Paraxial left-sided nodal expression and the start of left-right patterning in the early chick embryo

Molecular anatomy of emerging Xenopus left-right organizer at successive developmental stages

BACKGROUND

Vertebrate left-right symmetry breaking is preceded by formation of left-right organizer. In Amphibian, this structure is formed by gastrocoel roof plate, which emerges from superficial suprablastoporal cells. GRP is subdivided into medial area, which generates leftward flow by rotating monocilia and lateral Nodal1 expressing areas, which are involved in sensing of the flow. After successful symmetry breaking, medial cells are incorporated into a deep layer where they contribute to the axial mesoderm, while lateral domains join somitic mesoderm.

RESULTS

Here, we performed detailed analysis of spatial and temporal gene expression of important markers and the corresponding morphology of emerging GRP. Endodermal marker Sox17 and markers of superficial mesoderm display complementary patterns at all studied stages. At early stages, GRP forms Tekt2 positive epithelial domain clearly separated from underlying deep layers, while at later stages, this separation disappears. Marker of early somitic mesoderm MyoD1 was absent in emerging GRP and was induced together with Nodal1 during early neurulation. Decreasing morphological separation is accompanied by lateral to medial covering of GRP by endoderm.

CONCLUSION

Our data supports continuous link between superficial mesoderm at the start of gastrulation and mature GRP and suggests late induction of somitic fate in lateral GRP.

Development of node architecture and emergence of molecular organizer characteristics in the pig embryo

BACKGROUND

The avian node is the equivalent of the amphibian Spemann's organizer, as indicated by its ability to induce a secondary axis, cellular contribution, and gene expression, whereas the node of the mouse, which displays limited inductive capacities, was suggested to be a part of spatially distributed signaling. Furthermore, the structural identity of the mouse node is subject of controversy, while little is known about equivalent structures in other mammals.

RESULTS

We analyzed the node and emerging organizer in the pig using morphology and the expression of selected organizer genes prior to and during gastrulation. The node was defined according to the "four-quarter model" based on comparative consideration. The node of the pig displays a multilayered, dense structure that includes columnar epithelium, bottle-like cells in the dorsal part, and mesenchymal cells ventrally. Expression of goosecoid (gsc), chordin, and brachyury, together with morphology, reveal the consecutive emergence of three distinct domains: the gastrulation precursor domain, the presumptive node, and the mature node. Additionally, gsc displays a ventral expression domain prior to epiblast epithelialization.

CONCLUSION

Our study defines the morphological and molecular context of the emerging organizer equivalent in the pig and suggests a sequential development of its function.

Abnormal left-right organizer and laterality defects in Xenopus embryos after formin inhibitor SMIFH2 treatment

Left-right symmetry breaking in most studied vertebrates makes use of so-called leftward flow, a mechanism which was studied in detail especially in mouse and Xenopus laevis embryos and is based on rotation of monocilia on specialized epithelial surface designated as left-right organizer or laterality coordinator. However, it has been argued that prior to emergence of leftward flow an additional mechanism operates during early cleavage stages in Xenopus embryo which is based on cytoskeletal processes. Evidence in favour of this early mechanism was supported by left-right abnormalities after chemical inhibition of cytoskeletal protein formin. Here we analyzed temporal dimension of this effect in detail and found that reported abnormalities arise only after treatment at gastrula-neurula stages, i.e. just prior to and during the operation of left-right organizer. Moreover, molecular and morphological analysis of the left-right organizer reveals its abnormal development. Our results strongly indicate that left-right abnormalities reported after formin inhibition cannot serve as support of models based on early symmetry breaking event in Xenopus embryo.

