Distribution of tissue progenitors within the shield region of the zebrafish gastrula

Evolution of Developmental Programs for the Midline Structures in Chordates: Insights From Gene Regulation in the Floor Plate and Hypochord Homologues of Ciona Embryos

In vertebrate embryos, dorsal midline tissues, including the notochord, the prechordal plate, and the floor plate, play important roles in patterning of the central nervous system, somites, and endodermal tissues by producing extracellular signaling molecules, such as Sonic hedgehog (Shh). In Ciona, hedgehog.b, one of the two hedgehog genes, is expressed in the floor plate of the embryonic neural tube, while none of the hedgehog genes are expressed in the notochord. We have identified a cis-regulatory region of hedgehog.b that was sufficient to drive a reporter gene expression in the floor plate. The hedgehog.b cis-regulatory region also drove ectopic expression of the reporter gene in the endodermal strand, suggesting that the floor plate and the endodermal strand share a part of their gene regulatory programs. The endodermal strand occupies the same topographic position of the embryo as does the vertebrate hypochord, which consists of a row of single cells lined up immediately ventral to the notochord. The hypochord shares expression of several genes with the floor plate, including Shh and FoxA, and play a role in dorsal aorta development. Whole-embryo single-cell transcriptome analysis identified a number of genes specifically expressed in both the floor plate and the endodermal strand in Ciona tailbud embryos. A Ciona FoxA ortholog FoxA.a is shown to be a candidate transcriptional activator for the midline gene battery. The present findings suggest an ancient evolutionary origin of a common developmental program for the midline structures in Olfactores.

Neuromesodermal Lineage Contribution to CNS Development in Invertebrate and Vertebrate Chordates

Ascidians are invertebrate chordates and the closest living relative to vertebrates. In ascidian embryos a large part of the central nervous system arises from cells associated with mesoderm rather than ectoderm lineages. This seems at odds with the traditional view of vertebrate nervous system development which was thought to be induced from ectoderm cells, initially with anterior character and later transformed by posteriorizing signals, to generate the entire anterior-posterior axis of the central nervous system. Recent advances in vertebrate developmental biology, however, show that much of the posterior central nervous system, or spinal cord, in fact arises from cells that share a common origin with mesoderm. This indicates a conserved role for bi-potential neuromesoderm precursors in chordate CNS formation. However, the boundary between neural tissue arising from these distinct neural lineages does not appear to be fixed, which leads to the notion that anterior-posterior patterning and neural fate formation can evolve independently.

Maternal contributions to gastrulation in zebrafish

Gastrulation is a critical early morphogenetic process of animal development, during which the three germ layers; mesoderm, endoderm and ectoderm, are rearranged by internalization movements. Concurrent epiboly movements spread and thin the germ layers while convergence and extension movements shape them into an anteroposteriorly elongated body with head, trunk, tail and organ rudiments. In zebrafish, gastrulation follows the proliferative and inductive events that establish the embryonic and extraembryonic tissues and the embryonic axis. Specification of these tissues and embryonic axes are controlled by the maternal gene products deposited in the egg. These early maternally controlled processes need to generate sufficient cell numbers and establish the embryonic polarity to ensure normal gastrulation. Subsequently, after activation of the zygotic genome, the zygotic gene products govern mesoderm and endoderm induction and germ layer patterning. Gastrulation is initiated during the maternal-to-zygotic transition, a process that entails both activation of the zygotic genome and downregulation of the maternal transcripts. Genomic studies indicate that gastrulation is largely controlled by the zygotic genome. Nonetheless, genetic studies that investigate the relative contributions of maternal and zygotic gene function by comparing zygotic, maternal and maternal zygotic mutant phenotypes, reveal significant contribution of maternal gene products, transcripts and/or proteins, that persist through gastrulation, to the control of gastrulation movements. Therefore, in zebrafish, the maternally expressed gene products not only set the stage for, but they also actively participate in gastrulation morphogenesis.

Notch signaling in the division of germ layers in bilaterian embryos

Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/β-catenin pathway.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Establishment of the Vertebrate Germ Layers

The process of germ layer formation is a universal feature of animal development. The germ layers separate the cells that produce the internal organs and tissues from those that produce the nervous system and outer tissues. Their discovery in the early nineteenth century transformed embryology from a purely descriptive field into a rigorous scientific discipline, in which hypotheses could be tested by observation and experimentation. By systematically addressing the questions of how the germ layers are formed and how they generate overall body plan, scientists have made fundamental contributions to the fields of evolution, cell signaling, morphogenesis, and stem cell biology. At each step, this work was advanced by the development of innovative methods of observing cell behavior in vivo and in culture. Here, we take an historical approach to describe our current understanding of vertebrate germ layer formation as it relates to the long-standing questions of developmental biology. By comparing how germ layers form in distantly related vertebrate species, we find that highly conserved molecular pathways can be adapted to perform the same function in dramatically different embryonic environments.

Gastrulation: making and shaping germ layers

Gastrulation is a fundamental phase of animal embryogenesis during which germ layers are specified, rearranged, and shaped into a body plan with organ rudiments. Gastrulation involves four evolutionarily conserved morphogenetic movements, each of which results in a specific morphologic transformation. During emboly, mesodermal and endodermal cells become internalized beneath the ectoderm. Epibolic movements spread and thin germ layers. Convergence movements narrow germ layers dorsoventrally, while concurrent extension movements elongate them anteroposteriorly. Each gastrulation movement can be achieved by single or multiple motile cell behaviors, including cell shape changes, directed migration, planar and radial intercalations, and cell divisions. Recent studies delineate cyclical and ratchet-like behaviors of the actomyosin cytoskeleton as a common mechanism underlying various gastrulation cell behaviors. Gastrulation movements are guided by differential cell adhesion, chemotaxis, chemokinesis, and planar polarity. Coordination of gastrulation movements with embryonic polarity involves regulation by anteroposterior and dorsoventral patterning systems of planar polarity signaling, expression of chemokines, and cell adhesion molecules.

