry. Our studies also show that the shape and position of the notochord are dependent on the shape and embryonic placement of the node. Our studies suggest that fibronectin regulates the shape of the node by affecting apico-basal polarity of the nodal cells. Taken together, our data indicate that cell–extracellular matrix interactions mediated by integrin α5β1 and fibronectin regulate the geometry of the node as well as the placement of the node and notochord along the plane of bilateral symmetry in the mammalian embryo.

Vascular development in the zebrafish

The zebrafish has emerged as an excellent vertebrate model system for studying blood and lymphatic vascular development. The small size, external and rapid development, and optical transparency of zebrafish embryos are some of the advantages the zebrafish model system offers. Multiple well-established techniques have been developed for imaging and functionally manipulating vascular tissues in zebrafish embryos, expanding on and amplifying these basic advantages and accelerating use of this model system for studying vascular development. In the past decade, studies performed using zebrafish as a model system have provided many novel insights into vascular development. In this article we discuss the amenability of this model system for studying blood vessel development and review contributions made by this system to our understanding of vascular development.

Cranial vasculature in zebrafish forms by angioblast cluster-derived angiogenesis

Formation of embryonic vasculature involves vasculogenesis as endothelial cells differentiate and aggregate into vascular cords and angiogenesis which includes branching from the existing vessels. In the zebrafish which has emerged as an advantageous model to study vasculogenesis, cranial vasculature is thought to originate by a combination of vasculogenesis and angiogenesis, but how these processes are coordinated is not well understood. To determine how angioblasts assemble into cranial vasculature, we generated an etsrp:GFP transgenic line in which GFP reporter is expressed under the promoter control of an early regulator of vascular and myeloid development, etsrp/etv2. By utilizing time-lapse imaging we show that cranial vessels originate by angiogenesis from angioblast clusters, which themselves form by the mechanism of vasculogenesis. The two major pairs of bilateral clusters include the rostral organizing center (ROC) which gives rise to the most rostral cranial vessels and the midbrain organizing center (MOC) which gives rise to the posterior cranial vessels and to the myeloid and endocardial lineages. In Etsrp knockdown embryos initial cranial vasculogenesis proceeds normally but endothelial and myeloid progenitors fail to initiate differentiation, migration and angiogenesis. Such angioblast cluster-derived angiogenesis is likely to be involved during vasculature formation in other vertebrate systems as well.

Hedgehog signaling induces arterial endothelial cell formation by repressing venous cell fate

In vertebrate embryos, the dorsal aorta and the posterior cardinal vein form in the trunk to comprise the original circulatory loop. Previous studies implicate Hedgehog (Hh) signaling in the development of the dorsal aorta. However, the mechanism controlling specification of artery versus vein remains unclear. Here, we investigated the cell-autonomous mechanism of Hh signaling in angioblasts (endothelial progenitor cells) during arterial-venous specification utilizing zebrafish mutations in Smoothened (Smo), a G protein-coupled receptor essential for Hh signaling. smo mutants exhibit an absence of the dorsal aorta accompanied by a reciprocal expansion of the posterior cardinal vein. The increased number of venous cells is equivalent to the loss of arterial cells in embryos with loss of Smo function. Activation of Hh signaling expands the arterial cell population at the expense of venous cell fate. Time-lapse imaging reveals two sequential waves of migrating progenitor cells that contribute to the dorsal aorta and the posterior cardinal vein, respectively. Angioblasts deficient in Hh signaling fail to contribute to the arterial wave; instead, they all migrate medially as a single population to form the venous wave. Cell transplantation analyses demonstrate that Smo plays a cell-autonomous role in specifying angioblasts to become arterial cells, and Hh signaling-depleted angioblasts differentiate into venous cells instead. Collectively, these studies suggest that arterial endothelial cells are specified and formed via repressing venous cell fate at the lateral plate mesoderm by Hh signaling during vasculogenesis.