Nodal asymmetry and hedgehog signaling during vertebrate left–right symmetry breaking

Development of visceral left–right asymmetry in bilateria is based on initial symmetry breaking followed by subsequent asymmetric molecular patterning. An important step is the left-sided expression of transcription factor pitx2 which is mediated by asymmetric expression of the nodal morphogen in the left lateral plate mesoderm of vertebrates. Processes leading to emergence of the asymmetric nodal domain differ depending on the mode of symmetry breaking. In Xenopus laevis and mouse embryos, the leftward fluid flow on the ventral surface of the left–right organizer leads through intermediate steps to enhanced activity of the nodal protein on the left side of the organizer and subsequent asymmetric nodal induction in the lateral plate mesoderm. In the chick embryo, asymmetric morphogenesis of axial organs leads to paraxial nodal asymmetry during the late gastrulation stage. Although it was shown that hedgehog signaling is required for initiation of the nodal expression, the mechanism of its asymmetry remains to be clarified. In this study, we established the activation of hedgehog signaling in early chick embryos to further study its role in the initiation of asymmetric nodal expression. Our data reveal that hedgehog signaling is sufficient to induce the nodal expression in competent domains of the chick embryo, while treatment of Xenopus embryos led to moderate nodal inhibition. We discuss the role of symmetry breaking and competence in the initiation of asymmetric gene expression.

Flipping Shells! Unwinding LR Asymmetry in Mirror-Image Molluscs

In seeking to understand the establishment of left-right (LR) asymmetry, a limiting factor is that most animals are ordinarily invariant in their asymmetry, except when manipulated or mutated. It is therefore surprising that the wider scientific field does not appear to fully appreciate the remarkable fact that normal development in molluscs, especially snails, can flip between two chiral types without pathology. Here, I describe recent progress in understanding the evolution, development, and genetics of chiral variation in snails, and place it in context with other animals. I argue that the natural variation of snails is a crucial resource towards understanding the invariance in other animal groups and, ultimately, will be key in revealing the common factors that define cellular and organismal LR asymmetry.

Matrix-filled microcavities in the emerging avian left-right organizer

BACKGROUND

Hensen node of the amniote embryo plays a central role in multiple developmental processes, especially in induction and formation of axial organs. In the chick, it is asymmetrical in shape and has recently been considered to represent the left-right organizer. As mechanisms of breaking the initial left-right symmetry of the embryo are still ill-understood, analyzing the node's microarchitecture may provide insights into functional links between symmetry breaking and asymmetric morphology.

RESULTS

In the course of a light- and electron-microscopic study addressing this issue we discovered novel intercellular matrix-filled cavities in the node of the chick during gastrulation and during early neurulation stages; measuring up to 45 μm, they are surrounded by densely packed cells and filled with nanoscale fibrils, which immunostaining suggests to consist of the basement membrane-related proteins fibronectin and perlecan. The cavities emerge immediately prior to node formation in the epiblast layer adjacent to the tip of the primitive streak and later, with emerging node asymmetry, they are predominantly located in the right part of the node. Almost identical morphological features of microcavities were found in the duck node.

CONCLUSIONS

We address these cavities as "nodal microcavities" and propose their content to be involved in the function of the avian node by mediating morphogen signaling and storage.

Nodal and Hedgehog synergize in gill slit formation during development of the cephalochordate Branchiostoma floridae

The larval pharynx of the cephalochordate Branchiostoma (amphioxus) is asymmetrical. The mouth is on the left, and endostyle and gill slits are on the right. At the neurula, Nodal and Hedgehog (Hh) expression becomes restricted to the left. To dissect their respective roles in gill slit formation, we inhibited each pathway separately for 20 min at intervals during the neurula stage, before gill slits penetrate, and monitored the effects on morphology and expression of pharyngeal markers. The results pinpoint the short interval spanning the gastrula/neurula transition as the critical period for specification and positioning of future gill slits. Thus, reduced Nodal signaling shifts the gill slits ventrally, skews the pharyngeal domains of Hh, Pax1/9, Pax2/5/8, Six1/2 and IrxC towards the left, and reduces Hh and Tbx1/10 expression in endoderm and mesoderm, respectively. Nodal auto-regulates. Decreased Hh signaling does not affect gill slit positions or Hh or Nodal expression, but it does reduce the domain of Gli, the Hh target, in the pharyngeal endoderm. Thus, during the neurula stage, Nodal and Hh cooperate in gill slit development - Hh mediates gill slit formation and Nodal establishes their left-right position.