Notch signalling regulates the contribution of progenitor cells from the chick Hensen's node to the floor plate and notochord

Hensen's node of the chick embryo contains multipotent self-renewing progenitor cells that can contribute to either the floor plate or the notochord. Floor plate cells are a population of epithelial cells that lie at the ventral midline of the developing neural tube, whereas the notochord is a rod of axial mesoderm that lies directly beneath the floor plate. These two tissues serve as a source of a potent signalling morphogen, sonic hedgehog (Shh), which patterns the dorsoventral axis of the neural tube. We show, through both gain- and loss-of-function approaches, that Notch signalling promotes the contribution of chick axial progenitor cells to the floor plate and inhibits contribution to the notochord. Thus, we propose that Notch regulates the allocation of appropriate numbers of progenitor cells from Hensen's node of the chick embryo to the notochord and the floor plate.

Fate mapping embryonic blood in zebrafish: multi- and unipotential lineages are segregated at gastrulation

Vertebrate hematopoiesis first produces primitive (embryonic) lineages and ultimately generates the definitive (adult) blood. Whereas definitive hematopoiesis may produce many diverse blood types via a common multipotent progenitor, primitive hematopoiesis has been thought to produce only erythrocytes or macrophages via progenitors that are unipotent for single blood lineages. Using a variety of in vivo cell-tracing techniques, we show that primitive blood in zebrafish derives from two different progenitor types. On the dorsal gastrula, blood progenitors are unipotential cells that divide infrequently, populate the rostral blood islands, and differentiate into macrophages. In contrast, on the ventral gastrula, blood progenitors are multipotential cells with rapid cell cycles; populate the intermediate cell mass; and differentiate into erythrocytes, neutrophils, and thrombocytes. Our results demonstrate the existence of primitive hematopoietic progenitors that are segregated very early in development and that are specified to produce either a unipotent or a multipotent blood cell lineage.

Two deltaC splice-variants have distinct signaling abilities during somitogenesis and midline patterning

Notch signaling is required for many developmental processes, yet differences in the signaling abilities of various Notch ligands are poorly understood. Here, we have isolated a splice variant of the zebrafish Notch ligand deltaC in which the inclusion of the last intron leads to a truncation of the C-terminal 39 amino acids (deltaC(tv2)). We show that, unlike deltaC(tv1), deltaC(tv2) cannot function effectively in somitogenesis but has an enhanced ability to signal during midline development. Additionally, over-expression of deltaC(tv2) preferentially affects anterior midline development, while another Notch ligand, deltaD, shows a posterior bias. Using chimeric Deltas we show that the intracellular domain is responsible for the strength of signal in midline development, while the extracellular domain influences the anterior-posterior bias of the effect. Together our data show that different deltas can signal in biologically distinct ways in both midline formation and somitogenesis. Moreover, it illustrates the importance of cell-type-dependent modifiers of Notch signaling in providing ligand specificity.

Origin of the zebrafish endocrine and exocrine pancreas

Here, we report a detailed fate map of the zebrafish pancreas at the early gastrula stage of development (6 hours postfertilization; hpf). We show that, at this stage, both pancreas and liver progenitors are symmetrically localized in two broad domains relative to the dorsal organizer. We demonstrate that the dorsal and ventral pancreatic buds can derive from common progenitor pools at 6 hpf, but often derive from independent populations. Endocrine vs. exocrine pancreas show a similar pattern of progenitors, consistent with descriptions of the dorsal bud being strictly endocrine and the ventral bud primarily exocrine. In general, we find that endocrine/dorsal bud progenitors are located more dorsally than the exocrine pancreas/ventral bud progenitors. Later in gastrulation (10 hpf), pancreas progenitors have migrated to bilateral domains at the equator of the embryo. Our fate map will assist with design and interpretation of future experiments to understand early pancreas development.

Zebrafish Gastrulation: Cell Movements, Signals, and Mechanisms

Gastrulation is a morphogenetic process that results in the formation of the embryonic germ layers. Here we detail the major cell movements that occur during zebrafish gastrulation: epiboly, internalization, and convergent extension. Although gastrulation is known to be regulated by signaling pathways such as the Wnt/planar cell polarity pathway, many questions remain about the underlying molecular and cellular mechanisms. Key factors that may play a role in gastrulation cell movements are cell adhesion and cytoskeletal rearrangement. In addition, some of the driving force for gastrulation may derive from tissue interactions such as those described between the enveloping layer and the yolk syncytial layer. Future exploration of gastrulation mechanisms relies on the development of sensitive and quantitative techniques to characterize embryonic germ-layer properties.

Notch signaling regulates midline cell specification and proliferation in zebrafish

Notochord and floor plate cells are sources of molecules that pattern tissues near the midline, including the spinal cord. Hypochord cells are also found at the midline of anamniote embryos and are important for aorta development. Delta-Notch signaling regulates midline patterning in the dorsal organizer by inhibiting notochord formation and promoting hypochord and possibly floor plate development, but the precise mechanisms by which this regulation occurs are unknown. We demonstrate here that floor plate and hypochord cells arise from distinct regions of the zebrafish shield. Blocking Notch signaling during gastrulation entirely prevented hypochord specification but only reduced the number of floor plate cells that developed compared to control embryos. In contrast, elevation of Notch signaling at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not affected. A cell proliferation assay revealed that Notch signaling maintains dividing floor plate progenitors. Together, our results indicate that Notch signaling regulates allocation of appropriate numbers of different midline cells by different mechanisms.

Molecular genetics of axis formation in zebrafish

The basic vertebrate body plan of the zebrafish embryo is established in the first 10 hours of development. This period is characterized by the formation of the anterior-posterior and dorsal-ventral axes, the development of the three germ layers, the specification of organ progenitors, and the complex morphogenetic movements of cells. During the past 10 years a combination of genetic, embryological, and molecular analyses has provided detailed insights into the mechanisms underlying this process. Maternal determinants control the expression of transcription factors and the location of signaling centers that pattern the blastula and gastrula. Bmp, Nodal, FGF, canonical Wnt, and retinoic acid signals generate positional information that leads to the restricted expression of transcription factors that control cell type specification. Noncanonical Wnt signaling is required for the morphogenetic movements during gastrulation. We review how the coordinated interplay of these molecules determines the fate and movement of embryonic cells.

her9 promotes floor plate development in zebrafish

Notochord, floor plate, and in anamniotes hypochord, are vertebrate embryonic midline structures that are the sources of molecules that pattern the nervous system, somites, and dorsal aorta. Midline precursor cells arise from the dorsal organizer during gastrulation, and Notch signaling is an important regulator of midline cell fate specification. To understand fully how Notch signaling regulates midline development, we investigated the role of potential Notch target genes. We show here that midline precursors express her9, a member of the hairy/Enhancer of split gene family. Although her9 inhibits notochord development and promotes floor plate specification, her9 expression in floor plate cells appears not to require Notch signaling. We show that, instead, her9 is a downstream effector of Nodal signaling for floor plate specification.

Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development

Epithelial-mesenchymal transitions (EMTs) are an important mechanism for reorganizing germ layers and tissues during embryonic development. They have both a morphogenic function in shaping the embryo and a patterning function in bringing about new juxtapositions of tissues, which allow further inductive patterning events to occur [Genesis 28 (2000) 23]. Whereas the mechanics of EMT in cultured cells is relatively well understood [reviewed in Biochem. Pharmacol. 60 (2000) 1091; Cell 105 (2001) 425; Bioessays 23 (2001) 912], surprisingly little is known about EMTs during embryonic development [reviewed in Acta Anat. 154 (1995) 8], and nowhere is the entire process well characterized within a single species. Embryonic (developmental) EMTs have properties that are not seen or are not obvious in culture systems or cancer cells. Developmental EMTs are part of a specific differentiative path and occur at a particular time and place. In some types of embryos, a relatively intact epithelium must be maintained while some of its cells de-epithelialize during EMT. In most cases de-epithelialization (loss of apical junctions) must occur in an orderly, patterned fashion in order that the proper morphogenesis results. Interestingly, we find that de-epithelialization is not always necessarily tightly coupled to the expression of mesenchymal phenotypes.Developmental EMTs are multi-step processes, though the interdependence and obligate order of the steps is not clear. The particulars of the process vary between tissues, species, and specific embryonic context. We will focus on 'primary' developmental EMTs, which are those occurring in the initial epiblast or embryonic epithelium. 'Secondary' developmental EMT events are those occurring in epithelial tissues that have reassembled within the embryo from mesenchymal cells. We will review and compare a number of primary EMT events from across the metazoans, and point out some of the many open questions that remain in this field.

Pattern and morphogenesis of presumptive superficial mesoderm in two closely related species, Xenopus laevis and Xenopus tropicalis

The mesoderm, comprising the tissues that come to lie entirely in the deep layer, originates in both the superficial epithelial and the deep mesenchymal layers of the early amphibian embryo. Here, we characterize the mechanisms by which the superficial component of the presumptive mesoderm ingresses into the underlying deep mesenchymal layer in Xenopus tropicalis and extend our previous findings for Xenopus laevis. Fate mapping the superficial epithelium of pregastrula stage embryos demonstrates ingression of surface cells into both paraxial and axial mesoderm (including hypochord), in similar patterns and amounts in both species. Superficial presumptive notochord lies medially, flanked by presumptive hypochord and both overlie the deep region of the presumptive notochord. These tissues are flanked laterally by superficial presumptive somitic mesoderm, the anterior tip of which also appears to overlay the presumptive deep notochord. Time-lapse recordings show that presumptive somitic and notochordal cells move out of the roof of the gastrocoel and into the deep region during neurulation, whereas hypochordal cells ingress after neurulation. Scanning electron microscopy at the stage and position where ingression occurs suggests that superficial presumptive somitic cells in X. laevis ingress into the deep region as bottle cells whereas those in X. tropicalis ingress by "relamination" (e.g., [Dev. Biol. 174 (1996) 92]). In both species, the superficially derived presumptive somitic cells come to lie in the medial region of the presumptive somites during neurulation. By the early tailbud stages, these cells lie at the horizontal myoseptum of the somites. The morphogenic pathway of these cells strongly resembles that of the primary slow muscle pioneer cells of the zebrafish. We present a revised fate map of Xenopus, and we discuss the conservation of superficial mesoderm within amphibians and across the chordates and its implications for the role of this tissue in patterning the mesoderm.

How we are shaped: the biomechanics of gastrulation

Although it is rarely considered so in modern developmental biology, morphogenesis is fundamentally a biomechanical process, and this is especially true of one of the first major morphogenic transformations in development, gastrulation. Cells bring about changes in embryonic form by generating patterned forces and by differentiating the tissue mechanical properties that harness these forces in specific ways. Therefore, biomechanics lies at the core of connecting the genetic and molecular basis of cell activities to the macroscopic tissue deformations that shape the embryo. Here we discuss what is known of the biomechanics of gastrulation, primarily in amphibians but also comparing similar morphogenic processes in teleost fish and amniotes, and selected events in several species invertebrates. Our goal is to review what is known and identify problems for further research.

From cells to circuits: development of the zebrafish spinal cord

The ability of an animal to carry out its normal behavioral repertoire requires generation of an enormous diversity of neurons and glia. The relative simplicity of the spinal cord makes this an especially attractive part of the nervous system for addressing questions about the development of vertebrate neural specification and function. The last decade has witnessed an explosion in our understanding of spinal cord development and the functional interactions among spinal cord neurons and glia. Cellular, genetic, molecular, physiological and behavioral studies in zebrafish have all been important in providing insights into questions that remained unanswered by studies from other vertebrate model organisms. This is the case because many zebrafish spinal neurons can be individually identified and followed over time in living embryos and larvae. In this review, we discuss what is currently known about the cellular, genetic and molecular mechanisms involved in specifying distinct cell types in the zebrafish spinal cord and how these cells establish the functional circuitry that mediates particular behaviors. We start by describing the early signals and morphogenetic movements that form the nervous system, and in particular, the spinal cord. We then provide an overview of the cell types within the spinal cord and describe how they are specified and patterned. We begin ventrally with floor plate and proceed dorsally, through motoneurons and oligodendrocytes, interneurons, astrocytes and radial glia, spinal sensory neurons and neural crest. We next describe axon pathfinding of spinal neurons. Finally, we discuss the roles of particular spinal cord neurons in specific behaviors.

