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PubMed1993

Inductive events in the patterning of the Xenopus laevis hatching and cement glands, two cell types which delimit head boundaries.

In Xenopus, the hatching and cement glands form at important boundaries in the head. We have examined induction of these glands in order to see the basis for this ectodermal patterning. Ectoderm can be induced to form hatching and cement glands by dorsal mesoderm and by the neural plate. Because the neural plate is sufficient to induce hatching and cement glands and lies adjacent to presumptive hatching and cement glands at the time of induction, it is the best candidate for the in vivo inducer of these tissues. The cement gland is restricted to the front of the head in part because the anterior but not posterior neural plate is capable of inducing it. The hatching gland is also restricted to the head, but can be induced by both anterior and posterior neural plates. Therefore, some factor suppresses hatching gland differentiation in the trunk. Transplanted neural plate pieces induced hatching gland cells in the ectoderm of embryos, suggesting that the inductive signal is planar. Treatment with retinoic acid or lithium at the start of gastrulation caused a loss of head structures. The presence of hatching gland in lithium-treated embryos suggests that these two agents have distinct effects and supports the idea that the induction of hatching and cement glands involves different pathways.

Animals↗
PubMed1996

Inductive processes leading to inner ear formation during Xenopus development.

This study examines the spatial and temporal attributes of inner ear induction in Xenopus embryos. These results are compared to recent experiments concerning lens induction to assess whether head sensory structures share common ontogenetic features. Ectoderm from different regions and stages was transplanted to the presumptive ear region of hosts of either early (neural plate) or late (neural tube) stages. Explants of the presumptive ear ectoderm were also taken from embryos at these stages to establish the time of otic ectoderm specification. We find that ectodermal competence for otic vesicle formation extends through neural plate stages, far longer than for lens formation. Otic vesicle specification occurs substantially earlier, at neural plate stages, than lens specification. Competent ectoderm forms otic vesicles in a high fraction of cases when exposed to the ear-inducing environment of either neural plate stages or neural tube stages, a result which contrasts with lens induction where the neural tube stage embryo provides a much weaker inducing environment than earlier stages. Otic vesicles induced in neural tube stage hosts are primarily in contact with presumptive hindbrain, suggesting that this neural tissue may be sufficient for otic vesicle induction. These studies reveal overall similarities between lens and inner ear induction, but sufficient differences to propose that some facets of determination of these sensory tissues may occur by independent mechanisms and not via a common developmental state.

Animals↗
PubMed1982

On the morphogenesis of the early rudiments of the developing central nervous system.

Neurulation consists of a complex series of events that result in formation of the neural tube, the rudiment of the entire adult central nervous system. This article discusses primary neurulation, a process that occurs in three major stages: formation of the neural plate; bending of the neural plate, with formation of the neural groove and neural folds; and fusion of the neural folds, with formation of the roof of the neural tube and overlying surface epithelium. In addition, new information is presented on the morphogenesis of the chick neuroepithelium during bending of the neural plate. These data show that bending of the neural plate occurs in two distinct steps at all craniocaudal levels: elevation of the lateral parts of the neural plate and the incipient neural folds, and formation and convergence of the definitive neural folds. Elevation is associated with the formation of a midline, ventral locus of bending, and convergence, with the formation of bilateral loci of bending on the future luminal sides of the dorsolateral walls of the neural groove. Several mechanisms possibly involved in the stages and steps of primary neurulation are discussed.

Animals↗
PubMed1996

The genesis of avian neural crest cells: a classic embryonic induction.