Pitx2 and nodal as conserved early markers of the anterior-posterior axis in the rabbit embryo

Attaining molecular and morphological axial polarity during gastrulation is a fundamental early requirement for normal development of the embryo. In mammals, the first morphological sign of the anterior-posterior axis appears anteriorly in the form of the anterior marginal crescent (or anterior visceral endoderm) while in the avian the first such sign is the Koller's sickle at the posterior pole of the embryonic disc. Despite this inverse mode of axis formation many genes and molecular pathways involved in various steps of this process seem to be evolutionarily conserved amongst amniotes, the nodal gene being a well-known example with its functional involvement prior and during gastrulation. The pitx2 gene, however, is a new candidate described in the chick as an early marker for anterior-posterior polarity and as a regulator of axis formation including twinning. To find out whether pitx2 has retained its inductive and early marker function during the evolution of mammals this study analyses pitx2 and nodal expression at parallel stages during formation of the anterior-posterior polarity in the early rabbit embryo using whole-mount in situ hybridization and serial light-microscopical sections. At a late pre-gastrulation stage a localized reduction of nodal expression presages the position of the anterior pole of the embryonic disc and thus serves as the earliest molecular marker of anterior-posterior polarity known so far. Pitx2 is expressed in a polarized manner in the anterior marginal crescent and in the posterior half of the embryonic disc during further development. In the anterior segment of the posterior pitx2 expression domain, the anterior streak domain (ASD) is defined by nodal expression as a hypothetical progenitor region of the anterior half of the primitive streak. The expression patterns of both genes thus serve as signs of a conserved involvement in early axis formation in amniotes and, possibly, in twinning in mammals as well.

Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate

Background

The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken).

Results

Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation.

Conclusions

While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of "ventral to dorsal induction" of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction.

An Early Chick Embryo Culture Device for Extended Continuous Observation

Appropriate mechanical tension of the vitelline membrane as the culture substrate for the early chick embryo is frequently reported to be required for successful in vitro development. Here we describe a modified device, made of anodized aluminum, for in vitro culture which we used for studies of left-right symmetry breaking with emphasis on morphology and gene expression as readouts. The technique allows for easy, high-throughput tissue handling and provides a suitable tension in a stable and easily reproducible manner proven to be suitable for correct molecular left-right patterning and heart looping after long-term culture.

Paraxial Nodal Expression Reveals a Novel Conserved Structure of the Left-Right Organizer in Four Mammalian Species

Nodal activity in the left lateral plate mesoderm is a conserved sign of irreversible left-right asymmetry at early somite stages of the vertebrate embryo. An earlier, paraxial nodal domain accompanies the emergence and initial extension of the notochord and is either left-sided, as in the chick and pig, or symmetrical, as in the mouse and rabbit; intriguingly, this interspecific dichotomy is mirrored by divergent morphological features of the posterior notochord (also known as the left-right organizer), which is ventrally exposed to the yolk sac cavity and carries motile cilia in the latter 2 species only. By introducing the cattle embryo as a new model organism for early left-right patterning, we present data to establish 2 groups of mammals characterized by both the morphology of the left-right organizer and the dynamics of paraxial nodal expression: presence and absence of a ventrally open surface of the early (plate-like) posterior notochord correlates with a symmetrical (in mice and rabbits) versus an asymmetrical (in pigs and cattle) paraxial nodal expression domain next to the notochordal plate. High-resolution histological analysis reveals that the latter domain defines in all 4 mammals a novel 'parachordal' axial mesoderm compartment, the topography of which changes according to the specific regression of the similarly novel subchordal mesoderm during the initial phases of notochord development. In conclusion, the mammalian axial mesoderm compartment (1) shares critical conserved features despite the marked differences in early notochord morphology and early left-right patterning and (2) provides a dynamic topographical framework for nodal activity as part of the mammalian left-right organizer.