Gbx2 interacts with Otx2 and patterns the anterior-posterior axis during gastrulation in Xenopus

Anterior-posterior patterning of the embryo requires the activity of multiple homeobox genes among them Hox, caudal (Cdx, Xcad) and Otx2. During early gastrulation, Otx2 and Xcad2 establish a cross-regulatory network, which is an early event in the anterior-posterior patterning of the embryo. As gastrulation proceeds and the embryo elongates, a new domain forms, which expresses neither, Otx2 nor Xcad2 genes. Early transcription of the Xenopus Gbx2 homologue, Xgbx2a, is spatially restricted between Otx2 and Xcad2. When overexpressed, Otx2 and Xcad2 repress Xgbx2a transcription, suggesting their role in setting the early Xgbx2a expression domain. Homeobox genes have been shown to play crucial roles in the specification of the vertebrate brain. The border between the transcription domains of Otx2 and Gbx2 is the earliest known marker of the region where the midbrain/hindbrain boundary (MHB) organizer will develop. Xgbx2a is a negative regulator of Otx2 and a weak positive regulator of Xcad2. Using obligatory activator and repressor versions of Xgbx2a, we demonstrate that, during early embryogenesis, Xgbx2a acts as a transcriptional repressor. In addition, taking advantage of hormone-inducible versions of Xgbx2a and its antimorph, we show that the ability of Xgbx2a to induce head malformations is restricted to gastrula stages and correlates with its ability to repress Otx2 during the same developmental stages. We therefore suggest that the earliest known step of the MHB formation, the establishment of Otx2/Gbx2 boundary, takes place via mutual inhibitory interactions between these two genes and this process begins as early as at midgastrulation.

Zebrafish tiggy-winkle hedgehog promoter directs notochord and floor plate green fluorescence protein expression in transgenic zebrafish embryos

Zebrafish tiggy-winkle hedgehog (twhh) is a member of the hedgehog gene family that plays an important role in patterning brain, neural tube, somites, and eyes. To better understand the regulation of its tissue-specific expression, the activity of the twhh promoter was determined in zebrafish embryos by transient and transgenic expression analysis. Transient expression studies revealed that the 5.2-kb twhh promoter drove green fluorescence protein (GFP) expression in the notochord, floor plate, and branchial arches. Deletion analysis showed that distinct regions of the twhh promoter regulated the respective notochord or floor plate specific expression. To confirm the tissue specificity of the twhh promoter, transgenic zebrafish containing the twhh-GFP transgene were generated. GFP expression was analyzed in the F1, F2, and F3 generations of the transgenic embryos. The results confirmed the tissue-specific expression of the transgene in the notochord, floor plate, and branchial arches. In addition, GFP expression was also found in the pectoral fin buds, retina, and epithelial lining cells of the Kupffer's vesicle in the transgenic fish embryos. The expression pattern of the twhh-GFP transgene mimicked the expression of the endogenous twhh mRNAs in the floor plate, fin buds, branchial arches, retina, and epithelial lining cells of the Kupffer's vesicle. The expression in the notochord, however, did not mimic the pattern of the endogenous twhh expression. To determine whether no tail (ntl) or floating head (flh) mutants that have developmental defect in the notochord or the Kupffer's vesicle may affect the GFP expression in these regions, GFP expression was analyzed in ntl or flh transgenic embryos. No GFP expression could be detected in the midline region of the ntl transgenic embryos. However, in flh transgenic embryos, although GFP expression was affected in the midline region, its expression in the Kupffer's vesicle appeared normal. Together, these data indicated that the 5.2-kb twhh promoter contains regulatory elements for tissue-specific expression of twhh in the floor plate, pectoral fin bud, branchial arches, retina, and Kupffer's vesicle.

Single-cell internalization during zebrafish gastrulation

During gastrulation, germ layers are formed as prospective mesodermal and endodermal cells internalize and come to underlie the ectoderm [1-9]. Despite the pivotal role of gastrulation in animal development, the cellular interactions underlying this process are poorly understood. In zebrafish, mesoderm and endoderm formation requires the Nodal signals Cyclops and Squint and their cofactor One-eyed pinhead (Oep) [10-14]. We found that marginal cells in maternal-zygotic oep (MZoep) mutants do not internalize during gastrulation and acquire neural and tail fates at the expense of head and trunk mesendoderm. The lack of internalization in MZoep embryos and the cell-autonomous requirement for oep in Nodal signaling enabled us to test whether internalization can be achieved by individual cells or whether it depends on interactions within a group of cells. We found that individual MZoep mutant cells transplanted to the margin of wild-type blastula embryos initially internalize with their neighbors but are unable to contribute to the mesendoderm. In the reciprocal experiment, single wild-type cells transplanted to the margin of MZoep mutant embryos autonomously internalize and can express the mesendodermal markers axial/foxA2 and sox17. These results suggest that internalization and mesendoderm formation in zebrafish can be attained autonomously by single cells.

Tracing transgene expression in living zebrafish embryos

Ectopic expression by injection of plasmid DNA is rarely used in zebrafish embryos due to a low frequency of cells expressing a transgene of interest at detectable levels. Furthermore, the mosaic nature of ectopic expression by plasmid injection requires the direct detection of transgene-expressing cells. We have used the transcriptional activator Gal4-VP16 to amplify transgene expression in living zebrafish embryos. In comparison to conventional expression vectors, Gal4-VP16-amplified expression results in a significant higher number of cells which express a transgene at detectable levels. The Gal4-VP16-activator and the Gal4-VP16-dependent transgene can be placed on a single expression vector. Using tissue-specific regulatory elements, we show that expression of a Gal4-VP16-dependent transgene can be reliably restricted to muscle, notochordal, or neuronal tissues. Furthermore, Gal4-VP16 can drive the expression of two or more transgenes from the same construct resulting in simultaneous coexpression of both genes in virtually all expressing cells. The reported expression system works effectively not only in zebrafish embryos but also in Xenopus embryos, chicken, mouse, and human cultured cells and is thus applicable to a broad variety of vertebrates. The high frequency of transgene expression together with the linked coexpression of more than one transgene opens the possibility of easily monitoring the behavior of individual transgene-expressing cells in real time by labeling them with the fluorescent reporter GFP. The combinatorial nature of the expression system greatly facilitates changing the tissue-specificity, the transgene expressed, or the cell compartment-specific GFP reporter, making it simpler to address a gene's function in different tissues as well as its cell biological consequences.