Chemokine signaling guides regional patterning of the first embryonic artery

The aorta traverses the body, yet little is known about how it is patterned in different anatomical locations. Here, we show that the aorta develops from genetically distinct endothelial cells originating from diverse locations within the embryo. Furthermore, chemokine (C-X-C motif) receptor 4a (cxcr4a) is restricted to endothelial cells derived from anterior mesoderm, and is required specifically for formation of the lateral aortae. Cxcl12b, a cxcr4a ligand, is expressed in endoderm underlying the lateral aortae, and loss of cxcl12b phenocopies cxcr4a deficiency. These studies reveal unexpected endothelial diversity within the aorta that is necessary to facilitate its regional patterning by local cues.

Analysis of heart valve development in larval zebrafish

Malformations of the cardiac endocardial cushions (ECs) and valves are common congenital dysmorphisms in newborn infants. Many regulators of EC development have been identified, but the process of valve maturation is less well understood. Zebrafish have been used to understand cardiogenesis through 6 days postfertilization, yet mature heart valves are not present at this stage. By analyzing valve development in larval zebrafish, we identify that valve development proceeds in two phases. Valve elongation occurs through 16 dpf independently of localized cell division. Valve maturation then ensues, resulting from deposition of extracellular matrix and thickening of the valves. Whereas elongation is consistent between larvae, maturation varies based on larval size, suggesting that maturation occurs in response to mechanical forces. Taken together, our studies indicate that zebrafish valve morphogenesis occurs in the larval period, and that zebrafish may provide a unique opportunity to study epigenetic mechanisms leading to human congenital valvular disease, when studied at the appropriate developmental stages.

Arterial-venous specification during development

The major arteries and veins of the vertebrate circulatory system are formed early in embryonic development, before the onset of circulation, following de novo aggregation of "angioblast" progenitors in a process called vasculogenesis. Initial embryonic determination of artery or vein identity is regulated by variety of genetic factors that work in concert to specify endothelial cell fate, giving rise to 2 distinct components of the circulatory loop possessing unique structural characteristics. Work in multiple in vivo animal model systems has led to a detailed examination of the interacting partners that determine arterial and venous specification. We discuss the hierarchical arrangement of many signaling molecules, including Hedgehog (Hh), vascular endothelial growth factor (VEGF), Notch, and chicken ovalbumin upstream-transcription factor II (COUP-TFII) that promote or inhibit divergent pathways of endothelial cell fate. Elucidation of the functional role of these genetic determinants of blood vessel specification together with the epigenetic factors involved in subsequent modification of arterial-venous identity will allow for potential new therapeutic targets for vascular disorders.

Egfl7 knockdown causes defects in the extension and junctional arrangements of endothelial cells during zebrafish vasculogenesis

The endothelial cell (EC) -specific secreted protein EGFL7 is important for tubulogenesis in newly forming blood vessels. We studied its role in vascular tube formation by a quantitative ultrastructural analysis of Egfl7-knockdown zebrafish embryos. At 24 hours postfertilization, the endothelia of dorsal aorta (DA) and posterior cardinal vein (PCV) were correctly anchored to the hypochord and endoderm, respectively, but failed to expand into the vascular area. This resulted in vessels with reduced or split lumen and open sheets of ECs. Concomitantly, the organization of hematopoietic cells-identified by the presence of previously undescribed membrane tubules-between DA and PCV, and within the vessels, was severely disturbed. Strikingly, ectopic cell junctions occurred across the obstructed vessel lumen, on the luminal EC surfaces, which in control conditions never display junctions of any kind. These data suggest that Egfl7 provides ECs with a cue for their extension into the vascular area and in establishing EC cell polarity.

Chapter 4. Using the zebrafish to study vessel formation

Danio rerio, commonly referred to as the zebrafish, is a powerful animal model for studying the formation of the vasculature. Zebrafish offer unique opportunities for in vivo analysis of blood and lymphatic vessels formation because of their accessibility to large-scale genetic and experimental analysis as well as the small size, optical clarity, and external development of zebrafish embryos and larvae. A wide variety of established techniques are available to study vessel formation in the zebrafish, from early endothelial cell differentiation to adult vessel patterning. In this chapter, we review methods used to functionally manipulate and visualize the vasculature in the zebrafish and illustrate how these methods have helped further understanding of the genetic components regulating formation and patterning of developing vessels.

