Somitogenesis in zebrafish

Tert-butylphenol exposure alters cartilage and bone development in zebrafish

Phenolic antioxidants, such as 2,4-di-tert-butylphenol (2,4-DTBP), 2-tert-butyl phenol (2-BP), and 4-tert-butyl phenol (4-BP), are additives used in domestic water pipes, food packaging, paints, and other industrial products. As additives, they can leach from products and are frequently found in both environmental and human biological samples. Previous studies have demonstrated that 2,4-DTBP exposure can impair the differentiation of human iPS cells into somite- and sclerotome-like cells, and reduce key processes involved in osteoblast formation. Therefore, the goal of this study is to determine if 2-BP, 4-BP, 2,4-DTBP, and its metabolite 3,5-di-tert-butylcatechol (3,5-DTBC) impacts the development of cartilage and bone in vivo, using zebrafish as a model organism. Zebrafish embryos were exposed to increasing concentrations of each of the four chemicals from 1 h post fertilization (hpf) until 5 days post fertilization (dpf), and analyzed for markers of bone and cartilage development. At their highest concentrations tested, both 2-BP and 2,4-DTBP altered axial skeleton formation, with 76% and 61% of the zebrafish showing spinal curvatures, respectively. To corroborate these changes, the expression of marker transcripts were examined. 2-BP exposure reduced mRNA expression of the bone mineralization marker sparc by 1.6-fold. In contrast, 2,4-DTBP increased sparc transcript expression by 1.4-fold. All four compounds significantly upregulated sox9a, a chondrogenesis marker, between 1.4- to 5-fold. Changes in tail cartilage formation were noted using Alician blue staining, with 2,4-DTBP reducing width, length, and cartilage area of the tail, while 2-BP reduced the tail width but with increased the tail base, yielding a more straightened tail. Principle component analysis (PCA) demonstrated associations between sox9a, sparc, nrf2a, reactive oxygen species (ROS), and tail cartilage measurements, particularly in the 2,4-DTBP exposures, suggesting the involvement of nrf2a signaling in impairing cartilage formation. Overall, the study shows that each of the phenolic antioxidants differentially affects the development of bone and cartilage structures in zebrafish.

Nifedipine toxicity is exacerbated by acetyl l-carnitine but alleviated by low-dose ketamine in zebrafish in vivo

Calcium channel blocker (CCB) poisoning is a common and sometimes life-threatening emergency. Our previous studies have shown that acetyl l-carnitine (ALCAR) prevents cardiotoxicity and developmental toxicity induced by verapamil, a CCB used to treat patients with hypertension. Here, we tested whether toxicities of nifedipine, a dihydropyridine CCB used to treat hypertension, can also be mitigated by co-treatment with ALCAR. In the zebrafish embryos at three different developmental stages, nifedipine induced developmental toxicity with pericardial sac edema in a dose-dependent manner, which were surprisingly exacerbated with ALCAR co-treatment. Even with low-dose nifedipine (5 μm), when the pericardial sac looked normal, ALCAR co-treatment showed pericardial sac edema. We hypothesized that toxicity by nifedipine, a vasodilator, may be prevented by ketamine, a known vasoconstrictor. Nifedipine toxicity in the embryos was effectively prevented by co-treatment with low (subanesthetic) doses (25-100 μm added to the water) of ketamine, although a high dose of ketamine (2 mm added to the water) partially prevented the toxicity.As expected of a CCB, nifedipine either in the presence or absence of ketamine-reduced metabolic reactive oxygen species (ROS), a downstream product of calcium signaling, in the rapidly developing digestive system. However, nifedipine induced ROS in the trunk region that showed significantly stunted growth indicating that the tissues under stress potentially produced pathologic ROS. To the best of our knowledge, these studies for the first time show that nifedipine and the dietary supplement ALCAR together induce adverse effects while providing evidence on the therapeutic efficacy of subanesthetic doses of ketamine against nifedipine toxicity in vivo.