Cloning and expression pattern of a zebrafish homolog of forkhead activin signal transducer (FAST), a transcription factor mediating Nodal-related signals

Forkhead activin signal transducer (FAST) is a member of the winged-helix family of DNA-binding proteins that has been implicated in mesoderm induction and left-right axis specification during embryonic development in Xenopus and mouse. We have cloned and characterized a zebrafish FAST homolog. Zebrafish fast is expressed maternally and zygotically. Transcripts start regionalizing and decline in level during gastrulation. During somitogenesis, fast is expressed bilaterally in the lateral plate mesoderm, like its mouse homolog. In addition, zebrafish fast is also expressed bilaterally in the dorsal diencephalon, where the nodal-related cyclops gene is only expressed on the left side. It remains to be demonstrated whether FAST expression in the brain can mediate Nodal-induced asymmetric development.

Axis-inducing activities and cell fates of the zebrafish organizer

We have investigated axis-inducing activities and cellular fates of the zebrafish organizer using a new method of transplantation that allows the transfer of both deep and superficial organizer tissues. Previous studies have demonstrated that the zebrafish embryonic shield possesses classically defined dorsal organizer activity. When we remove the morphologically defined embryonic shield, embryos recover and are completely normal by 24 hours post-fertilization. We find that removal of the morphological shield does not remove all goosecoid- and floating head-expressing cells, suggesting that the morphological shield does not comprise the entire organizer region. Complete removal of the embryonic shield and adjacent marginal tissue, however, leads to a loss of both prechordal plate and notochord. In addition, these embryos are cyclopean, show a significant loss of floor plate and primary motorneurons and display disrupted somite patterning. Motivated by apparent discrepancies in the literature we sought to test the axis-inducing activity of the embryonic shield. A previous study suggested that the shield is capable of only partial axis induction, specifically being unable to induce the most anterior neural tissues. Contrary to this study, we find shields can induce complete secondary axes when transplanted into host ventral germ-ring. In induced secondary axes donor tissue contributes to notochord, prechordal plate and floor plate. When explanted shields are divided into deep and superficial fragments and separately transplanted we find that deep tissue is able to induce the formation of ectopic axes with heads but lacking posterior tissues. We conclude that the deep tissue included in our transplants is important for proper head formation.

Coregulation of anterior and posterior mesendodermal development by a hairy-related transcriptional repressor

During embryonic development in vertebrates, the endoderm becomes patterned along the anteroposterior axis to produce distinct derivatives. How this regulation is controlled is not well understood. We report that the zebrafish hairy/enhancer of split [E(spl)]-related gene her5 plays a critical role in this process. At gastrulation, following endoderm induction and further cell interaction processes including a local release of Notch/Delta signaling, her5 expression is progressively excluded from the presumptive anterior- and posteriormost mesendodermal territories to become restricted to an adjacent subpopulation of dorsal endodermal precursors. Ectopic misexpressions of wild-type and mutant forms of her5 reveal that her5functions primarily within the endodermal/endmost mesendodermal germ layer to inhibit cell participation to the endmost-fated mesendoderm. In this process, her5 acts as an active transcriptional repressor. These features are strikingly reminiscent of the function of Drosophila Hairy/E(spl) factors in cell fate decisions. Our results provide the first model for vertebrate endoderm patterning where an early regulatory step at gastrulation, mediated by her5 controls cell contribution jointly to the anterior- and posteriormost mesendodermal regions.

Two distinct cell populations in the floor plate of the zebrafish are induced by different pathways

The floor plate is a morphologically distinct structure of epithelial cells situated along the midline of the ventral spinal cord in vertebrates. It is a source of guidance molecules directing the growth of axons along and across the midline of the neural tube. In the zebrafish, the floor plate is about three cells wide and composed of cuboidal cells. Two cell populations can be distinguished by the expression patterns of several marker genes, including sonic hedgehog (shh) and the fork head-domain gene fkd4: a single row of medial floor plate (MFP) cells, expressing both shh and fkd4, is flanked by rows of lateral floor plate (LFP) cells that express fkd4 but not shh. Systematic mutant searches in zebrafish embryos have identified a number of genes, mutations in which visibly reduce the floor plate. In these mutants either the MFP or the LFP cells are absent, as revealed by the analysis of the shh and fkd4 expression patterns. MFP cells are absent, but LFP cells are present, in mutants of cyclops, one-eyed pinhead, and schmalspur, whose development of midline structures is affected. LFP cells are absent, but MFP cells are present, in mutants of four genes, sonic you, you, you-too, and chameleon, collectively called the you-type genes. This group of mutants also shows defects in patterning of the paraxial mesoderm, causing U- instead of V-shaped somites. One of the you-type genes, sonic you, was recently shown to encode the zebrafish Shh protein, suggesting that the you-type genes encode components of the Shh signaling pathway. It has been shown previously that in the zebrafish shh is required for the induction of LFP cells, but not for the development of MFP cells. This conclusion is supported by the finding that injection of shh RNA causes an increase in the number of LFP, but not MFP cells. Embryos mutant for iguana, detour, and umleitung share the lack of LFP cells with you-type mutants while somite patterning is not severely affected. In mutants that fail to develop a notochord, MFP cells may be present, but are always surrounded by LFP cells. These data indicate that shh, expressed in the notochord and/or the MFP cells, induces the formation of LFP cells. In embryos doubly mutant for cyclops (cyc) and sonic you (syu) both LFP and MFP cells are deleted. The number of primary motor neurons is strongly reduced in cyc;syu double mutants, while almost normal in single mutants, suggesting that the two different pathways have overlapping functions in the induction of primary motor neurons.