DNA repair protein involved in heart and blood development

Apurinic/apyrimidinic endonuclease 1, a key enzyme in repairing abasic sites in DNA, is an embryonic lethal in mice. We are examining its role in embryogenesis in zebra fish. Zebra fish contain two genomic copies (zfAPEX1a and zfAPEX1b) with identical coding sequences. zfAPEX1b lacks introns. Recombinant protein (ZAP1) is highly homologous with and has the same enzymatic properties as its human orthologue. ZAP1 is highly expressed throughout development. Embryos microinjected with morpholino oligonucleotide (MO) targeting the translation start site die at approximately the midblastula transition (MBT) without apoptosis. They are rescued with mRNA for human wild-type APEX1 but not for APEX1 encoding endonuclease-defective protein. Rescued embryos develop dysmorphic hearts, pericardial edema, few erythrocytes, small eyes, and abnormal notochords. Although the hearts in rescued embryos form defective loops ranging from no loop to one that is abnormally shaped, cardiac myosin (cmlc2) is present and contraction occurs. Embryos microinjected with MO targeting zfAPEX1a intron-exon junctions also pass the MBT with similar abnormalities. We conclude that AP endonuclease 1 is involved in both repairing DNA and regulating specific early stages of embryonic development.

Endothelium is required for the promotion of interrenal morphogenetic movement during early zebrafish development

The adrenal cortex has a complex vasculature that is essential for growth, tissue maintenance, and access of secreted steroids to the bloodstream. However, the interaction between vasculature and adrenal cortex during early organogenesis remains largely unclear. In this study, we focused on the zebrafish counterpart of adrenal cortex, interrenal tissue, to explore the possible role of endothelium in the development of steroidogenic tissues. The ontogeny of interrenal tissue was found to be tightly associated with the endothelial cells (ECs) that constitute the axial vessels. The early interrenal primordia emerge as two clusters of cells that migrate centrally and converge at the midline, whereas the central convergence was abrogated in the avascular cloche (clo) mutant. Neither loss of blood circulation nor perturbations of vessel assembly could account for the interrenal convergence defect, implying a role of endothelial signaling prior to the formation of axial blood vessels. Moreover, as the absence of trunk endothelium in clo mutant was rescued by the forced expression of SCL, the interrenal fusion defect could be alleviated. We thus conclude that endothelial signaling is involved in the morphogenetic movement of early interrenal tissue.

Sustained Bmp signaling is essential for cloaca development in zebrafish

Bone morphogenetic protein (Bmp) signaling has long been known to be important for the early development of the ventral mesoderm, including blood,vasculature and kidney cells. Although Bmp genes are continually expressed in the ventral cells throughout gastrulation and somitogenesis, previous studies in zebrafish have not addressed how the role of Bmp signaling changes over time to regulate ventral mesoderm development. Here, we describe the use of a transgenic inducible dominant-negative Bmp receptor line to examine the temporal roles of Bmp signaling in ventral mesoderm patterning. Surprisingly,we find that Bmp signaling from the mid-gastrula stage through early somitogenesis is important for excluding blood and vascular precursors from the extreme ventral mesoderm, and we show that this domain is normally required for development of the cloaca (the common gut and urogenital opening). Using a novel assay for cloacal function, we find that larvae with reduced mid-gastrula Bmp signaling cannot properly excrete waste. We show that the cloacal defects result from alterations in the morphogenesis of the cloaca and from changes in the expression of genes marking the excretory system. Finally, we show that HrT, a T-box transcription factor, is a Bmp-regulated gene that has an essential function in cloacal development. We conclude that sustained Bmp signaling plays an important role in specification of the zebrafish cloaca by maintaining the fate of extreme ventral cells during the course of gastrulation and early somitogenesis. Furthermore, our data suggest that alterations in Bmp signaling are one possible cause of anorectal malformations during human embryogenesis.