Neural crest cells arise from the ectoderm and are first recognizable as discrete cells in the chicken embryo when they emerge from the neural tube. Despite the classical view that neural crest precursors are a distinct population lying between epidermis and neuroepithelium, our results demonstrate that they are not a segregated population. Cell lineage analyses have demonstrated that individual precursor cells within the neural folds can give rise to epidermal, neural crest, and neural tube derivatives. Interactions between the neural plate and epidermis can generate neural crest cells, since juxtaposition of these tissues at early stages results in the formation of neural crest cells at the interface. Inductive interactions between the epidermis and neural plate can also result in "dorsalization" of the neural plate, as assayed by the expression of the Wnt transcripts characteristic of the dorsal neural tube. The competence of the neural plate changes with time, however, such that interaction of early neural plate with epidermis generates only neural crest cells, whereas interaction of slightly older neural plate with epidermis generates neural crest cells and Wnt-expressing cells. At cranial levels, neuroepithelial cells can regulate to generate neural crest cells when the endogenous neural folds are removed, probably via interaction of the remaining neural tube with the epidermis. Taken together, these experiments demonstrate that: (i) progenitor cells in the neural folds are multipotent, having the ability to form multiple ectodermal derivatives, including epidermal, neural crest, and neural tube cells; (ii) the neural crest is an induced population that arises by interactions between the neural plate and the epidermis; and (iii) the competence of the neural plate to respond to inductive interactions changes as a function of embryonic age.

Animals↗
PubMed2006

The role of XBtg2 in Xenopus neural development.

In early neural development, active cell proliferation and apoptosis take place concurrent with cell differentiation, but how these processes are coordinated remains unclear. In this study, we characterized the role of XBtg2 in Xenopus neural development. XBtg2 transcripts were detected at the edge of the anterolateral neural plate and the neural crest region at the midneurula stage, and in eyes and in part of the neural tube at the tailbud stage. Translational inhibition of XBtg2 affected anterior neural development and impaired eye formation. XBtg2 depletion altered the expression patterns of the early neural genes, Zic3 and SoxD, at the midneurula stage, but not at the early neurula stage. At the midneurula stage, XBtg2-depleted embryos exhibited a marked decrease in the expression of anterior neural genes, En2, Otx2, and Rx1, without any changes in neural crest genes, Slug and Snail, or an epidermal gene, XK81. These results suggest that XBtg2 is required for the differentiation of the anterior neural plate from the midneurula stage, but not for the specification of the fate and patterning of the neural plate. XBtg2-depleted embryos also exhibited an increase in both proliferation and apoptosis in the anterior neural plate; however, the altered expression patterns of neural markers were not reversed by inhibition of either the cell cycle or apoptosis. Based on these data, we propose that XBtg2 plays an essential role in the anterior neural development, by regulating neural cell differentiation, and, independently, cell proliferation and survival.

Animals↗
PubMed2004

Multiple roles of Sox2, an HMG-box transcription factor in avian neural crest development.

Expression of Sox2, which encodes an HMG-box-type transcription factor, is down-regulated in the neural plate when neural crest segregates from dorsal neural tube and remains low during crest cell migration. Sox2 expression is subsequently up-regulated in some crest-derived cells in the developing peripheral nervous system and is later restricted to glial sublineages. Misexpression of Sox2 and mutant forms of Sox2 both in neural plate explants and in embryonic ectoderm reveals that Sox2 inhibits neural crest formation as a transcriptional activator. Similar manipulation of Sox2 function in migratory and postmigratory neural crest-derived cells indicates that Sox2 regulates proliferation and differentiation in developing peripheral nervous system. Developmental Dynamics 229:74-86, 2004.

Animals↗
PubMed1988

Microsurgical analyses of avian neurulation: separation of medial and lateral tissues.

Neurulation, formation of the neural tube, is a complex process involving shaping and bending of the neural plate and closure of the neural groove. We have used avian embryos as model systems to study this process. In the present investigation, blastoderms were cut parasagittally through their entire thickness, either unilaterally or bilaterally, at two mediolateral locations: 1) at the juncture between prospective neural plate and prospective surface epithelium, and 2) at the juncture between the midline strip of prospective neural plate and more lateral prospective neural plate. In the first experiment, shaping of the neural plate seemed normal, but elevation and convergence of the neural folds and closure of the neural groove were inhibited (except at the forebrain level). This result demonstrates that extrinsic forces generated by lateral tissues are required for neural plate bending and neural groove closure. In the second experiment, neuroepithelial cells within the isolated, midline strip became wedge shaped. This result indicates that neuroepithelial cell "wedging" is an active event occurring independently of forces generated by elevation of the neural folds. Additional studies are required to define the natures of neurulation forces and the mechanisms by which they are generated.