BMP and retinoic acid regulate anterior-posterior patterning of the non-axial mesoderm across the dorsal-ventral axis

Despite the fundamental importance of patterning along the dorsal–ventral (DV) and anterior–posterior (AP) axes during embryogenesis, uncertainty exists in the orientation of these axes for the mesoderm. Here we examine the origin and formation of the zebrafish kidney, a ventrolateral mesoderm derivative, and show that AP patterning of the non-axial mesoderm occurs across the classic gastrula stage DV axis while DV patterning aligns along the animal–vegetal pole. We find that BMP signalling acts early to establish broad anterior and posterior territories in the non-axial mesoderm while retinoic acid (RA) functions later, but also across the classic DV axis. Our data support a model in which RA on the dorsal side of the embryo induces anterior kidney fates while posterior kidney progenitors are protected ventrally by the RA-catabolizing enzyme Cyp26a1. This work clarifies our understanding of vertebrate axis orientation and establishes a new paradigm for how the kidney and other mesodermal derivatives arise during embryogenesis.

It is unclear how the dorsal-ventral (DV) and anterior-posterior (AP) axes established in the gastrula affect tissues. Here, the authors show that in zebrafish kidney development, with regard to non-axial mesoderm, the classic DV axis corresponds to the AP axis, and is regulated by BMP and retinoic acid.

Metabolic regulation of the ultradian oscillator Hes1 by reactive oxygen species

Ultradian oscillators are cyclically expressed genes with a period of less than 24h, found in the major signalling pathways. The Notch effector hairy and enhancer of split Hes genes are ultradian oscillators. The physiological signals that synchronise and entrain Hes oscillators remain poorly understood. We investigated whether cellular metabolism modulates Hes1 cyclic expression. We demonstrated that, in mouse myoblasts (C2C12), Hes1 oscillation depends on reactive oxygen species (ROS), which are generated by the mitochondria electron transport chain and by NADPH oxidases NOXs. In vitro, the regulation of Hes1 by ROS occurs via the calcium-mediated signalling. The modulation of Hes1 by ROS was relevant in vivo, since perturbing ROS homeostasis was sufficient to alter Medaka (Oryzias latipes) somitogenesis, a process that is dependent on Hes1 ultradian oscillation during embryo development. Moreover, in a Medaka model for human microphthalmia with linear skin lesions syndrome, in which mitochondrial ROS homeostasis was impaired, we documented important somitogenesis defects and the deregulation of Hes homologues genes involved in somitogenesis. Notably, both molecular and developmental defects were rescued by antioxidant treatments. Our studies provide the first evidence of a coupling between cellular redox metabolism and an ultradian biological oscillator with important pathophysiological implication for somitogenesis.

On the origin of vertebrate somites

INTRODUCTION

Somites, blocks of mesoderm tissue located on either side of the neural tube in the developing vertebrate embryo, are derived from mesenchymal cells in the presomitic mesoderm (PSM) and are a defining characteristic of vertebrates. In vertebrates, the somite segmental boundary is determined by Notch signalling and the antagonistic relationship of the downstream targets of Notch, Lfng, and Delta1 in the anterior PSM. The presence of somites in the basal chordate amphioxus (Branchiostoma floridae) indicates that the last common ancestor of chordates also had somites. However, it remains unclear how the genetic mechanisms underlying somitogenesis in vertebrates evolved from those in ancestral chordates.