The caudal-related homeobox genes cdx1a and cdx4 act redundantly to regulate hox gene expression and the formation of putative hematopoietic stem cells during zebrafish embryogenesis

The hox genes play a central role in organogenesis and are implicated in the formation of hematopoietic stem cells (HSCs). The cdx genes encode homeodomain transcription factors that act as master regulators of the hox genes. In zebrafish, mutations in cdx4 cause a severe, but not complete, deficit in embryonic blood cells. Here, we report the expression and function of cdx1a, a zebrafish Cdx1 paralogue. Using morpholino-mediated knockdown of cdx1a in a cdx4 mutant background, we show that a deficiency in both cdx genes causes a severe perturbation of hox gene expression and a complete failure to specify blood. The hematopoietic defect in cdx-deficient embryos does not result from a general block in posterior mesoderm differentiation as endothelial cells and kidney progenitors are still formed in the doubly deficient embryos. In addition, cdx-deficient embryos display a significant reduction in runx1a(+) putative HSCs in the zebrafish equivalent to the aorta-gonad-mesonephros (AGM) region. Overexpressing hoxa9a in cdx-deficient embryos rescues embryonic erythropoiesis in the posterior mesoderm as well as the formation of HSCs in the AGM region. Taken together, these results suggest that the cdx-hox pathway plays an essential role in the formation of both embryonic erythroid cells and definitive HSCs during vertebrate embryogenesis.

Endothelial cells and VEGF in vascular development

The intricate patterning processes that establish the complex vascular system during development depend on a combination of intrinsic pre-patterning and extrinsic responses to environmental parameters. Mutational studies in mice and fish have shown that the vascular system is highly sensitive to genetic disruption and have identified potential targets for therapeutic interventions. New insights into non-vascular roles of vascular endothelial growth factor and the requirement for endothelial cells in adult organs and stem-cell niches highlight possible side effects of anti-angiogenic therapy and the need for new targets.

[Vascularization of the head and neck during development]

One of the earliest priorities of the embryonic vascular system is to ensure the metabolic needs of the head. This review covers some of the principles that govern the cellular assembly and localization of blood vessels in the head. In order to understand the development and organization of the cephalic vascular tree, one needs to recall the morphogenetic movements underlying vertebrate head formation and giving rise to the constituent cells of the vascular system. Some of the major signaling molecules involved in vascular development are discussed, including the angiopoietins, the endothelins, the FGFs, the Notch receptors, the PDGFs, Sonic hedgehog, the TGF family and the VEGFs, in order to underline similarities between embryonic and postnatal vascular development, even in the context of increasingly divergent form.

Transgenic zebrafish reveal stage-specific roles for Bmp signaling in ventral and posterior mesoderm development

Bone morphogenetic protein (Bmp) signaling is crucial for the formation and patterning of zebrafish ventral and posterior mesoderm. Mutants defective in the Bmp pathway have expanded trunk muscle, abnormal tails and severely impaired development of ventral mesodermal derivatives such as vasculature, blood and pronephros. As Bmps continue to be expressed in the ventral and posterior mesoderm after gastrulation, it is likely that Bmp signaling continues to play an important developmental role during outgrowth of the posterior body. However, because Bmp signaling plays an essential role during the gastrula stages, it has not been possible with mutants or standard disruption techniques to determine the later functions of the Bmp pathway. To study the role of Bmp signaling in the ventral and posterior mesoderm during trunk and tail outgrowth, we generated a transgenic zebrafish line containing a heatshock-inducible dominant-negative Bmp receptor-GFP fusion. Our data show that Bmps are important for tail organizer formation and for patterning the ventral mesoderm during early gastrulation. However, from mid-gastrulation to the early somitogenesis stages, Bmp signaling is important for ventral tail fin development and for preventing secondary tail formation. We conclude that the role of Bmp signaling in the ventral and posterior mesoderm changes as gastrulation proceeds.