Animals↗
PubMed2003

A 90-degree rotation of the mitotic spindle changes the orientation of mitoses of zebrafish neuroepithelial cells.

In the neural plate and neural tube in the trunk region of the zebrafish embryo, dividing cells are oriented parallel to the plane of the neuroepithelium, while in neural keel/rod, cells divide perpendicular to it. This change in the orientation of mitosis is brought about by a 90 degrees rotation of the mitotic spindle. As the two halves of the neural primordium in keel/rod stage are in apposition, the perpendicular orientation of mitoses in this stage determines that daughter cells become allocated to both sides of the neural tube. To assess the role played by cell junctions in controlling the orientation of dividing cells, we studied the expression of components of adherens and tight junctions in the neuroepithelial cells. We find that these proteins are distributed irregularly at the neural plate stage and become polarised apically in the cell membrane only during the keel/rod stage. The stereotypic orientation of mitoses is perturbed only weakly upon loss of function of the cell junction components ASIP and aPKClambda, suggesting that mitotic orientation depends in part on the integrity of cell junctions and the polarity of the epithelium as a whole. However, the 90-degree rotation of the spindle does not require perfectly polarised cell junctions between the neuroepithelial cells.

Animals↗
PubMed1999

Axial mesendoderm refines rostrocaudal pattern in the chick nervous system.

There has long been controversy concerning the role of the axial mesoderm in the induction and rostrocaudal patterning of the vertebrate nervous system. Here we investigate the neural inducing and regionalising properties of defined rostrocaudal regions of head process/prospective notochord in the chick embryo by juxtaposing these tissues with extraembryonic epiblast or neural plate explants. We localise neural inducing signals to the emerging head process and using a large panel of region-specific neural markers, show that different rostrocaudal levels of the head process derived from headfold stage embryos can induce discrete regions of the central nervous system. However, we also find that rostral and caudal head process do not induce expression of any of these molecular markers in explants of the neural plate. During normal development the head process emerges beneath previously induced neural plate, which we show has already acquired some rostrocaudal character. Our findings therefore indicate that discrete regions of axial mesendoderm at headfold stages are not normally responsible for the establishment of rostrocaudal pattern in the neural plate. Strikingly however, we do find that caudal head process inhibits expression of rostral genes in neural plate explants. These findings indicate that despite the ability to induce specific rostrocaudal regions of the CNS de novo, signals provided by the discrete regions of axial mesendoderm do not appear to establish regional differences, but rather refine the rostrocaudal character of overlying neuroepithelium.

Animals↗
PubMed2001

Induction and development of neural crest in Xenopus laevis.

Neural crest cells are a migratory embryonic cell population that form at the border between the neural plate and the future epidermis. This border, the neural plate border, corresponds to the neural fold. The neural fold surrounds the entire neural plate, but only the lateral and posterior portions of the fold give rise to neural crest cells, while the anterior neural fold differentiates as forebrain. This review focuses on neural crest development in Xenopus laevis embryos, and analyzes aspects of the induction of the neural crest in Xenopus, summarizing available information relating to the expression of several genes in the neural crest. Two models for neural crest induction are discussed. In the first model, the neural crest is induced by the interaction between the neural plate and the epidermis. In the second, the specification of the neural plate border arises as a consequence of a gradient of BMP activity. The role of posteriorizing signals on neural crest specification is also discussed. Finally, we propose that the specification and differentiation of the neural crest is controlled by a cascade of transcription factors, encoded and expressed from a hierarchy of genes. A set of extracellular signals establishes the positional information in the ectoderm, which activates Prepattern genes (Gli, Xiro, Zic, Dlx, etc.) across extended and overlapping domains. A local combination of these genes at the neural plate border activates the cascade of neural crest specification, while different sets of genes are activated at both sides of the neural folds (in the epidermis and the neural plate). The genes activated in regions adjacent to the neural plate border have an inhibitory effect on the neural crest transcription program.

Animals↗
PubMed1995

Tissue boundaries and cell behavior during neurulation.

