The origin and morphogenesis of amphibian somites

Waiting on the Fringe: cell autonomy and signaling delays in segmentation clocks

The rhythmic and sequential segmentation of the vertebrate body axis into somites during embryogenesis is governed by a multicellular, oscillatory patterning system called the segmentation clock. Despite many overt similarities between vertebrates, differences in genetic and dynamic regulation have been reported, raising intriguing questions about the evolution and conservation of this fundamental patterning process. Recent studies have brought insights into two important and related issues: (1) whether individual cells of segmentation clocks are autonomous oscillators or require cell-cell communication for their rhythm; and (2) the role of delays in the cell-cell communication that synchronizes the population of genetic oscillators. Although molecular details differ between species, conservation may exist at the level of the dynamics, hinting at rules for evolutionary trajectories in the system.

Cdc42 Effector Protein 3 Interacts With Cdc42 in Regulating Xenopus Somite Segmentation

Somitogenesis is a critical process during vertebrate development that establishes the segmented body plan and gives rise to the vertebra, skeletal muscles, and dermis. While segmentation clock and wave front mechanisms have been elucidated to control the size and time of somite formation, regulation of the segmentation process that physically separates somites is not understood in detail. Here, we identified a cytoskeletal player, Cdc42 effector protein 3 (Cdc42ep3, CEP3) that is required for somite segmentation in Xenopus embryos. CEP3 is specifically expressed in somite tissue during somite segmentation. Loss-of-function experiments showed that CEP3 is not required for the specification of paraxial mesoderm, nor the differentiation of muscle cells, but is required for the segmentation process. Live imaging analysis further revealed that CEP3 is required for cell shape changes and alignment during somitogenesis. When CEP3 was knocked down, somitic cells did not elongate efficiently along the mediolateral axis and failed to undertake the 90° rotation. As a result, cells remained in a continuous sheet without an apparent segmentation cleft. CEP3 likely interacts with Cdc42 during this process, and both increased and decreased Cdc42 activity led to defective somite segmentation. Segmentation defects caused by Cdc42 knockdown can be partially rescued by the overexpression of CEP3. Conversely, loss of CEP3 resulted in the maintenance of high levels of Cdc42 activity at the cell membrane, which is normally reduced during and after somite segmentation. These results suggest that there is a feedback regulation between Cdc42 and CEP3 during somite segmentation and the activity of Cdc42 needs to be fine-tuned to control the coordinated cell shape changes and movement required for somite segmentation.

Multiscale analysis of architecture, cell size and the cell cortex reveals cortical F-actin density and composition are major contributors to mechanical properties during convergent extension

The large-scale movements that construct complex three-dimensional tissues during development are governed by universal physical principles. Fine-grained control of both mechanical properties and force production is crucial to the successful placement of tissues and shaping of organs. Embryos of the frog Xenopus laevis provide a dramatic example of these physical processes, as dorsal tissues increase in Young's modulus by six-fold to 80 Pascal over 8 h as germ layers and the central nervous system are formed. These physical changes coincide with emergence of complex anatomical structures, rounds of cell division, and cytoskeletal remodeling. To understand the contribution of these diverse structures, we adopt the cellular solids model to relate bulk stiffness of a solid foam to the unit size of individual cells, their microstructural organization, and their material properties. Our results indicate that large-scale tissue architecture and cell size are not likely to influence the bulk mechanical properties of early embryonic or progenitor tissues but that F-actin cortical density and composition of the F-actin cortex play major roles in regulating the physical mechanics of embryonic multicellular tissues.

Primordial germ cells: the first cell lineage or the last cells standing?

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The 'last cell standing' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

Paraxis is required for somite morphogenesis and differentiation in Xenopus laevis

BACKGROUND

In most vertebrates, the segmentation of the paraxial mesoderm involves the formation of metameric units called somites through a mesenchymal-epithelial transition. However, this process is different in Xenopus laevis because it does not form an epithelial somite. Xenopus somitogenesis is characterized by a complex cells rearrangement that requires the coordinated regulation of cell shape, adhesion, and motility. The molecular mechanisms that control these cell behaviors underlying somite formation are little known. Although the Paraxis has been implicated in the epithelialization of somite in chick and mouse, its role in Xenopus somite morphogenesis has not been determined.

RESULTS

Using a morpholino and hormone-inducible construction approaches, we showed that both gain and loss of function of paraxis affect somite elongation, rotation and alignment, causing a severe disorganization of somitic tissue. We further found that depletion or overexpression of paraxis in the somite led to the downregulation or upregulation, respectively, of cell adhesion expression markers. Finally, we demonstrated that paraxis is necessary for the proper expression of myotomal and sclerotomal differentiation markers.

CONCLUSIONS

Our results demonstrate that paraxis regulates the cell rearrangements that take place during the somitogenesis of Xenopus by regulating cell adhesion. Furthermore, paraxis is also required for somite differentiation.