MODULATION OF ACTIVIN A–INDUCED DIFFERENTIATION IN VITRO BY VASCULAR ENDOTHELIAL GROWTH FACTOR IN XENOPUS PRESUMPTIVE ECTODERMAL CELLS

We have previously demonstrated that activin A at low concentrations induced ventral mesoderm including blood-like cells from Xenopus animal caps and that beating heart could be also induced from animal caps treated with 100 ng/ml activin A, suggesting that activin A might be involved in cardiac vasculogenesis. A vascular endothelial growth factor (VEGF) is a powerful mitogen for endothelial cells and is an inducer and regulator of angiogenesis. However, VEGF function in Xenopus development is not clearly identified. In this study, we determined the effect of VEGF on activin A-induced differentiation of animal cap. The VEGF induced duct-like structure composed of Flk-1-positive cells together with the induction of nonvascular tissues, such as neural tissues. This histological result was coincident with our reverse transcriptase-polymerase chain reaction analysis that VEGF together with activin A promoted the expression of Xenopus N-CAM and Xenopus brachyury. This study suggests that VEGF has additional biological activities besides angiogenesis, and arises a different function that VEGF induces stroma cell migration or recruitment that are required for blood vessel formation. This differentiation system will aid in the understanding of angiogenesis during early development.

Vegfc is required for vascular development and endoderm morphogenesis in zebrafish

During embryogenesis, complex morphogenetic events lead endodermal cells to coalesce at the midline and form the primitive gut tube and associated organs. While several genes have recently been implicated in endoderm differentiation, we know little about the genes that regulate endodermal morphogenesis. Here, we show that vascular endothelial growth factor C (Vegfc), an angiogenic as well as a lymphangiogenic factor, is unexpectedly involved in this process in zebrafish. Reducing Vegfc levels using morpholino antisense oligonucleotides, or through overexpression of a soluble form of the VEGFC receptor, VEGFR-3, affects the coalescence of endodermal cells in the anterior midline, leading to the formation of a forked gut tube and the duplication of the liver and pancreatic buds. Further analyses indicate that Vegfc is additionally required for the initial formation of the dorsal endoderm. We also demonstrate that Vegfc is required for vasculogenesis as well as angiogenesis in the zebrafish embryo. These data argue for a requirement of Vegfc in the developing vasculature and, more surprisingly, implicate Vegfc signalling in two distinct steps during endoderm development, first during the initial differentiation of the dorsal endoderm, and second in the coalescence of the anterior endoderm to the midline.

The 'definitive' (and 'primitive') guide to zebrafish hematopoiesis

Progressive advances using zebrafish as a model organism have provided hematologists with an additional genetic system to study blood cell formation and hematological malignancies. Despite extensive evolutionary divergence between bony fish (teleosts) and mammals, the molecular pathways governing hematopoiesis have been highly conserved. As a result, most (if not all) of the critical hematopoietic transcription factor genes identified in mammals have orthologues in zebrafish. As in other vertebrates, all of the teleost blood lineages are believed to originate from a pool of pluripotent, self-renewing hematopoietic stem cells. Here, we provide a detailed review of the timing, anatomical location, and transcriptional regulation of zebrafish 'primitive' and 'definitive' hematopoiesis as well as discuss a model of T-cell leukemia and recent advances in blood cell transplantation. Given that many of the regulatory genes that control embryonic hematopoiesis have been implicated in oncogenic pathways in adults, an understanding of blood cell ontogeny is likely to provide insights into the pathophysiology of human leukemias.

Modeling human hematopoietic and cardiovascular diseases in zebrafish

Zebrafish have emerged as a useful vertebrate model system in which unbiased large-scale screens have revealed hundreds of mutations affecting vertebrate development. Many zebrafish mutants closely resemble known human disorders, thus providing intriguing prospects for uncovering the genetic basis of human diseases and for the development of pharmacologic agents that inhibit or correct the progression of developmental disorders. The rapid pace of advances in genomic sequencing and map construction, in addition to morpholino targeting and transgenic techniques, have facilitated the identification and analysis of genes associated with zebrafish mutants, thus promoting the development of zebrafish as a model for human disorders. This review aims to illustrate how the zebrafish has been used to identify unknown genes, to assign function to known genes, and to delineate genetic pathways, all contributing valuable leads toward understanding human pathophysiology.