We have analyzed the dynamics of the boundaries between the neural plate and the epidermis and between the neural plate and the notoplate. Our experiments confirm that these two boundaries have important roles in neurulation. Measurements of the lengths of neural fold (the boundary between epidermis and neural plate) in embryos of axolotls and newts reveal that neural folds abutting the prospective brain decrease in length while neural folds abutting the prospective spinal cord increase in length during neurulation. We tested the proposition that boundaries of the neural plate with epidermis and with notoplate are essential for proper neurulation. Cuts made along the boundaries with epidermis or with notoplate stop, or greatly diminish, neural plate elongation and tube formation. Explanting the axolotl neural plate without any bordering epidermis stops plate elongation and prevents neural tube closure, but neural plates explanted with a rim of epidermis elongate and close into tubes. Cutting the notoplate boundary stops midline elongation in the newt embryo or diminishes it in the axolotl embryo. We conclude that the notoplate boundary and part of the boundary of the epidermis that abuts the prospective spinal cord organize cell behavior to elongate the neural plate and help close the neural tube. The boundary of the neural plate with the epidermis is essential for tube closure both because it organizes plate elongation in the spinal cord region and because cell behavior becomes organized at the boundary such that neural folds are raised and a rolling moment is produced that helps form the neural tube.

Ambystoma↗
PubMed1987

Shaping of the chick neuroepithelium during primary and secondary neurulation: role of cell elongation.

Cell elongation is a cardinal event in formation and shaping of the neuroepithelium during both primary and secondary neurulation. This study had three purposes. The first was to clarify the role of microtubules in maintaining the elongated configurations of chick neuroepithelial cells. Neuroepithelial cells of the neural plate (the rudiment of the primary neural tube) and medullary cord (the rudiment of the secondary neural tube) reduced their heights an average of approximately 25% when their microtubules were depolymerized, but most cells remained considerably elongated and columnar. Complete rounding up occurred only as cells entered metaphase where they arrested. These results suggest that microtubules as well as other factors are required to maintain the fully elongated configurations of cells composing epithelial sheets. The second purpose of this study was to evaluate the hypothesis that neuroepithelial cell elongation plays a major role in narrowing of the neural plate. To do this, the width of the neural plate was examined after microtubule depolymerization and repolymerization. As the heights of neuroepithelial cells decreased with loss of their microtubules, the width of the neuroepithelium increased roughly proportionately; subsequent repolymerization with concomitant cell elongation resulted again in neural plate narrowing. Thus, the hypothesis is supported. The third purpose of this study was to examine the roles of cell rearrangement and change in neuroepithelial cell or extracellular volume in neural plate narrowing and extension. Extensive cell rearrangement, resulting in net cell loss from the width of selected, representative levels of the neural plate, does not seem to play a major role in plate narrowing, but decreases in cell or extracellular volume are likely involved. Further studies are necessary to complete our understanding of the mechanisms driving neural plate shaping and bending.

Animals↗
PubMed1992

Mechanism of anteroposterior axis specification in vertebrates. Lessons from the amphibians.

Interest in the problem of anteroposterior specification has quickened because of our near understanding of the mechanism in Drosophila and because of the homology of Antennapedia-like homeobox gene expression patterns in Drosophila and vertebrates. But vertebrates differ from Drosophila because of morphogenetic movements and interactions between tissue layers, both intimately associated with anteroposterior specification. The purpose of this article is to review classical findings and to enquire how far these have been confirmed, refuted or extended by modern work. The "pre-molecular" work suggests that there are several steps to the process: (i) Formation of anteroposterior pattern in mesoderm during gastrulation with posterior dominance. (ii) Regional specific induction of ectoderm to form neural plate. (iii) Reciprocal interactions from neural plate to mesoderm. (iv) Interactions within neural plate with posterior dominance. Unfortunately, almost all the observable markers are in the CNS rather than in the mesoderm where the initial specification is thought to occur. This has meant that the specification of the mesoderm has been assayed indirectly by transplantation methods such as the Einsteckung. New molecular markers now supplement morphological ones but they are still mainly in the CNS and not the mesoderm. A particular interest attaches to the genes of the Antp-like HOX clusters since these may not only be markers but actual coding factors for anteroposterior levels. We have a new understanding of mesoderm induction based on the discovery of activins and fibroblast growth factors (FGFs) as candidate inducing factors. These factors have later consequences for anteroposterior pattern with activin tending to induce anterior, and FGF posterior structures. Recent work on neural induction has implicated cAMP and protein kinase C (PKC) as elements of the signal transduction pathway and has provided new evidence for the importance of tangential neural induction. The regional specificity of neural induction has been reinvestigated using molecular markers and provides conclusions rather similar to the classical work. Defects in the axial pattern may be produced by retinoic acid but it remains unclear whether its effects are truly coordinate ones or are concentrated in certain regions of high sensitivity. In general the molecular studies have supported and reinforced the "pre-molecular ones". Important questions still remain: (i) How much pattern is there in the mesoderm (how many states?) (ii) How is this pattern generated by the invaginating organizer? (iii) Is there one-to-one transmission of codings to the neural plate? (iv) What is the nature of the interactions within the neural plate? (v) Are the HOX cluster genes really the anteroposterior codings?