The tissue mechanics of vertebrate body elongation and segmentation

England's King Richard III, whose skeleton was recently discovered lying ignobly beneath a parking lot, suffered from a lateral curvature of his spinal column called scoliosis. We now know that his scoliosis was not caused by 'imbalanced bodily humors', rather vertebral defects arise from defects in embryonic elongation and segmentation. This review highlights recent advances in our understanding of post-gastrulation biomechanics of the posteriorly advancing tailbud and somite morphogenesis. These processes are beginning to be deciphered from the level of gene networks to a cross-scale physical model incorporating cellular mechanics, the extracellular matrix, and tissue fluidity.

'Monster… -omics': on segmentation, re-segmentation, and vertebrae formation in amphibians and other vertebrates

Background

The axial skeleton is one of the defining evolutionary landmarks of vertebrates. How this structure develops and how it has evolved in the different vertebrate lineages is, however, a matter of debate. Vertebrae and vertebral structures are derived from the embryonic somites, although the mechanisms of development are different between lineages.

Discussion

Using the anecdotal description of a teratological newt (Triturus dobrogicus) with an unusual malformation in its axial skeleton, we review, compare, and discuss the development of vertebral structures and, in particular, the development of centra from somitic cellular domains in different vertebrate groups. Vertebrae development through re-segmentation of the somitic sclerotomal cells is considered the general mechanism among vertebrates, which has been generalized from studies in amniotic model organisms. The prevalence of this mechanism among anamniotes is, however, controversial. We propose alternative developmental mechanisms for vertebrae formation that should be experimentally tested.

Summary

Research in model organisms, especially amniotes, is laying the foundations for a thorough understanding of the mechanisms of development of the axial skeleton in vertebrates, foundations that should expand the extent of future comparative studies. Although immersed in the ‘-omics’ era, we emphasize the need for an integrative and organismal approach in evolutionary developmental biology for a better understanding of the causal role of development in the evolution of morphological diversity in nature.

Characterization of pax1, pax9, and uncx sclerotomal genes during Xenopus laevis embryogenesis

BACKGROUND

The axial skeleton develops from the sclerotome, a mesenchymal cell population derived from somites. Sclerotomal cells migrate from somites to the perinotochordal and perineural space where they differentiate into chondrocytes to form cartilage and bone. In anurans, little is known about the way how the sclerotome changes as development proceeds and how these events are regulated at the molecular level. Pax1, Pax9, and Uncx4.1 genes play a central role in the morphogenesis of the axial skeleton in vertebrates, regulating cell proliferation and chondrogenic specification of the sclerotome.

RESULTS

In this work, we cloned and examined through whole-mount in situ hybridization and reverse transcriptase-polymerase chain reaction the expression patterns of pax1, pax9, and uncx transcription factors in the anuran Xenopus laevis.

CONCLUSIONS

We found that these genes are similarly expressed in the sclerotome and in the pharyngeal pouch. A detailed analysis of the location of these transcripts showed that they are expressed in different subdomains of the sclerotomal compartment and differ from that observed in other vertebrates.

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs.

RESULTS

The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis.

CONCLUSIONS

The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages.

Developmental expression and cardiac transcriptional regulation of Myh7b, a third myosin heavy chain in the vertebrate heart

The mammalian heart expresses two myosin heavy chain (MYH) genes (Myh6 and Myh7), which are major components of the thick filaments of the sarcomere. We have determined that a third MYH, MYH7B, is also expressed in the myocardium. Developmental analysis shows Myh7b expression in cardiac and skeletal muscle of Xenopus, chick and mouse embryos, and in smooth muscle tissues during later stages of mouse embryogenesis. Myh7b is also expressed in the adult human heart. The promoter region of the Myh7b gene shows remarkable similarity between diverse species, suggesting that transcriptional control mechanisms have been conserved. Using luciferase reporter analysis in rat cardiomyocytes, it can be shown that MEF2, GATA, and E-box regulatory elements are essential for efficient expression of the Myh7b gene. In addition two conserved elements that do not correspond to consensus binding sites for known transcription factors are also essential for full transcriptional activity of the Myh7b reporter. Finally, the Myh7b gene shows a transcriptional response similar to Myh6 in response to cardiac hypertrophy.

The translational repressor 4E-BP mediates the hypoxia-induced defects in myotome cells

Cell growth, proliferation, differentiation, and survival are influenced by the availability of oxygen. The effect of hypoxia on embryonic cells and the underlying molecular mechanisms to maintain cellular viability are still poorly understood. In this study, we show that hypoxia during Xenopus embryogenesis rapidly leads to a significant developmental delay and to cell apoptosis after prolonged exposure. We provide strong evidence that hypoxia does not affect somitogenesis but affects the number of mitotic cells and muscle-specific protein accumulation in somites, without interfering with the expression of MyoD and MRF4 transcription factors. We also demonstrate that hypoxia reversibly decreases Akt phosphorylation and increases the total amount of the translational repressor 4E-BP, in combination with an increase of the 4E-BP associated with eIF4E. Interestingly, the inhibition of PI3-Kinase or mTOR, with LY29002 or rapamycin respectively, triggers the 4E-BP accumulation in Xenopus embryos. Finally, the overexpression of the non-phosphorylatable 4E-BP protein induces, similar to hypoxia, a decrease in mitotic cells and a decrease in muscle-specific protein accumulation in somites. Taken together, our studies suggest that 4E-BP plays a central role under hypoxia in promoting the cap-independent translation at the expense of cap-dependent translation and triggers specific defects in muscle development.