RESULTS

We demonstrate that during the gastrula stages of amphioxus embryos, BfFringe expression in the endoderm of the archenteron is detected ventrally to the ventral limit of BfDelta expression in the presumptive rostral somites along the dorsal/ventral (D/V) body axis. Suppression of Notch signalling by DAPT (a γ-secretase inhibitor that indirectly inhibits Notch) treatment from the late blastula stage reduced late gastrula stage expression of BfFringe in the endodermal archenteron and somite markers BfDelta and BfHairy-b in the mesodermal archenteron. Later in development, somites in the DAPT-treated embryo did not separate completely from the dorsal roof of the archenteron. In addition, clear segmental boundaries between somites were not detected in DAPT-treated amphioxus embryos at the larva stage. Similarly, in vertebrates, DAPT treatment from the late blastula stage in Xenopus (Xenopus laevis) embryos resulted in disruption of somite XlDelta-2 expression at the late gastrula stage. At the tail bud stage, the segmental expression of XlMyoD in myotomes was diminished.

CONCLUSIONS

We propose that Notch signalling and the Fringe/Delta cassette for dorso-ventral boundary formation in the archenteron that separates somites from the gut in an amphioxus-like ancestral chordate were co-opted for anteroposterior segmental boundary formation in the vertebrate anterior PSM during evolution.

Membrane-myofibril cross-talk in myofibrillogenesis and in muscular dystrophy pathogenesis: lessons from the zebrafish

Striated muscle has a highly ordered structure in which specialized domains of the cell membrane involved in force transmission (costameres) and excitation-contraction coupling (T tubules) as well as the internal membranes of the sarcoplasmic reticulum are organized over specific regions of the sarcomere. Optimal muscle function is dependent on this high level of organization but how it established and maintained is not well understood. Due to its ex utero development and transparency, the zebrafish embryo is an excellent vertebrate model for the study of dynamic relationships both within and between cells during development. Transgenic models have allowed the delineation of cellular migration and complex morphogenic rearrangements during the differentiation of skeletal myocytes and the assembly and organization of new myofibrils. Molecular targeting of genes and transcripts has allowed the identification of key requirements for myofibril assembly and organization. With the recent advances in gene editing approaches, the zebrafish will become an increasingly important model for the study of human myopathies and muscular dystrophies. Its high fecundity and small size make it well suited to high-throughput screenings to identify novel pharmacologic and molecular therapies for the treatment of a broad range of neuromuscular conditions. In this review, we examine the lessons learned from the zebrafish model regarding the complex interactions between the sarcomere and the sarcolemma that pattern the developing myocyte and discuss the potential for zebrafish as a model system to examine the pathophysiology of, and identify new treatments for, human myopathies and muscular dystrophies.

Visualization of Ca²+ signaling during embryonic skeletal muscle formation in vertebrates

Dynamic changes in cytosolic and nuclear Ca(2+) concentration are reported to play a critical regulatory role in different aspects of skeletal muscle development and differentiation. Here we review our current knowledge of the spatial dynamics of Ca(2+) signals generated during muscle development in mouse, rat, and Xenopus myocytes in culture, in the exposed myotome of dissected Xenopus embryos, and in intact normally developing zebrafish. It is becoming clear that subcellular domains, either membrane-bound or otherwise, may have their own Ca(2+) signaling signatures. Thus, to understand the roles played by myogenic Ca(2+) signaling, we must consider: (1) the triggers and targets within these signaling domains; (2) interdomain signaling, and (3) how these Ca(2+) signals integrate with other signaling networks involved in myogenesis. Imaging techniques that are currently available to provide direct visualization of these Ca(2+) signals are also described.

Targeted gene knockdown in zebrafish reveals distinct intraembryonic functions for insulin-like growth factor II signaling