Manipulating angiogenesis in medicine

Blood vessels nourish organs with vital nutrients and oxygen and, thus, new vessels form when the embryo needs to grow or wounds are to heal. However, forming new blood vessels is a complex and delicate process, which, unfortunately, is often derailed. Thus, when insufficient vessels form, the tissue becomes ischaemic and stops to function adequately. Conversely, when vessels grow excessively, malignant and inflamed tissues grow faster. It is now becoming increasingly evident that abnormal vessel growth contributes to the pathogenesis of numerous malignant, ischaemic, inflammatory, infectious and immune disorders. With an in-depth molecular understanding, we should be better armamented to combat such angiogenic disorders in the future. That such therapeutic strategies might change the face of medicine is witnessed by initial evidence of success in the clinic.

The endothelial-cell-derived secreted factor Egfl7 regulates vascular tube formation

Vascular development is a complex but orderly process that is tightly regulated. A number of secreted factors produced by surrounding cells regulate endothelial cell (EC) differentiation, proliferation, migration and coalescence into cord-like structures. Vascular cords then undergo tubulogenesis to form vessels with a central lumen. But little is known about how tubulogenesis is regulated in vivo. Here we report the identification and characterization of a new EC-derived secreted factor, EGF-like domain 7 (Egfl7). Egfl7 is expressed at high levels in the vasculature associated with tissue proliferation, and is downregulated in most of the mature vessels in normal adult tissues. Loss of Egfl7 function in zebrafish embryos specifically blocks vascular tubulogenesis. We uncover a dynamic process during which gradual separation and proper spatial arrangement of the angioblasts allow subsequent assembly of vascular tubes. This process fails to take place in Egfl7 knockdown embryos, leading to the failure of vascular tube formation. Our study defines a regulator that controls a specific and important step in vasculogenesis.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

Blood Vessel Patterning at the Embryonic Midline

The reproducible pattern of blood vessels formed in vertebrate embryos has been described extensively, but only recently have we obtained the genetic and molecular tools to address the mechanisms underlying these processes. This review describes our current knowledge regarding vascular patterning around the vertebrate midline and presents data derived from frogs, zebrafish, avians, and mice. The embryonic structures implicated in midline vascular patterning, the hypochord, endoderm, notochord, and neural tube, are discussed. Moreover, several molecular signaling pathways implicated in vascular patterning, VEGF, Tie/tek, Notch, Eph/ephrin, and Semaphorin, are described. Data showing that VEGF is critical to patterning the dorsal aorta in frogs and zebrafish, and to patterning the vascular plexus that forms around the neural tube in amniotes, is presented. A more complete knowledge of vascular patterning is likely to come from the next generation of experiments using ever more sophisticated tools, and these results promise to directly impact on clinically important issues such as forming new vessels in the human body and/or in bioreactors.

Lmo2 and Scl/Tal1 convert non-axial mesoderm into haemangioblasts which differentiate into endothelial cells in the absence of Gata1

The LIM domain protein Lmo2 and the basic helix-loop-helix transcription factor Scl/Tal1 are expressed in early haematopoietic and endothelial progenitors and interact with each other in haematopoietic cells. While loss-of-function studies have shown that Lmo2 and Scl/Tal1 are essential for haematopoiesis and angiogenic remodelling of the vasculature, gain-of-function studies have suggested an earlier role for Scl/Tal1 in the specification of haemangioblasts, putative bipotential precursors of blood and endothelium. In zebrafish embryos, Scl/Tal1 can induce these progenitors from early mesoderm mainly at the expense of the somitic paraxial mesoderm. We show that this restriction to the somitic paraxial mesoderm correlates well with the ability of Scl/Tal1 to induce ectopic expression of its interaction partner Lmo2. Co-injection of lmo2 mRNA with scl/tal1 dramatically extends its effect to head, heart, pronephros and pronephric duct mesoderm inducing early blood and endothelial genes all along the anteroposterior axis. Erythroid development, however, is expanded only into pronephric mesoderm,remaining excluded from head, heart and somitic paraxial mesoderm territories. This restriction correlates well with activation of gata1transcription and co-injection of gata1 mRNA along with scl/tal1 and lmo2 induces erythropoiesis more broadly without ventralising or posteriorising the embryo. While no ectopic myeloid development from the Scl/Tal1-Lmo2-induced haemangioblasts was observed, a dramatic increase in the number of endothelial cells was found. These results suggest that, in the absence of inducers of erythroid or myeloid haematopoiesis, Scl/Tal1-Lmo2-induced haemangioblasts differentiate into endothelial cells.