Animals↗
PubMed1994

Molecular genetics of neurulation.

The formation of the neural tube begins during gastrulation when ectoderm, an epithelial sheet on the outside of the embryo, is induced to form the neural plate. During the process of neural induction, the epithelium of the neural plate is regionalized along both the dorsoventral and anteroposterior axes of the embryo; this regionalization is likely to contribute to the cellular processes underlying neurulation. Genes whose expression marks the formation and regionalization of the neural plate and which encode cell adhesion molecules or putative transcription factors have been recently identified. The differential expression of these genes apparently subdivides the epithelium of the neural plate into small regions. Evidence from transgenic embryo experiments supports the idea that the differential expression of these genes in the neural plate plays a role in neural tube formation.

Animals↗
PubMed2004

dlx3b and dlx4b function in the development of Rohon-Beard sensory neurons and trigeminal placode in the zebrafish neurula.

Rohon-Beard sensory neurons, neural crest cells, and sensory placodes can be distinguished at the boundary of the embryonic epidermis (skin) and the neural plate. The inductive signals at the neural plate border region are likely to involve a gradient of bone morphogenic protein (BMP) in conjunction with FGF and Wnts and other signals. However, how these signals are transduced to produce the final cell fate remains to be determined. Recent evidence from Xenopus and chick suggest that Dlx genes are required for the generation of cell fates at the neural plate border (McLarren, K.W., Litsiou, A., Streit, A., 2003. DLX5 positions the neural crest and preplacode region at the border of the neural plate. Dev. Biol. 259, 34-47; Woda, J.M., Pastagia, J., Mercola, M., Artinger, K.B., 2003. Dlx proteins position the neural plate border and determine adjacent cell fates. Development 130, 331-342). In the present study, we extend these findings to zebrafish, where we unequivocally demonstrate that dlx3b and dlx4b function in a dose-dependent manner to specify cell fates such as Rohon-Beard sensory neurons and trigeminal sensory placodes. dlx function was examined by inhibiting: (1) protein levels with antisense morpholino oligonucleotides (MOs), and (2) activity by repressing the ability of dlx-homeodomain to bind to downstream targets (EnR-dlx3bhd mRNA; dlx3b homeodomain fused to Engrailed transcriptional repressor domain). Inhibition of dlx3b and dlx4b protein and activity resulted in the reduction or complete loss of Rohon-Beard (RB) sensory neurons and trigeminal (TG) sensory placodes. These data suggest that dlx3b and dlx4b function in the specification of RB neurons and trigeminal sensory placodes in zebrafish. Further, we have shown that dlx3b and dlx4b function in a non-cell-autonomous manner for RB neuron development; dlx3b and dlx4b act to regulate bmp2b expression at the non-neural ectodermal border. These data suggest that the contribution of dlx3b and dlx4b to neural plate border formation is partially non-cell-autonomous acting via BMP activity.

Animals↗
PubMed1998

Paraxial-fated mesoderm is required for neural crest induction in Xenopus embryos.