Skeletal muscle differentiation and fusion are regulated by the BAR-containing Rho-GTPase-activating protein (Rho-GAP), GRAF1

Although RhoA activity is necessary for promoting myogenic mesenchymal stem cell fates, recent studies in cultured cells suggest that down-regulation of RhoA activity in specified myoblasts is required for subsequent differentiation and myotube formation. However, whether this phenomenon occurs in vivo and which Rho modifiers control these later events remain unclear. We found that expression of the Rho-GTPase-activating protein, GRAF1, was transiently up-regulated during myogenesis, and studies in C2C12 cells revealed that GRAF1 is necessary and sufficient for mediating RhoA down-regulation and inducing muscle differentiation. Moreover, forced expression of GRAF1 in pre-differentiated myoblasts drives robust muscle fusion by a process that requires GTPase-activating protein-dependent actin remodeling and BAR-dependent membrane binding or sculpting. Moreover, morpholino-based knockdown studies in Xenopus laevis determined that GRAF1 expression is critical for muscle development. GRAF1-depleted embryos exhibited elevated RhoA activity and defective myofibrillogenesis that resulted in progressive muscle degeneration, defective motility, and embryonic lethality. Our results are the first to identify a GTPase-activating protein that regulates muscle maturation and to highlight the functional importance of BAR domains in myotube formation.

Looking at the origin of phenotypic variation from pattern formation gene networks

This article critically reviews some widespread views about the overall functioning of development. Special attention is devoted to views in developmental genetics about the superstructure of developmental gene networks. According to these views gene networks are hierarchic and multilayered. The highest layers partition the embryo in large coarse areas and control downstream genes that subsequently subdivide the embryo into smaller and smaller areas. These views are criticized on the bases of developmental and evolutionary arguments. First, these views, although detailed at the level of gene identities, do not incorporate morphogenetic mechanisms nor do they try to explain how morphology changes during development. Often, they assume that morphogenetic mechanisms are subordinate to cell signaling events. This is in contradiction to the evidence reviewed herein. Experimental evidence on pattern formation also contradicts the view that developmental gene networks are hierarchically multilayered and that their functioning is decodable from promoter analysis. Simple evolutionary arguments suggest that, indeed, developmental gene networks tend to be non-hierarchic. Re-use leads to extensive modularity in gene networks while developmental drift blurs this modularity. Evolutionary opportunism makes developmental gene networks very dependent on epigenetic factors.

Delta-Notch signaling is involved in the segregation of the three germ layers in Xenopus laevis

In vertebrates, the induction of the three germ layers (ectoderm, mesoderm and endoderm) has been extensively studied, but less is known about how they segregate. Here, we investigated whether Delta-Notch signaling is involved in this process. Activating the pathway in the marginal zone with Notch(ICD) resulted in an expansion of endodermal and neural ectoderm precursors, leaving a thinner mesodermal ring around the blastopore at gastrula stage, when germ layers are segregated. On the other hand, when the pathway was blocked with Delta-1(STU) or with an antisense morpholino oligonucleotide against Notch, the pan-mesodermal brachyury (bra) domain was expanded and the neural border was moved animalwards. Strikingly, the suprablastoporal endoderm was either expanded when Delta-1 signaling was blocked, or reduced after the general knock-down of Notch. In addition, either activating or blocking the pathway delays the blastopore closure. We conclude that the process of delimiting the three germ layers requires Notch signaling, which may be finely regulated by ligands and/or involve non-canonical components of the pathway. Moreover, Notch activity must be modulated at appropriate levels during this process in order to keep normal morphogenetic movements during gastrulation.

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

In vivo analyzes of dystroglycan function during somitogenesis in Xenopus laevis

Dystroglycan (Dg) is a cell adhesion receptor for laminin that has been reported to play a role in skeletal muscle cell stability, cytoskeletal organization, cell polarity, and signaling. Here we show that Dg is expressed at both the notochord/somite and the intersomitic boundaries, where laminin and fibronectin are accumulated during somitogenesis. Inhibition of Dg function with morpholino antisense oligonucleotides or a dominant negative mutant results in the normal segmentation of the presomitic mesoderm but affects the number, the size, and the integrity of somites. Depletion of Dg disrupts proliferation and alignment of myoblasts without affecting XMyoD and XMRF4 expression. It also leads to defects in laminin deposition at the intersomitic junctions, whereas expression of integrin beta1 subunits and fibronectin assembly occur normally. Our results show that Dg is critical for both proliferation and elongation of somitic cells and that the Dg-cytoplasmic domain is required for the laminin assembly at the intersomitic boundaries. Developmental Dynamics 238:1332-1345, 2009. (c) 2008 Wiley-Liss, Inc.