IGF-II is the predominant IGF ligand regulating prenatal growth in all vertebrates, including humans, but its central role in placental development has confounded efforts to fully elucidate its functions within the embryo. Here we use a nonplacental model vertebrate (zebrafish) to interrogate the intraembryonic functions of IGF-II signaling. The zebrafish genome contains two coorthologs of mammalian IGF2 (igf2a, igf2b), which exhibit distinct patterns of expression during embryogenesis. Expression of igf2a mRNA is restricted to the notochord, primarily during segmentation/neurulation. By contrast, igf2b mRNA is expressed in midline tissues adjacent to the notochord, with additional sites of expression in the ventral forebrain, and the pronephros. To identify their intraembryonic functions, we suppressed the expression of each gene with morpholino oligonucleotides. Knockdown of igf2a led to defects in dorsal midline development, characterized by delayed segmentation, notochord undulations, and ventral curvature. Similarly, suppression of igf2b led to defects in dorsal midline development but also induced ectopic fusion of the nephron primordia, and defects in ventral forebrain development. Subsequent onset of severe body edema in igf2b, but not igf2a morphants, further suggested a distinct role for igf2b in development of the embryonic kidney. Simultaneous knockdown of both genes increased the severity of dorsal midline defects, confirming a conserved role for both genes in dorsal midline development. Collectively, these data provide evidence that the zebrafish orthologs of IGF2 function in dorsal midline development during segmentation/neurulation, whereas one paralog, igf2b, has evolved additional, distinct functions during subsequent organogenesis.

Znrg, a novel gene expressed mainly in the developing notochord of zebrafish

The notochord, a defining characteristic of the chordate embryo is a critical midline structure required for axial skeletal formation in vertebrates, and acts as a signaling center throughout embryonic development. We utilized the digital differential display program of the National Center for Biotechnology Information, and identified a contig of expressed sequence tags (no. Dr. 83747) from the zebrafish ovary library in Genbank. Full-length cDNA of the identified gene was cloned by 5'- and 3'- RACE, and the resulting sequence was confirmed by polymerase chain reaction and sequencing. The cDNA clone contains 2,505 base pairs and encodes a novel protein of 707 amino acids that shares no significant homology with any known proteins. This gene was expressed in mature oocytes and at the one-cell stage, and persisted until the 5th day of development, as determined by RT-PCR. Transcripts were detected by whole-mount RNA in situ hybridization from the two-cell stage to 72 h of embryonic development. This gene was uniformly distributed from the cleavage stage up to the blastula stage. During early gastrulation, it was present in the dorsal region, and became restricted to the notochord and pectoral fin at 48 and 72 h of embryonic development. Based on its abundance in the notochord, we hypothesized that the novel gene may play an important role in notochord development in zebrafish; we named this gene, zebrafish notochord-related gene, or znrg.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Building an evolutionary innovation: differential growth in the modified vertebral elements of the zebrafish Weberian apparatus

The Weberian apparatus, a complex assemblage of greatly modified vertebral elements, significantly enhances hearing within Otophysi. Ultimately we are interested in investigating the genetic mechanisms responsible for the origin, development and morphological diversification of these vertebral elements in the Weberian apparatus of otophysan fishes. However, a necessary first step involves identifying changes in growth of this region as compared with the vertebrae from which these modified elements purportedly derive. Using an ontogenetic series of the zebrafish, Danio rerio, we collected growth data for specific elements within the Weberian apparatus, including neural arches, ribs, and parapophyses. These data are compared to both serially homologous structures in posterior thoracic vertebrae (which act as internal controls) and vertebral elements from the same axial levels in three other non-otophysan teleosts. Significant differences in growth rate were found among serially homologous structures, as well as at equivalent axial levels in different species. Uniform changes in growth rates (in which all structures derived from a specific somite were equally affected) were not found, suggesting precise targeting of morphological change to specific structures. The variation in growth of anterior vertebrae in and among species was greater than expected. This variation in growth rates created developmental patterns unique to each species. Such patterns of growth may help illuminate the specific heterochronic mechanisms required for the origin and subsequent morphological diversification of the Weberian apparatus. This morphological diversity is exemplified by the multitude of forms seen in the cypriniform Weberian apparatus. Understanding patterns of growth in discrete elements of the Weberian apparatus allows us to hypothesize as to the specific developmental changes, likely constituting differences in gene expression in pathways involved in bone and cartilage differentiation, responsible for this morphological diversity.