Blood vessels and nerves: common signals, pathways and diseases

Both blood vessels and nerves are vital channels to and from tissues. Recent genetic insights show that they have much more in common than was originally anticipated. They use similar signals and principles to differentiate, grow and navigate towards their targets. Moreover, the vascular and nervous systems cross-talk and, when dysregulated, this contributes to medically important diseases. The realization that both systems use common genetic pathways should not only form links between vascular biology and neuroscience, but also promises to accelerate the discovery of new mechanistic insights and therapeutic opportunities.

Plumbing the mysteries of vascular development using the zebrafish

The zebrafish has recently emerged as an advantageous model organism for studying how the stereotypic and evolutionarily conserved network of vertebrate blood vessels arises during development. The ability to screen for vascular-specific mutants and to image and experimentally manipulate blood vessels throughout living embryos has already yielded new insights into the anatomy of the early vasculature, the dynamics of growing blood vessels, the specification of early vascular progenitors, and arterial-venous differentiation of blood vessels.

Radar is required for the establishment of vascular integrity in the zebrafish

The precise assembly of an integrated network of blood vessels is essential for the survival of vertebrate embryos. However, the processes by which primitive endothelial cells form mature vessels capable of supplying oxygen and nutrients to developing tissues remain incompletely understood. Here, we propose a role for Radar, one of the zebrafish orthologues of gdf6, in establishing integrity of the trunk vasculature in zebrafish embryos. We show that radar expression is appropriately placed, both spatially and temporally, to perform such a role. Transcripts for radar are detected in the hypochord and the primitive gut endoderm. These tissues intimately flank developing axial vessels in the trunk and have been previously implicated in the regulation of vascular development. Morpholino-based targeted gene knock-down has generated a Radar-specific loss-of-function zebrafish model. These embryos display normal initiation of vascular patterning and commencement of circulation. However, by day 2 of development, the integrity of the axial vasculature is compromised with hemorrhages and circulation short-circuits throughout the developing trunk. We show that this aberrant vascular development is specific to a reduction of the radar gene product. These results suggest that Radar is involved in a signaling pathway required for establishing the integrity of the axial vessels during zebrafish development.

Analysis of a zebrafish VEGF receptor mutant reveals specific disruption of angiogenesis

Blood vessels form either by the assembly and differentiation of mesodermal precursor cells (vasculogenesis) or by sprouting from preexisting vessels (angiogenesis). Endothelial-specific receptor tyrosine kinases and their ligands are known to be essential for these processes. Targeted disruption of vascular endothelial growth factor (VEGF) or its receptor kdr (flk1, VEGFR2) in mouse embryos results in a severe reduction of all blood vessels, while the complete loss of flt1 (VEGFR1) leads to an increased number of hemangioblasts and a disorganized vasculature. In a large-scale forward genetic screen, we identified two allelic zebrafish mutants in which the sprouting of blood vessels is specifically disrupted without affecting the assembly and differentiation of angioblasts. Molecular cloning revealed nonsense mutations in flk1. Analysis of mRNA expression in flk1 mutant embryos showed that flk1 expression was severely downregulated, while the expression of other genes (scl, gata1, and fli1) involved in vasculogenesis or hematopoiesis was unchanged. Overexpression of vegf(121+165) led to the formation of additional vessels only in sibling larvae, not in flk1 mutants. We demonstrate that flk1 is not required for proper vasculogenesis and hematopoiesis in zebrafish embryos. However, the disruption of flk1 impairs the formation or function of vessels generated by sprouting angiogenesis.

Building the house around the plumbing

Signaling between growing blood vessels and the tissues that they innervate has traditionally been viewed as a one-way conversation, with organs and tissues supplying important cues for the growth and anatomical patterning of the blood vessels supplying them, but not vice-versa. Two recent papers now provide evidence that blood vessels can have an important role in promoting the assembly of organs and tissues. These papers show that proper formation of the pancreas and liver and induction of endocrine and hepatic cell types in these endodermal organs requires inductive signals from blood vessels.