Diversification of the expression patterns and developmental functions of the dishevelled gene family during chordate evolution

Dishevelled (Dvl) proteins are key transducers of Wnt signaling encoded by members of a multi-gene family in vertebrates. We report here the divergent, tissue-specific expression patterns for all three Dvl genes in Xenopus embryos, which contrast dramatically with their expression patterns in mice. Moreover, we find that the expression patterns of Dvl genes in the chick diverge significantly from those of Xenopus. In addition, in hemichordates, an outgroup to chordates, we find that the one Dvl gene is dynamically expressed in a tissue-specific manner. Using knockdowns, we find that Dvl1 and Dvl2 are required for early neural crest specification and for somite segmentation in Xenopus. Most strikingly, we report a novel role for Dvl3 in the maintenance of gene expression in muscle and in the development of the Xenopus sclerotome. These data demonstrate that the expression patterns and developmental functions of specific Dvl genes have diverged significantly during chordate evolution.

Actomyosin stiffens the vertebrate embryo during crucial stages of elongation and neural tube closure

Physical forces drive the movement of tissues within the early embryo. Classical and modern approaches have been used to infer and, in rare cases, measure mechanical properties and the location and magnitude of forces within embryos. Elongation of the dorsal axis is a crucial event in early vertebrate development, yet the mechanics of dorsal tissues in driving embryonic elongation that later support neural tube closure and formation of the central nervous system is not known. Among vertebrates, amphibian embryos allow complex physical manipulation of embryonic tissues that are required to measure the mechanical properties of tissues. In this paper, we measure the stiffness of dorsal isolate explants of frog (Xenopus laevis) from gastrulation to neurulation and find dorsal tissues stiffen from less than 20 Pascal (Pa) to over 80 Pa. By iteratively removing tissues from these explants, we find paraxial somitic mesoderm is nearly twice as stiff as either the notochord or neural plate, and at least 10-fold stiffer than the endoderm. Stiffness measurements from explants with reduced fibronectin fibril assembly or disrupted actomyosin contractility suggest that it is the state of the actomyosin cell cortex rather than accumulating fibronectin that controls tissue stiffness in early amphibian embryos.

The genetics of vertebrate myogenesis

The molecular, genetic and cellular bases for skeletal muscle growth and regeneration have been recently documented in a number of vertebrate species. These studies highlight the role of transient subcompartments of the early somite as a source of distinct waves of myogenic precursors. Individual myogenic progenitor populations undergo a complex series of cell rearrangements and specification events in different regions of the body, all of which are controlled by distinct gene regulatory networks. Collectively, these studies have opened a window into the morphogenetic and molecular bases of the different phases of vertebrate myogenesis, from embryo to adult.

Regionally autonomous segmentation within zebrafish presomitic mesoderm

Similar to chick, mouse, and Xenopus, a number of intercellular signaling pathways are required for zebrafish segmentation. However, the spatial scales over which these signaling pathways operate in zebrafish remain largely unknown. During zebrafish segmentation, waves of her1 transcription (a) initiate within the tailbud region, (b) propagate anteriorly through presomitic mesoderm (PSM), and (c) terminate in the anlage of newly-forming somites. These observations raise the question of whether the tailbud region serves as a "pacemaker" or "organizing center" for the initiation of propagating her1 expression waves. Microsurgical manipulations reveal that the anteriorly waves of her1 transcription are not perturbed by removal of the zebrafish tailbud. Furthermore, expression patterns of deltaD, paraxial protocadherin C (papc), and myoD within recently formed somites also appear to be relatively unperturbed by either removal of the tailbud, or by removal of lateral plate mesoderm. Although dynamic gene networks rapidly specify mesenchymal or epithelial cellular identities within forming somites, this specification is plastic. Time-lapse analysis has shown that the cellular progeny of mitotically-active epithelial border cells within newly formed somites can adopt different cellular identities than their precursor cells. Overall, these results indicate that sustained long-range intercellular communication with the tailbud, anterior somites, or lateral plate mesoderm is not necessary for segmentation or somitogenesis to proceed within the PSM. The basic segmentation and somitogenesis processes in zebrafish presomitic mesoderm appear to be largely regionally autonomous and governed by local morphogenetic cell behaviors.

Morphogenic machines evolve more rapidly than the signals that pattern them: lessons from amphibians

The induction of mesoderm and the patterning of its dorsal-ventral and anterior-posterior axes seems to be relatively conserved throughout the chordates, as do the morphogenic movements that produce a phylotypic stage embryo. What is not conserved is the initial embryonic architecture of the fertilized egg, and the specific cell behaviors used to drive mesoderm morphogenesis. How then do conserved patterning pathways adapt to diverse architectures and where do they diverge to direct the different cell behaviors used to shape the phylotypic body plan? Amphibians in particular, probably because of their broad range of reproductive strategies, show diverse embryonic architectures across their class and use diverse cell behaviors during their early morphogenesis, making them an interesting comparative group. We examine three examples from our work on amphibians that show variations in the use of cell behaviors to drive the morphogenesis of the same tissues. We also consider possible points where the conserved patterning pathways might diverge to produce different cell behaviors.