Ca2+SIGNALLING AND EARLY EMBRYONIC PATTERNING DURING ZEBRAFISH DEVELOPMENT

1. It has been proposed that Ca2+ signalling, in the form of pulses, waves and steady gradients, may play a crucial role in key pattern-forming events during early vertebrate development. 2. With reference to the embryo of the zebrafish (Danio rerio), herein we review the Ca2+ transients reported from the cleavage to segmentation periods. This time-window includes most of the major pattern-forming events of early development, which transform a single-cell zygote into a complex multicellular embryo with established primary germ layers and body axes. 3. Data are presented to support our proposal that intracellular Ca2+ waves are an essential feature of embryonic cytokinesis and that propagating intercellular Ca2+ waves (both long and short range) may play a crucial role in: (i) the establishment of the embryonic periderm and the coordination of cell movements during epiboly, convergence and extension; (ii) the establishment of the basic embryonic axes and germ layers; and (iii) definition of the morphological boundaries of specific tissue domains and embryonic structures, including future organ anlagen. 4. The potential downstream targets of these Ca2+ transients are also discussed, as well as how they may integrate with other pattern-forming signalling pathways known to modulate early developmental events.

Signaling networks that regulate muscle development: Lessons from zebrafish

Locomotion mediated by skeletal muscle provides a basis for the behavioral repertoire of most animals. Embryological and genetic studies of mouse, bird, fish and frog embryos are providing insights into the functions of the myogenic regulatory factors (MRFs) and the signaling molecules that regulate activity of MRFs. Nevertheless, our understanding of muscle development remains somewhat limited. Fundamental goals are to elucidate how mesodermal cells are induced during gastrulation to form muscle precursor cells and how muscle precursor cells acquire specific cell fates, such as slow and fast muscle cells. In this review, we focus on studies of zebrafish muscle development that have advanced our understanding of the molecular genetics of muscle cell induction and specification.

Somite compartments in anamniotes

Somites are a common feature of the phylotypic stage of embryos of all higher chordates. In amniote species like mouse and chick, somite development has been the subject of intense research over many decades, giving insight into the morphological and molecular processes leading to somite compartmentalization and subsequent differentiation. In anamniotes, somite development is much less understood. Except for recent data from zebrafish, and morphological studies in Xenopus, very little is known about the formation of somite compartments and the differentiation of somite derivatives in anamniotes. Here, we give a brief overview on the development of myotome, sclerotome and dermomyotome in various anamniote organisms, and point out the different mechanisms of somite development between anamniotes and the established amniote model systems.

Myocyte-specific enhancer factor 2A is essential for zebrafish posterior somite development

Somite development is governed tightly by genetic factors. In the large-scale mutagenesis screens of zebrafish, no mutations were linked to myocyte enhancer factor 2A (MEF2A) locus. In this study, we find that MEF2A knock-down embryos display a downward tail curvature and have U-shaped posterior somites. Furthermore, we demonstrate that MEF2A is required for Hedgehog signaling. MEF2A inhibition results in induction of apoptosis in the posterior somites. We further find that Hedgehog signaling can negatively regulate MEF2A expression in the somites. Microarray studies reveal a number of genes that are differentially expressed in the MEF2A morphants. Our studies suggest that MEF2A is essential for zebrafish posterior somite development.

Ca2+ signaling during vertebrate somitogenesis

A variety of Ca2+ signals, in the form of intercellular pulses and waves, have been reported to be associated with the various sequential stages of somitogenesis: from convergent extension and the formation of the paraxial mesoderm; during the patterning of the paraxial mesoderm to establish segmental units; throughout the formation of the morphological boundaries that delineate the segmental units, and finally from within the maturing somites as they undergo subsequent development and differentiation. Due to both the technical challenges presented in imaging intact, developing embryos, and the subtle nature of the Ca2+ transients generated, they have proved to be difficult to visualize. However, a combination of cultured cell preparations and improvements in explant and whole embryo imaging techniques has begun to reveal a new and exciting class of developmental Ca2+ signals. In this chapter, we review the small, but expanding, number of reports in the literature and attempt to identify common characteristics of the somitogenic Ca2+ transients, such as their mode of generation, as well as their spatial and temporal features. This may help to elucidate the significance and function of these intriguing Ca2+ transients and thus integrate them into the complex signaling networks that orchestrate early developmental events.