Nodal signaling patterns the organizer

Spemann's organizer plays an essential role in patterning the vertebrate embryo. During gastrulation, organizer cells involute and form the prechordal plate anteriorly and the notochord more posteriorly. The fate mapping and gene expression analyses in zebrafish presented in this study reveal that this anteroposterior polarity is already initiated in the organizer before gastrulation. Prechordal plate progenitors reside close to the blastoderm margin and express the homeobox gene goosecoid, whereas notochord precursors are located further from the margin and express the homeobox gene floating head. The nodal-related genes cyclops and squint are expressed at the blastoderm margin and are required for prechordal plate and notochord formation. We show that differential activation of the Nodal signaling pathway is essential in establishing anteroposterior pattern in the organizer. First, overexpression of cyclops and squint at different doses leads to the induction of floating head at low doses and the induction of both goosecoid and floating head at higher doses. Second, decreasing Nodal signaling using different concentrations of the antagonist Antivin inhibits goosecoid expression at low doses and blocks expression of both goosecoid and floating head at higher doses. Third, attenuation of Nodal signaling in zygotic mutants for the EGF-CFC gene one-eyed pinhead, an essential cofactor for Nodal signaling, leads to the loss of goosecoid expression and expansion of floating head expression in the organizer. Concomitantly, cells normally fated to become prechordal plate are transformed into notochord progenitors. Finally, activation of Nodal signaling at different times suggests that prechordal plate specification requires sustained Nodal signaling, whereas transient signaling is sufficient for notochord development. Together, these results indicate that differential Nodal signaling patterns the organizer before gastrulation, with the highest level of activity required for anterior fates and lower activity essential for posterior fates.

Mesendoderm and left-right brain, heart and gut development are differentially regulated by pitx2 isoforms

The pitx2 gene is a member of the bicoid-homeodomain class of transcription factors that has been implicated in the control of left-right asymmetry during organogenesis. Here we demonstrate that in zebrafish there are two pitx2 isoforms, pitx2a and pitx2c, which show distinct expression patterns and have non-overlapping functions during mesendoderm and asymmetric organ development. pitx2c is expressed symmetrically in presumptive mesendoderm during late blastula stages and in the prechordal plate during late gastrulation. pitx2a expression is first detected at bud stage in the anterior prechordal plate. The regulation of early mesendoderm pitx2c expression is dependent on one-eyed pinhead (EGF-CFC-related gene) and spadetail (tbx-transcription factor) and can be induced by ectopic goosecoid expression. Maintenance of pitx2c midline expression is dependent on cyclops (nodal) and schmalspur, but not no tail (brachyury). Ectopic expression of pitx2 isoforms results in distinct morphological and molecular phenotypes, indicating that pitx2a and pitx2c have divergent regulatory functions. Both isoforms downregulate goosecoid on the dorsal side, but in contrast to earlier reports that nodal and lefty are upstream of pitx2, ectopic pitx2c in other regions induces cyclops, lefty2 and goosecoid expression. Asymmetric isoform expression occurs in non-overlapping domains, with pitx2c in left dorsal diencephalon and developing gut and pitx2a in left heart primordium. Targeted asymmetric expression in Xenopus shows that both isoforms can alter left-right development, but pitx2a has a slightly stronger effect on heart laterality. Our results indicate that distinct genetic pathways regulate pitx2a and pitx2c isoform expression, and each isoform regulates different downstream pathways during mesendoderm and asymmetric organ development.

The nieuwkoid/dharma homeobox gene is essential for bmp2b repression in the zebrafish pregastrula

Dorsoventral specification of the zebrafish gastrula is governed by the functions of the dorsal shield, a region of the embryo functionally analogous to the amphibian Spemann organizer. We report that the bozozok locus encodes the transcription factor nieuwkoid/dharma, a homeobox gene with non-cell-autonomous organizer-inducing activity. The nieuwkoid/dharma gene is expressed prior to the onset of gastrulation in a restricted region of an extraembryonic tissue, the yolk syncytial layer, that directly underlies the presumptive organizer cells. A single base-pair substitution in the nieuwkoid/dharma gene results in a premature stop codon in boz(m168) mutants, leading to the generation of a truncated protein product which lacks the homeodomain and fails to induce a functional organizer in misexpression assays. Embryos homozygous for the boz(m168) mutation exhibit impaired dorsal shield specification often leading to the loss of shield derivatives, such as prechordal plate in the anterior and notochord in the posterior, along the entire anteroposterior axis. Furthermore, boz homozygotes feature a loss of neural fates anterior to the midbrain/hindbrain boundary. Characterization of homozygous mutant embryos using molecular markers indicates that the boz ventralized phenotype may be due, in part, to the derepression of a secreted antagonizer of dorsal fates, zbmp2b, on the dorsal side of the embryo prior to the onset of gastrulation. Furthermore, ectopic expression of nieuwkoid/dharma RNA is sufficient to lead to the down regulation of zbmp2b expression in the pregastrula. Based on these results, we propose that gastrula organizer specification requires the Nieuwkoop center-like activity mediated by the nieuwkoid/dharma/bozozok homeobox gene and that this activity reveals the role of a much earlier than previously suspected inhibition of ventral determinants prior to dorsal shield formation.

Early appearance of pancreatic hormone-expressing cells in the zebrafish embryo

Adult pancreatic islets comprise four cell types, alpha, beta, delta and PP, expressing glucagon, insulin, somatostatin and pancreatic-polypeptide, respectively, arising from cell lineages whose relationships during endocrine pancreas differentiation are still uncertain [Edlund, 1998. Diabetes 47, 1817-1823]. As zebrafish (Danio rerio) represents an attractive vertebrate model to study mutants affecting pancreatic organogenesis [Pack et al., 1996. Development 123, 321-328], we have investigated the expression patterns of islet hormones in zebrafish embryos, from the 16-somite (17 h) to 48-h stages, by whole-mount in situ hybridization and immunofluorescence. Results showed that in the zebrafish pancreatic primordium (a) insulin is the first hormone gene to be expressed, and (b) somatostatin colocalizes with insulin while glucagon-expressing cells, since their appearance, are distinct from insulin- or insulin/somatostatin-expressing cells. Notably, both somatostatin and glucagon, but not insulin, are first expressed in extrapancreatic regions.