Integrating morphogenesis with underlying mechanics and cell biology

Morphogenesis integrates a wide range of cellular processes into a self-organizing, self-deforming tissue. No single molecular "magic bullet" controls morphogenesis. Wide ranging cellular processes, often without parallels in conventional cell culture systems, work together to generate the architecture and modulate forces that produce and guide shape changes in the embryo. In this review we summarize the early development of the frog Xenopus laevis from a biomechanical perspective. We describe processes operating in the embryo from whole embryo scale, the tissue-scale, to the cellular and extracellular matrix scale. We focus on describing cells, their behaviors and the unique microenvironments they traverse during gastrulation and discuss the role of tissue mechanics in these processes.

Muscle development in a biphasic animal: the frog

Knowledge of muscle development in a vertebrate reflects strengths of the particular model system. For example, the origin of mesoderm is very well characterized in Xenopus laevis, where development of somites is less well understood. The major problem in muscle development, presented by frogs, is the complete replacement of larval muscles by adult muscles at thyroid hormone-dependent metamorphosis. All tail muscles die, all leg muscles form de novo, and muscles in the jaw and trunk show both processes. The nature of adult muscle progenitors remains unclear. Comparison of X. laevis development with divergent amphibian patterns, such as direct developers, which lack the larval tadpole, should highlight important steps in adult muscle formation.

The teleost dermomyotome

Recent work in teleosts has renewed interest in the dermomyotome, which was initially characterized in the late 19th century. We review the evidence for the teleost dermomyotome, comparing it to the more well-characterized amniote dermomyotome. We discuss primary myotome morphogenesis, the relationship between the primary myotome and the dermomyotome, the differentiation of axial muscle, appendicular muscle, and dermis from the dermomyotome, and the signaling molecules that regulate myotome growth from myogenic precursors within the dermomyotome. The recognition of a dermomyotome in teleosts provides a new perspective on teleost muscle growth, as well as a fruitful approach to understanding the vertebrate dermomyotome.

Whole-Somite Rotation Generates Muscle Progenitor Cell Compartments in the Developing Zebrafish Embryo

Somites are transient, mesodermally derived structures that give rise to a number of different cell types within the vertebrate embryo. To achieve this, somitic cells are partitioned into lineage-restricted domains, whose fates are determined by signals secreted from adjacent tissues. While the molecular nature of many of the inductive signals that trigger formation of different cell fates within the nascent somite has been identified, less is known about the processes that coordinate the formation of the subsomitic compartments from which these cells arise. Utilizing a combination of vital dye-staining and lineage-tracking techniques, we describe a previously uncharacterized, lineage-restricted compartment of the zebrafish somite that generates muscle progenitor cells for the growth of appendicular, hypaxial, and axial muscles during development. We also show that formation of this compartment occurs via whole-somite rotation, a process that requires the action of the Sdf family of secreted cytokines.

The anuran Bauplan: a review of the adaptive, developmental, and genetic underpinnings of frog and tadpole morphology

Anurans (frogs, toads, and their larvae) are among the most morphologically derived of vertebrates. While tightly conserved across the order, the anuran Bauplan (body plan) diverges widely from that of other vertebrates, particularly with respect to the skeleton. Here we address the adaptive, ontogenetic, and genetic bases of three such hallmark anuran features: (1) the absence of discrete caudal vertebrae, (2) a truncated axial skeleton, and (3) elongate hind limbs. We review the functional significance of each as it relates to the anuran lifestyle, which includes locomotor adaptations to both aquatic and terrestrial environments. We then shift our focus to the proximal origins of each feature, namely, ontogeny and its molecular regulation. Drawing on relatively limited data, we detail the development of each character and then, by extrapolating from comparative vertebrate data, propose molecular bases for these processes. Cast in this light, the divergent morphology of anurans emerges as a product of evolutionary modulation of the generalised vertebrate developmental machinery. Specifically, we hypothesise that: (1) the formation of caudal vertebrae is precluded due to a failure of sclerotomes to form cartilaginous condensations, perhaps resulting from altered expression of a suite of genes, including Pax1, Pax9, Msx1, Uncx-4.1, Sonic hedgehog, and noggin; (2) anteriorised Hox gene expression in the paraxial mesoderm has led to a rostral shift of morphological boundaries of the vertebral column; and, (3) spatial and temporal shifts in Hox expression may underlie the expanded tarsal elements of the anuran hind limb. Technology is currently in place to investigate each of these scenarios in the African clawed frog Xenopus. Experimental corroboration will further our understanding of the molecular regulation of the anuran Bauplan and provide insight into the origin of vertebrate morphological diversity as well as the role of development in evolution.