fused-somites–like mutants exhibit defects in trunk vessel patterning

We identified four mutants in two distinct loci exhibiting similar trunk vascular patterning defects in an F3 genetic screen for zebrafish vascular mutants. Initial vasculogenesis is not affected in these mutants, with proper specification and differentiation of endothelial cells. However, all four display severe defects in the growth and patterning of angiogenic vessels in the trunk, with ectopic branching and disoriented migration of intersegmental vessels. The four mutants are allelic to previously characterized mutants at the fused-somites (fss) and beamter (bea) loci, and they exhibit comparable defects in trunk somite boundary formation. The fss locus has been shown to correspond to tbx24; we show here that bea mutants are defective in the zebrafish dlC gene. Somitic expression of known vascular guidance factors efnb2a, sema3a1, and sema3a2 is aberrantly patterned in fss and bea mutants, suggesting that the vascular phenotype is due to loss of proper guidance cues provided by these factors.

Obscurin is required for the lateral alignment of striated myofibrils in zebrafish

Obscurin/obscurin-MLCK is a giant sarcomere-associated protein with multiple isoforms whose interactions with titin and small ankyrin-1 suggest that it has an important role in myofibril assembly, structural support, and the sarcomeric alignment of the sarcoplasmic reticulum. In this study, we characterized the zebrafish orthologue of obscurin and examined its role in striated myofibril assembly. Zebrafish obscurin was expressed in the somites and central nervous system by 24 hours post-fertilization (hpf) and in the heart by 48 hpf. Depletion of obscurin using two independent morpholino antisense oligonucleotides resulted in diminished numbers and marked disarray of skeletal myofibrils, impaired lateral alignment of adjacent myofibrils, disorganization of the sarcoplasmic reticulum, somite segmentation defects, and abnormalities of cardiac structure and function. This is the first demonstration that obscurin is required for vertebrate cardiac and skeletal muscle development. The diminished capacity to generate and organize new myofibrils in response to obscurin depletion suggests that it may have a vital role in the causation of or adaptation to cardiac and skeletal myopathies.

Expression patterns of collagen I (alpha1) encoding gene and muscle-specific genes reveal that the lateral domain of the fish somite forms a connective tissue surrounding the myotome

Somites are repeated, epithelial structures that are derived from the unsegmented paraxial mesoderm located lateral to the notochord. In higher vertebrates, somites differentiate into a sclerotome that subsequently forms the vertebrae and the ribs and into a dermomyotome that gives rise to a myotome, from which arises the skeletal muscle, and to a dermatome, from which arises the dermis. Fish somites have been shown to produce a sclerotome and a myotome, but very little is known regarding their participation in the formation of connective tissues, especially at the junction between the epidermis and the myotome. To investigate the formation of connective tissues in fish somites, we have examined the expression pattern of the collagen I (alpha1) chain. As somitogenesis proceeds rostrocaudally, collagen I (alpha1) expression marks the sclerotomal cells and delineates the formation of the vertebrae. Surprisingly, after the completion of the segmentation, transcript for the collagen I (alpha1) chain appeared in a distinct epithelial-like monolayer situated at the periphery of the developing somite facing the surface epidermis. This epithelial monolayer of somitic cells that covered the superficial slow muscle cells, did not express the myogenic transcriptional regulator myogenin and was devoid of contractile filament. As the somite increased in size, these collagen-expressing epithelial cells flattened, forming a thin cellular layer underlying the epidermis and recovering the lateral surface of the myotome. In conclusion, the lateral domain of the fish somite forms a distinct epithelial cell layer sharing many characteristics with amniote dermatome.