Regulation of midline development by antagonism of lefty and nodal signaling

The embryonic midline is crucial for the development of embryonic pattern including bilateral symmetry and left-right asymmetry. In zebrafish, lefty1 (lft1) and lefty2 (lft2) have distinct midline expression domains along the anteroposterior axis that overlap with the expression patterns of the nodal-related genes cyclops and squint. Altered expression patterns of lft1 and lft2 in zebrafish mutants that affect midline development suggests different upstream pathways regulate each expression domain. Ectopic expression analysis demonstrates that a balance of lefty and cyclops signaling is required for normal mesendoderm patterning and goosecoid, no tail and pitx2 expression. In late somite-stage embryos, lft1 and lft2 are expressed asymmetrically in the left diencephalon and left lateral plate respectively, suggesting an additional role in laterality development. A model is proposed by which the vertebrate midline, and thus bilateral symmetry, is established and maintained by antagonistic interactions among co-expressed members of the lefty and nodal subfamilies of TGF-beta signaling molecules.

Zebrafish tbx-c functions during formation of midline structures

Several genes containing the conserved T-box region in invertebrates and vertebrates have been reported recently. Here, we describe three novel members of the T-box gene family in zebrafish. One of these genes, tbx-c, is studied in detail. It is expressed in the axial mesoderm, notably, in the notochordal precursor cells immediately before formation of the notochord and in the chordoneural hinge of the tail bud, after the notochord is formed. In addition, its expression is detected in the ventral forebrain, sensory neurons, fin buds and excretory system. The expression pattern of tbx-c differs from that of the other two related genes, tbx-a and tbx-b. The developmental role of tbx-c has been analysed by overexpression of the full-length tbx-c mRNA and a truncated form of tbx-c mRNA, which encodes the dominant-negative Tbx-c. Overexpression of tbx-c causes expansion of the midline mesoderm and formation of ectopic midline structures at the expense of lateral mesodermal cells. In dominant-negative experiments, the midline mesoderm is reduced with the expansion of lateral mesoderm to the midline. These results suggest that tbx-c plays a role in formation of the midline mesoderm, particularly, the notochord. Moreover, modulation of tbx-c activity alters the development of primary motor neurons. Results of in vitro analysis in zebrafish animal caps suggest that tbx-c acts downstream of early mesodermal inducers (activin and ntl) and reveal an autoregulatory feedback loop between ntl and tbx-c. These data and analysis of midline (ntl−/− and flh−/−) and lateral mesoderm (spt−/−) mutants suggest that tbx-c may function during formation of the notochord.

An early developmental role for eph-ephrin interaction during vertebrate gastrulation

Eph receptor tyrosine kinases (RTK) and their ephrin ligands are involved in the transmission of signals which regulate cytoskeletal organisation and cell migration, and are expressed in spatially restricted patterns at discrete phases during embryogenesis. Loss of function mutants of Eph RTK or ephrin genes result in defects in neuronal pathfinding or cell migration. In this report we show that soluble forms of human EphA3 and ephrin-A5, acting as dominant negative inhibitors, interfere with early events in zebrafish embryogenesis. Exogenous expression of both proteins results in dose-dependent defects in somite development and organisation of the midbrain-hindbrain boundary and hindbrain. The nature of the defects as well as the distribution and timing of expression of endogenous ligands/receptors for both proteins suggest that Eph-ephrin interaction is required for the organisation of embryonic structures by coordinating the cellular movements of convergence during gastrulation.

Mosaic analysis with oep mutant reveals a repressive interaction between floor-plate and non-floor-plate mutant cells in the zebrafish neural tube

The floor plate is located at the ventral midline of the neural tube in vertebrates. Floor-plate development is severely impaired in zebrafish one-eyed pinhead (oep) mutants. oep encodes a membrane-bound protein with an epiblast growth factor (EGF) motif and functions autonomously in floor-plate precursors. To understand the cell behavior and cell-cell interaction during floor-plate development, the distribution and gene expression of wild-type and oep mutant cells in genetic mosaics were examined. When mutant shield cells were transplanted into a wild-type host, an ectopic neural tube with a floor plate was induced. However, the floor plate of the secondary axis was consistently devoid of mutant cells while its notochord was composed entirely of mutant cells. This indicates that oep shield cells adopt only a notochord fate in a wild-type environment. In reciprocal transplants (wild to oep), however, grafted shield cells frequently contributed to part of the floor-plate region of the secondary neural tube and expressed floor-plate markers. Careful examination of serial sections revealed that a mutant neural cell, when located next to the wild-type cells at the ventral midline, inhibited floor-plate differentiation of the adjacent wild-type cells. This inhibition was effective over an area only one- or two-cells wide along the anteroposterior axis. As the cells located at the ventral midline of the oep neural tube are thought to possess a neural character, similar to those located on either side of the floor plate in a wild-type embryo, this inhibition may play an important role during normal development in restricting the floor-plate region into the ventral-most midline by antagonizing homeogenetic signals from the floor-plate cells.

Delta-mediated specification of midline cell fates in zebrafish embryos

BACKGROUND

Fate mapping studies have shown that progenitor cells of three vertebrate embryonic midline structures - the floorplate in the ventral neural tube, the notochord and the dorsal endoderm - occupy a common region prior to gastrulation. This common region of origin raises the possibility that interactions between midline progenitor cells are important for their specification prior to germ layer formation.

RESULTS

One of four known zebrafish homologues of the Drosophila melanogaster cell-cell signaling gene Delta, deltaA (dlA), is expressed in the developing midline, where progenitor cells of the ectodermal floorplate, mesodermal notochord and dorsal endoderm lie close together before they occupy different germ layers. We used a reverse genetic strategy to isolate a missense mutation of dlA, dlAdx2, which coordinately disrupts the development of floorplate, notochord and dorsal endoderm. The dlAdx2 mutant embryos had reduced numbers of floorplate and hypochord cells; these cells lie above and beneath the notochord, respectively. In addition, mutant embryos had excess notochord cells. Expression of a dominant-negative form of Delta protein driven by mRNA microinjection produced a similar effect. In contrast, overexpression of dlA had the opposite effect: fewer trunk notochord cells and excess floorplate and hypochord cells.