Convergence and extension movements affect dynamic notochord-somite interactions essential for zebrafish slow muscle morphogenesis

During vertebrate gastrulation, convergence and extension (C&E) movements shape and position the somites that form the fast and slow muscles. In zebrafish knypek;trilobite non-canonical Wnt mutants, defective C&E movements cause misshapen somites and reduction of slow muscle precursors, the adaxial cells. Here, we demonstrate essential roles of C&E in slow muscle morphogenesis. During segmentation, the adaxial cells change shapes and migrate laterally to form slow muscles at the myotome surface. Using confocal imaging techniques, we show that the adaxial cells undergo three-step shape changes, including dorsoventral elongation, anterior-ward rotation, and anteroposterior elongation. The adaxial cells in knypek;trilobite double mutants maintain prolonged contact with the notochord and fail to rotate anteriorly. Such a defect was suppressed by physical removal of their notochord or by introducing wild-type notochord cells into the mutant. We propose that in the double mutants, impaired C&E movements disrupt notochord development, which impedes the adaxial cell shape changes.

Wnt11-R signaling regulates a calcium sensitive EMT event essential for dorsal fin development of Xenopus

In the frog embryo, a sub-population of trunk neural crest (NC) cells undergoes a dorsal route of migration to contribute to the mesenchyme in the core of the dorsal fin. Here we show that a second population of cells, originally located in the dorsomedial region of the somite, also contributes to the fin mesenchyme. We find that the frog orthologue of Wnt11 (Wnt11-R) is expressed in both the NC and somite cell populations that migrate into the fin matrix. Wnt11-R is expressed prior to migration and persists in the mesenchymal cells after they have distributed throughout the fin. Loss of function studies demonstrate that Wnt11-R activity is required for an epithelial to mesenchymal transformation (EMT) event that precedes migration of cells into the fin matrix. In Wnt11-R depleted embryos, the absence of fin core cells leads to defective dorsal fin development and to collapse of the fin structure. Experiments using small molecule inhibitors indicate that dorsal migration of fin core cells depends on calcium signaling through calcium/calmodulin-dependent kinase II (CaMKII). In Wnt11-R depleted embryos, normal migration of NC cells and dorsal somite cells into the fin and normal fin development can be rescued by stimulation of calcium release. These studies are consistent with a model in which Wnt11-R signaling, via a downstream calcium pathway, regulates fin cell migration and, more generally, indicates a role for non-canonical Wnt signaling in regulation of EMT.

Cell behaviors associated with somite segmentation and rotation inXenopus laevis

During vertebrate development the formation of somites is a critical step, as these structures will give rise to the vertebrae, muscle, and dermis. In Xenopus laevis, somitogenesis consists of the partitioning of the presomitic mesoderm into somites, which undergo a 90-degree rotation to become aligned parallel to the notochord. Using a membrane-targeted green fluorescent protein to visualize cell outlines, we examined the individual cell shape changes occurring during somitogenesis. We show that this process is the result of specific, coordinated cell behaviors beginning with the mediolateral elongation of cells in the anterior presomitic mesoderm and then the subsequent bending of these elongated cells to become oriented parallel with the notochord. By labeling a clonal population of paraxial mesoderm cells, we show that cells bend around their dorsoventral axis. Moreover, this cell bending correlates with an increase in the number of filopodial protrusions, which appear to be posteriorly directed toward the newly formed segmental boundary. By examining the formation of somites at various positions along the anteroposterior axis, we show that the general sequence of cell behaviors is the same; however, somite rotation in anterior somites is slower than in posterior somites. Lastly, this coordinated set of cell behaviors occurs in a dorsal-to-ventral progression within each somite such that cells in the dorsal aspect of the somite become aligned along the anteroposterior axis before cells in other regions of the same somite. Together, our data further define how these cell behaviors are temporally and spatially coordinated during somite segmentation and rotation.

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.

Urbilateria, un être évolué ?

In order to establish the portrait of Urbilateria, the common ancestor of triblastic metazoan, this paper focuses on the antero-posterior segmentation frequently considered as characterising the bilaterian bauplan. The synthesis presented here describes the morphological, anatomical and functional aspects of this organisation. Furthermore it analyses the conditions of its emergence during the ontogenesis of Annelids, Arthropods and Chordates and identifies its genetic bases. The provided data exhibit the unitary character of the segmentation modalities among protostomian and deuterostomian organisms. This process occurs in two phases, involving a posterior proliferative zone after the gastrulation. It shows the similarity of the expression patterns of orthologous genes, the implication of comparable signalisation and regulation pathways. The congruence of the results obtained at both structural and molecular levels reinforce the segmental organisation conception of the common ancestor of Bilaterians.

Tes regulates neural crest migration and axial elongation in Xenopus

Tes is a member of an emerging family of proteins sharing a set of protein motifs referred to as PET-LIM domains. PET-LIM proteins such as Prickle regulate cell behavior during gastrulation in Xenopus and zebrafish, and to ask whether Tes is also involved in controlling cell behavior, we isolated its Xenopus orthologue. Xtes is expressed as a maternal transcript that is maintained at low levels until neurula stages when expression is elevated in the head and axial structures. Depletion of Xtes leads to a foreshortened head and severe defects in axis elongation. The anterior defect is due in part to the inhibition of cranial neural crest migration while the defects in elongation may be due to perturbation of expression of XFGF8, Xdelta-1 and Xcad-3 and thereby to disruption of posterior somitogenesis. Finally, we note that simultaneous depletion of Xtes and Xenopus Prickle results in axial defects that are more severe than those resulting from depletion of Xtes alone, suggesting that the two proteins act together to control axial elongation.