Zebrafish ftz-f1a (nuclear receptor 5a2) functions in skeletal muscle organization

Fushi-tarazu factor 1a (Ftz-F1a, Ff1a, Nr5a2) is a nuclear receptor with diverse functions in many tissues. Here, we report the function of ff1a in zebrafish muscle differentiation. In situ hybridization revealed that ff1a mRNA was present in the adaxial and migrating slow muscle precursors and was down-regulated when slow muscle cells matured. This expression was under the control of hedgehog genes, expanded when hedgehog was increased and missing in mutants defective in genes in the Hedgehog pathway like you-too (yot), sonic you (syu), and u-boot (ubo). Blocking ff1a activity by injecting a deleted form of ff1a or an antisense ff1a morpholino oligo into fish embryos caused thinner and disorganized fibers of both slow and fast properties. Transient expression of ff1a in syu, ubo, and yot embryos led to more fibril bundles, even when slow myoblasts were transfated into fast properties. We showed that ff1a and prox1 complemented each other in slow myofibril assembly, but they did not affect the expression of each other. These results demonstrate that ff1a functions in both slow and fast muscle morphogenesis in response to Hedgehog signaling, and this function parallels the activity of another slow muscle gene, prox1.

Rotatin is a novel gene required for axial rotation and left-right specification in mouse embryos

The genetic cascade that governs left-right (L-R) specification is starting to be elucidated. In the mouse, the lateral asymmetry of the body axis is revealed first by the asymmetric expression of nodal, lefty2 and pitx2 in the left lateral plate mesoderm of the neurulating embryo. Here we describe a novel gene, rotatin, essential for the correct expression of the key L-R specification genes nodal, lefty and Pitx2. Embryos deficient in rotatin show also randomized heart looping and delayed neural tube closure, and fail to undergo the critical morphogenetic step of axial rotation. The amino acid sequence deduced from the cDNA is predicted to contain at least three transmembrane domains. Our results show a novel key player in the genetic cascade that determines L-R specification, and suggest a causal link between this process and axial rotation.

Vertebrate somitogenesis

In vertebrates, the paraxial mesoderm corresponds to the bilateral strips of mesodermal tissue flanking the notochord and neural tube and which are delimited laterally by the intermediate mesoderm and the lateral plate. The paraxial mesoderm comprises the head or cephalic mesoderm anteriorly and the somitic region throughout the trunk and the tail of the vertebrates. Soon after gastrulation, the somitic region of vertebrates starts to become segmented into paired blocks of mesoderm, termed somites. This process lasts until the number of somites characteristic of the species is reached. The somites later give rise to all skeletal muscles of the body, the axial skeleton, and part of the dermis. In this review I discuss the processes involved in the formation of the paraxial mesoderm and its segmentation into somites in vertebrates.

The winged helix transcription factor Foxc1a is essential for somitogenesis in zebrafish

Previous studies identified zebrafish foxc1a and foxc1b as homologs of the mouse forkhead gene, Foxc1. Both genes are transcribed in the unsegmented presomitic mesoderm (PSM), newly formed somites, adaxial cells, and head mesoderm. Here, we show that inhibiting synthesis of Foxc1a (but not Foxc1b) protein with two different morpholino antisense oligonucleotides blocks formation of morphological somites, segment boundaries, and segmented expression of genes normally transcribed in anterior and posterior somites and expression of paraxis implicated in somite epithelialization. Patterning of the anterior PSM is also affected, as judged by the absence of mesp-b, ephrinB2, and ephA4 expression, and the down-regulation of notch5 and notch6. In contrast, the expression of other genes, including mesp-a and papc, in the anterior of somite primordia, and the oscillating expression of deltaC and deltaD in the PSM appear normal. Nevertheless, this expression is apparently insufficient for the maturation of the presumptive somites to proceed to the stage when boundary formation occurs or for the maintenance of anterior/posterior patterning. Mouse embryos that are compound null mutants for Foxc1 and the closely related Foxc2 have no morphological somites and show abnormal expression of Notch signaling pathway genes in the anterior PSM. Therefore, zebrafish foxc1a plays an essential and conserved role in somite formation, regulating both the expression of paraxis and the A/P patterning of somite primordia.