Neural crest induction is thought to occur by a two-step process. Axially fated mesoderm induces neural plate, which is then recruited to neural crest by nonneural epidermal ectoderm at the neural plate border. This model suggests a rather indirect role for mesoderm in inducing neural crest. We extensively examined the role of mesoderm in neural crest induction by determining which types of mesoderm induce neural crest cells in Xenopus embryos. We found that noggin-dorsalized ventral marginal zone (VMZ) explants differentiate as melanocytes in the absence of axial mesoderm. Dorsalized VMZ is also a potent inducer of melanocytes when juxtaposed to animal cap ectoderm in recombinant explants. Dorsalized VMZ is analogous to the dorsal-lateral marginal zone (DLMZ) region of the embryo. Neural crest-inducing activities of gastrula stage DLMZ and dorsal marginal zone (DMZ) were also compared in recombinant explants. DLMZ was a stronger inducer of neural crest than was DMZ; DLMZ induced high levels of XSlug expression and melanocyte formation in recombinants, whereas DMZ weakly induced neural crest. In whole embryos lacking DLMZ, XSlug expression and melanocyte formation were significantly reduced; in contrast, no significant reduction of XSlug expression or melanocyte formation was seen in embryos lacking a DMZ. These results suggest that paraxial-fated mesoderm plays a central role in neural crest formation by inducing a novel type of lateral neural plate. This lateral neural plate is then recruited to neural crest by adjacent nonneural epidermal ectoderm.

Animals↗
PubMed1991

Homeogenetic neural induction in Xenopus.

Neural induction is known to involve an interaction of ectoderm with dorsal mesoderm during gastrulation, but several kinds of studies have argued that competent ectoderm can also be neutralized via an interaction with previously neuralized tissue, a process termed homeogenetic neural induction. Although homeogenetic neural induction has been proposed to play an important role in the normal induction of neural tissue, this process has not been subjected to detailed study using tissue recombinants and molecular markers. We have examined the question of homeogenetic neural induction in Xenopus embryos, both in transplant and recombinant experiments, using the expression of two neural antigens to assay the response. When ectoderm that is competent to be neuralized is transplanted to the region adjacent to the neural plate of early neurula embryos, it forms neural tissue, as assayed by staining with antibodies against the neural cell adhesion molecule, N-CAM. Transplants to the ventral region, far from the neural plate, do not express N-CAM, indicating that neuralization is not occurring as a result of the transplantation procedure itself. Because this response might be occurring as a result of interactions of ectoderm with either adjacent neural plate tissue, or with underlying dorsolateral mesoderm, recombinant experiments were performed to determine the source of the neuralizing signal. Ectoderm cultured in combination with neural plate tissue alone expresses neural markers, while ectoderm cultured in combination with dorsolateral mesoderm does not. We conclude that neural tissue can homeogenetically induce competent ectoderm to form neural tissue and argue that this induction occurs via planar signaling within the ectoderm, a mechanism that, in normal development, may be involved in interactions within presumptive neural ectoderm or in specifying structures that lie near the neural plate.

Animals↗
PubMed2004

Neural tissue continues its maturation at the site of neural tube closure defects: implications for prenatal intervention in human samples.

OBJECTIVE: Our objective was to investigate the relation between the embryological development and neural tissue maturation at the site where the neural plate failed to form a neural tube. MATERIAL AND METHODS: Samples from 15 aborted human fetuses with neural tube defects (NTD). All of the fetuses were between 20 and 25 gestational weeks old. Indicators of neural tissue maturation, formation of basal lamina, expression of integrins and neuron specific class III beta tubulin (tuj1) were investigated. To detect the adverse effects of the environment, if any, p53 and bcl-2 activity at both sites of the open and closed neural plate were investigated as well. RESULTS: No difference was found in the expression of maturation-related molecules at the site of the neural plate that remained open compared with the site where the neural tube is normally formed. While high p53 activity was noted in neural tissue at the site of the neural tube defect, no such activity was detected in the neural tissue where the neural tube is normally formed. CONCLUSION: Our results suggested that maturation and differentiation of neural tissue continued regardless of the failure of neural tube closure. Therefore, the neurological deficits that are encountered in NTD patients should be related to secondary damage such as amnion fluid toxicity, uterus contractions, labor, etc. It seems valuable to save the neural plate before the negative effects of the environment renders the neural tissue functionless.

Embryonic and Fetal Development↗