Interaction between X-Delta-2 and Hox genes regulates segmentation and patterning of the anteroposterior axis

In vertebrates, the paraxial mesoderm already exhibits a complex Hox gene pattern by the time that segmentation occurs and somites are formed. The anterior boundaries of the Hox genes are always maintained at the same somite number, suggesting coordination between somite formation and Hox expression. To study this interaction, we used morpholinos to knockdown either the somitogenesis gene X-Delta-2 or the complete Hox paralogous group 1 (PG1) in Xenopus laevis. When X-Delta-2 is knocked down, Hox genes from different paralogous groups are downregulated from the beginning of their expression at gastrula stages. This effect is not via the canonical Notch pathway, as it is independent of the Notch effector Su(H). We also reveal for the first time a clear role for Hox genes in somitogenesis, as loss of PG1 gene function results in the perturbation of somite formation and downregulation of the X-Delta-2 expression in the PSM. This effect on X-Delta-2 expression is also observed during neurula stages, before the somites are formed. These results show that somitogenesis and patterning of the anteroposterior axis are closely linked via a feedback loop involving Hox genes and X-Delta-2, suggesting the existence of a coordination mechanism between somite formation and anteroposterior patterning. Such a mechanism is likely to be functional during gastrulation, before the formation of the first pair of somites, as suggested by the early X-Delta-2 regulation of the Hox genes.

There's more than one frog in the pond: a survey of the Amphibia and their contributions to developmental biology

The study of developmental biology has benefited greatly from the insights gained using amphibians as experimental models. Although Xenopus is currently the predominant model, much of our embryological knowledge derives from research on other amphibians. I will review some of these discoveries, made through astute choice of model organism, and I will examine the reasons behind the adoption of Xenopus as the standard for amphibian research. Additionally, I will discuss the diversity in developmental and reproductive strategies that exists within the Amphibia, and consider some of the recent advances in our understanding of the mechanisms underlying this developmental diversity.

Restricted expression of reggie genes and proteins during early zebrafish development

Reggies are plasma membrane-associated proteins and characteristic markers of lipid-raft microdomains. They are highly conserved from flies to humans and have been implicated in axon regeneration and cell process and contact formation, possibly providing functional platforms for cell-signaling in neurons and other cell types. We analyzed reggie mRNA and protein expression patterns during early zebrafish development. All three zebrafish genes, re-1a, -2a, and -2b, span a considerably diverse set of expression patterns, and their proteins are induced maternally, showing ubiquitous expression at early stages. Although re-2a mRNA can be observed in differentiating neurons in the brain, spinal cord, and neurogenic placodes, re-2b is transcribed mainly in head mesoderm, in neural crest derivates, and along somite boundaries. re-1a mRNA is present at high levels in expression domains that overlap with the combined expression pattern of both re-2 genes except at the somites, where it complements the pattern of re-2b. Immunostaining on embryos reveals reggie protein localization at the cell membrane, at cell-cell contacts, and along all early axon tracts. The early phase of reggie expression suggests a basic and ubiquitous function during the first stages of embryogenesis and into the gastrula period. Upon segmentation, a second phase of expression shows distinctly localized expression patterns, indicating tissue-specific roles and an involvement of re-1a/re-2a in neural development.

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.

The evolution of arthropod segmentation mechanisms

The fruit fly, Drosophila melanogaster, patterns its segments rapidly and simultaneously, via a mechanism that relies on the ability of transcription factors to diffuse between blastoderm nuclei. Ancestral arthropods patterned posterior segments sequentially in a cellular environment, where free diffusion was likely to have been inhibited by the presence of cell membranes. Understanding how the Drosophila paradigm evolved is a problem that has interested evolutionary developmental biologists for some time. In this article, I review what is known about arthropod segmentation mechanisms, and present a model for the evolution of the Drosophila paradigm. The model predicts that the primary pair-rule genes of Drosophila ancestrally functioned within and/or downstream of a Notch-dependent segmentation clock, their striped expression gradually coming under the control of gap genes as the number of segments patterned simultaneously in the anterior increased and the number patterned sequentially via a segmentation clock mechanism in the posterior correspondingly decreased.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Evolution of predetermined germ cells in vertebrate embryos: implications for macroevolution