A molecular clock involved in somite segmentation

Somites are transient embryonic structures that are formed from the unsegmented presomitic mesoderm (PSM) in a highly regulated process called somitogenesis. Somite, formation can be considered as the result of several sequential processes: generation of a basic metameric pattern, specification of the antero-posterior identity of each somite, and, finally, formation of the somitic border. Evidence for the existence of a molecular clock or oscillator linked to somitogenesis has been provided by the discovery of the rhythmic and dynamic expression in the PSM of c-hairy1 and lunatic fringe, two genes potentially related to the Notch signaling pathway. These oscillating expression patterns suggest that an important role of the molecular clock could reside in the temporal control of periodic Notch activation, ultimately resulting in the regular array of the somites. We discuss both the importance of the Notch signaling pathway in the molecular events of somitogenesis and its relationship with the molecular clock, and, finally, in that context we review a number of other genes known to play a role in somitogenesis.

The role of the notochord in vertebral column formation

The backbone or vertebral column is the defining feature of vertebrates and is clearly metameric. Given that vertebrae arise from segmented paraxial mesoderm in the embryo, this metamerism is not surprising. Fate mapping studies in a variety of species have shown that ventromedial sclerotome cells of the differentiated somite contribute to the developing vertebrae and ribs. Nevertheless, extensive studies in amniote embryos have produced conflicting data on exactly how embryonic segments relate to those of the adult. To date, much attention has focused on the derivatives of the somites, while relatively little is known about the contribution of other tissues to the formation of the vertebral column. In particular, while it is clear that signals from the notochord induce and maintain proliferation of the sclerotome, and later promote chondrogenesis, the role of the notochord in vertebral segmentation has been largely overlooked. Here, we review the established role of the notochord in vertebral development, and suggest an additional role for the notochord in the segmental patterning of the vertebral column.

Vertebrate segmentation: is cycling the rule?

Vertebrate segmentation initiates with the subdivision of the paraxial mesoderm into a regular array of somites. Recent evidence suggests that the segmentation clock - a biochemical oscillator acting in the unsegmented paraxial mesoderm cells in most vertebrates - controls cyclic Notch signalling, resulting in periodic formation of somite boundaries.

Somite development in zebrafish

A full understanding of somite development requires knowledge of the molecular genetic pathways for cell determination as well as the cellular behaviors that underlie segmentation, somite epithelialization, and somite patterning. The zebrafish has long been recognized as an ideal organism for cellular and histological studies of somite patterning. In recent years, genetics has proven to be a very powerful complementary approach to these embryological studies, as genetic screens for zebrafish mutants defective in somitogenesis have identified over 50 genes that are necessary for normal somite development. Zebrafish is thus an ideal system in which to analyze the role of specific gene products in regulating the cell behaviors that underlie somite development. We review what is currently known about zebrafish somite development and compare it where appropriate to somite development in chick and mouse. We discuss the processes of segmentation and somite epithelialization, and then review the patterning of cell types within the somite. We show directly, for the first time, that muscle cell and sclerotome migrations occur at the same time. We end with a look at the many questions about somitogenesis that are still unanswered.

Control of her1 expression during zebrafish somitogenesis by a Delta-dependent oscillator and an independent wave-front activity

Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaDsignaling is required for this oscillation.aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream ofher1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression inaei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fssbut not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading ofher1 stripes in the PSM is governed by two separate activities.