Rho kinase activity controls directional cell movements during primitive streak formation in the rabbit embryo

During animal gastrulation, the specification of the embryonic axes is accompanied by epithelio-mesenchymal transition (EMT), the first major change in cell shape after fertilization. EMT takes place in disparate topographical arrangements, such as the circular blastopore of amphibians, the straight primitive streak of birds and mammals or in intermediate gastrulation forms of other amniotes such as reptiles. Planar cell movements are prime candidates to arrange specific modes of gastrulation but there is no consensus view on their role in different vertebrate classes. Here, we test the impact of interfering with Rho kinase-mediated cell movements on gastrulation topography in blastocysts of the rabbit, which has a flat embryonic disc typical for most mammals. Time-lapse video microscopy, electron microscopy, gene expression and morphometric analyses of the effect of inhibiting ROCK activity showed – besides normal specification of the organizer region – a dose-dependent disruption of primitive streak formation; this disruption resulted in circular, arc-shaped or intermediate forms, reminiscent of those found in amphibians, fishes and reptiles. Our results reveal a crucial role of ROCK-controlled directional cell movements during rabbit primitive streak formation and highlight the possibility that temporal and spatial modulation of cell movements were instrumental for the evolution of gastrulation forms.

Summary: ROCK regulates cell motility, polarisation, intercalation and division in gastrulating rabbit embryos, revealing a role for Wnt-PCP signalling in primitive streak formation.

The evolution and conservation of left-right patterning mechanisms

Morphological asymmetry is a common feature of animal body plans, from shell coiling in snails to organ placement in humans. The signaling protein Nodal is key for determining this laterality. Many vertebrates, including humans, use cilia for breaking symmetry during embryonic development: rotating cilia produce a leftward flow of extracellular fluids that induces the asymmetric expression of Nodal. By contrast, Nodal asymmetry can be induced flow-independently in invertebrates. Here, we ask when and why flow evolved. We propose that flow was present at the base of the deuterostomes and that it is required to maintain organ asymmetry in otherwise perfectly bilaterally symmetrical vertebrates.

Early left-right asymmetries during axial morphogenesis in the chick embryo

The primitive node is the "hub" of early left-right patterning in the chick embryo: (1) it undergoes asymmetrical morphogenesis immediately after its appearance at Stage 4; (2) it is closely linked to the emerging asymmetrical expression of nodal and shh at Stage 5; and (3) its asymmetry is spatiotemporally related to the emerging notochord, the midline barrier maintaining molecular left-right patterning from Stage 6 onward. Here, we study the correlation of node asymmetry to notochord marker expression using high-resolution histology, and we test pharmacological inhibition of shh signaling using cyclopamine at Stages 4 and 5. Just as noggin expression mirrors an intriguing structural continuity between the right node shoulder and the notochord, shh expression in the left node shoulder confirms a similar continuity with the future floor plate. Shh inhibition at Stage 4 or 5 suppressed nodal in both its paraxial or lateral plate mesoderm domains, respectively, and resulted in randomized heart looping. Thus, the "primordial" paraxial nodal asymmetry at Stage 4/5 (1) appears to be dependent on, but not instructed by, shh signaling and (2) may be fixed by asymmetrical roots of the notochord and the floor plate, thereby adding further twists to the node's pivotal role during left-right patterning.

Sept6 is required for ciliogenesis in Kupffer's vesicle, the pronephros, and the neural tube during early embryonic development

Septins are conserved filament-forming GTP-binding proteins that act as cellular scaffolds or diffusion barriers in a number of cellular processes. However, the role of septins in vertebrate development remains relatively obscure. Here, we show that zebrafish septin 6 (sept6) is first expressed in the notochord and then in nearly all of the ciliary organs, including Kupffer's vesicle (KV), the pronephros, eye, olfactory bulb, and neural tube. Knockdown of sept6 in zebrafish embryos results in reduced numbers and length of cilia in KV. Consequently, cilium-related functions, such as the left-right patterning of internal organs and nodal/spaw signaling, are compromised. Knockdown of sept6 also results in aberrant cilium formation in the pronephros and neural tube, leading to cilium-related defects in pronephros development and Sonic hedgehog (Shh) signaling. We further demonstrate that SEPT6 associates with acetylated α-tubulin in vivo and localizes along the axoneme in the cilia of zebrafish pronephric duct cells as well as cultured ZF4 cells. Our study reveals a novel role of sept6 in ciliogenesis during early embryonic development in zebrafish.