The germ line is established in animal embryos with the formation of primordial germ cells (PGCs), which give rise to gametes. Therefore, the need to form PGCs can act as a developmental constraint by inhibiting the evolution of embryonic patterning mechanisms that compromise their development. Conversely, events that stabilize the PGCs may liberate these constraints. Two modes of germ cell determination exist in animal embryos: (a) either PGCs are predetermined by the inheritance of germ cell determinants (germ plasm) or (b) PGCs are formed by inducing signals secreted by embryonic tissues (i.e., regulative determination). Surprisingly, among the major extant amphibian lineages, one mechanism is found in urodeles and the other in anurans. In anuran amphibians PGCs are predetermined by germ plasm; in urodele amphibians PGCs are formed by inducing signals. To determine which mechanism is ancestral to the tetrapod lineage and to understand the pattern of inheritance in higher vertebrates, we used a phylogenetic approach to analyze basic morphological processes in both groups and correlated these with mechanisms of germ cell determination. Our results indicate that regulative germ cell determination is a property of embryos retaining ancestral embryological processes, whereas predetermined germ cells are found in embryos with derived morphological traits. These correlations suggest that regulative germ cell formation is an important developmental constraint in vertebrate embryos, acting before the highly conserved pharyngula stage. Moreover, our analysis suggests that germ plasm has evolved independently in several lineages of vertebrate embryos.

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

Cyclic expression of esr9 gene in Xenopus presomitic mesoderm

During somitogenesis, the cycling expression of members of the Notch signalling cascade is involved in a segmentation clock that regulates the periodic budding of somites in chicken, mouse, and zebrafish. In frog, genes with cycling expression in the presomitic mesoderm have not been reported. Here, we describe the expression of Xenopus esr9 and esr10, two new members of the Hairy/Enhancer of split related family of bHLH proteins. We show that they are expressed in a highly dynamic fashion, with their mRNA levels oscillating periodically in the presomitic mesoderm during somitogenesis. This dynamic expression is independent of de novo protein synthesis. Thus, expression of esr9 and esr10 is an indicator of the segmentation clock in the amphibian embryo. This confirms the evolutionary conservation of a molecular pathway involved in vertebrate segmentation clock.

Cell dynamics during somite boundary formation revealed by time-lapse analysis

We follow somite segmentation in living chick embryos and find that the shaping process is not a simple periodic slicing of tissue blocks but a much more carefully choreographed separation in which the somite pulls apart from the segmental plate. Cells move across the presumptive somite boundary and violate gene expression boundaries thought to correlate with the site of the somite boundary. Similarly, cells do not appear to be preassigned to a given somite as they leave the node. The results offer a detailed picture of somite shaping and provide a spatiotemporal framework for linking gene expression with cell movements.

Temporal and spatial expression patterns of FoxD2 during the early development of Xenopus laevis

We have investigated the sequence and expression pattern of the Xenopus laevis FoxD2 gene, a member of the fork head/winged helix multigene family. The derived protein sequence is most closely related to FoxD2 factors known from other species. Maternal FoxD2 transcripts are degraded during early cleavage stages. Zygotic transcription is activated after the midblastula transition followed by a pronounced increase during neurulation. Whole mount in situ hybridisations reveal that FoxD2 is predominantly expressed in the paraxial mesoderm, but not within the myotome. In addition, FoxD2 transcripts are found within the migrating ventral abdominal muscle precursors, in cranial neural crest cells surrounding the eye and populating the second and third visceral arches as well as within restricted areas of the diencephalon. In hatched tadpoles, FoxD2 expression is also observed within the terminal part of the gut.

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.

Hypaxial muscle migration during primary myogenesis in Xenopus laevis

In contrast to many vertebrates, the ventral body wall muscles and limb muscles of Xenopus develop at different times. The ventral body wall forms in the tadpole, while limb (appendicular) muscles form during metamorphosis to the adult frog. In organisms that have been examined thus far, a conserved mechanism has been shown to control migratory muscle precursor specification, migration, and differentiation. Here, we show that the process of ventral body wall formation in Xenopus laevis is similar to hypaxial muscle development in chickens and mice. Cells specified for the migratory lineage display an upregulation of pax3 in the ventro-lateral region of the somite. These pax3-positive cells migrate ventrally, away from the somite, and undergo terminal differentiation with the expression of myf-5, followed by myoD. Several other genes are selectively expressed in the migrating muscle precursor population, including neural cell adhesion molecule (NCAM), Xenopus kit related kinase (Xkrk1), and Xenopus SRY box 5 (sox5). We have also found that muscle precursor migration is highly coordinated with the migration of neural crest-derived melanophores. However, by extirpating neural crest at an early stage and allowing embryos to develop, we determined that muscle precursor migration is not dependent on physical or genetic interaction with melanophores.

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.

An interferon regulatory factor-like binding element restricts Xmyf-5 expression in the posterior somites during Xenopus myogenesis

The expression of myf-5, a key component of myogenic regulatory genes, declines progressively in mature somitic cells during vertebrate myogenesis. Little is known about how this down-regulation takes place. Here we provide evidence that an interferon regulatory factor binding element (IRF element) within the Xenopus myf-5 promoter is responsible for the elimination of myf-5 transcription in mature somitic mesoderm of Xenopus embryos. We show that this IRF element mediates the down-regulation of Xmyf-5 transcription in gastrula embryos, and can specifically interact with nuclear proteins of early neurula. Moreover, deletion of this IRF element results in the anterior expansion of reporter gene transcripts within somitic mesoderm in transgenic embryos. Our results, therefore, provide insight into how the negative control of Xmyf-5 expression takes place.