CONCLUSION

Our results indicate that Delta signaling is important for the specification of midline cells. The results are most consistent with the hypothesis that developmentally equivalent midline progenitor cells require Delta-mediated signaling prior to germ layer formation in order to be specified as floorplate, notochord or hypochord.

Origin and development of the zebrafish endoderm

The segregation of cells into germ layers is one of the earliest events in the establishment of cell fate in the embryo. In the zebrafish, endoderm and mesoderm are derived from cells that involute into an internal layer, the hypoblast, whereas ectoderm is derived from cells that remain in the outer layer, the epiblast. In this study, we examine the origin of the zebrafish endoderm and its separation from the mesoderm. By labeling individual cells located at the margin of the blastula, we demonstrate that all structures that are endodermal in origin are derived predominantly from the more dorsal and lateral cells of the blastoderm margin. Frequently marginal cells give rise to both endodermal and mesodermal derivatives, demonstrating that these two lineages have not yet separated. Cells located farther than 4 cell diameters from the margin give rise exclusively to mesoderm, and not to endoderm. Following involution, we see a variety of cellular changes indicating the differentiation of the two germ layers. Endodermal cells gradually flatten and extend filopodial processes forming a noncontiguous inner layer of cells against the yolk. At this time, they also begin to express Forkhead-domain 2 protein. Mesodermal cells form a coherent layer of round cells separating the endoderm and ectoderm. In cyclops-mutant embryos that have reduced mesodermal anlage, we demonstrate that by late gastrulation not only mesodermal but also endodermal cells are fewer in number. This suggests that a common pathway initially specifies germ layers together before a progressive sequence of determinative events segregate endoderm and mesoderm into morphologically distinct germ layers.

Analysis of zebrafish development using explant culture assays

Two fundamental questions of developmental biology are when cells become committed to a certain lineage and what cell interactions are involved in establishing this commitment. These questions can be answered using explant or transplant assays. We have developed explant assays to study zebrafish development. These assays involve isolating by microdissection small regions of the embryo at specific times during development, and determining their fate after culture in isolation (lineage commitment assays) or after exposure to a putative inducing tissue (induction assays). In our laboratory, we have used these assays to address questions of neural development, including those of commitment to anterior and posterior neural lineages, and the signals involved in making these decisions. This chapter contains detailed guidelines for designing explant assays. These include suggestions for the isolation and successful culture of explants, and descriptions of the methods used to assay the final fate of explants after culture. Step-by-step protocols are given for the isolation of specific explants that can be used in specification and induction assays. The application of this technique is illustrated with descriptions of experiments. Explant assays will continue to generate key information concerning the establishment of lineage commitment of many embryonic tissues and will provide extremely valuable for analysis of new genes identified molecularly and in mutant screens.

Cell lineage tracing in heart development

Photolysis-based lineage determination is especially useful in addressing late-stage lineage definitions. Although we have focused on the heart lineage, this technique is readily applicable to any region of the embryo. Furthermore, the concomitant analysis of gene expression and lineage restriction within the same embryo will help define the signaling cascades that direct normal cell fate and the regulative changes that mark vertebrate organ development.

Determination of the zebrafish forebrain: induction and patterning

We report an analysis of forebrain determination and patterning in the zebrafish Danio rerio. In order to study these events, we isolated zebrafish homologs of two neural markers, odd-paired-like (opl), which encodes a zinc finger protein, and fkh5, which encodes a forkhead domain protein. At mid-gastrula, expression of these genes defines a very early pattern in the presumptive neurectoderm, with opl later expressed in the telencephalon, and fkh5 in the diencephalon and more posterior neurectoderm. Using in vitro explant assays, we show that forebrain induction has occurred even earlier, by the onset of gastrulation (shield stage). Signaling from the early gastrula shield, previously shown to be an organizing center, is sufficient for activation of opl expression in vitro. In order to determine whether the organizer is required for opl regulation, we removed from late blastula stage embryos either the presumptive prechordal plate, marked by goosecoid (gsc) expression, or the entire organizer, marked by chordin (chd) expression. opl was correctly expressed after removal of the presumptive prechordal plate and consistently, opl was correctly expressed in one-eyed pinhead (oep) mutant embryos, where the prechordal plate fails to form. However, after removal of the entire organizer, no opl expression was observed, indicating that this region is crucial for forebrain induction. We further show that continued organizer function is required for forebrain induction, since beads of BMP4, which promotes ventral fates, also prevented opl expression when implanted during gastrulation. Our data show that forebrain specification begins early during gastrulation, and that a wide area of dorsal mesendoderm is required for its patterning.

Pattern formation in zebrafish--fruitful liaisons between embryology and genetics

Vertebrate embryos, despite quite diverse early morphologies, appear to employ similar cellular strategies and conserved biochemical pathways in their development (Eyal-Giladi, 1997). In the past decade, a small tropical teleost, zebrafish (Danio rerio), became an important model system in which to study development (Streisinger et al., 1981). By combining embryology with molecular and classical genetic methods, our understanding of early inductive and morphogenetic events during vertebrate embryogenesis significantly advanced. In zebrafish, dorsal-ventral polarity is established during early cleavage and is dependent on microtubular transport of determinants from the vegetal pole to the blastomeres positioned on top of the yolk cell. The syncytium forming from these marginal blastomeres in the early blastula exhibits dorsal-ventral asymmetry with beta-catenin localized to the nuclei on the presumptive dorsal side of the syncytium. The yolk cell is a source of signals that induce and pattern overlying blastoderm. Therefore, the dorsal yolk syncytial layer is equivalent to the Nieuwkoop center of the amphibian embryo. The embryonic shield, a thickening of the dorsal blastoderm margin, exhibits properties similar to the amphibian Spemann organizer. However, certain inductive and patterning signals from the organizer might be produced before the shield forms or might originate outside of the shield. Similar to the amphibian embryo, the key patterning functions of the fish dorsal organizer (i.e., dorsalization of mesoderm, ectoderm, and coordination of gastrulation movements) are performed by secreted molecules that antagonize the ventralizing activity of the swil (zbmp-2) and zbmp-4 gene products expressed on the ventral side of the embryo. These functions of the dorsal organizer require the activity of the chordino gene (a zebrafish homologue of chordin), bozozok, mercedes and ogon loci.