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PubMed1998

Vascularization in the murine allantois occurs by vasculogenesis without accompanying erythropoiesis.

The aim of this study was to determine whether the blood vessels of the murine allantois are formed by vasculogenesis or angiogenesis. Morphological analysis revealed that differentiation of allantoic mesoderm into an outer layer of mesothelium and an inner vascular network begins in the distal region of the allantois, which is most remote from other tissues, as early as the late neural plate stage (approximately 7.75 days postcoitum). Nascent blood vessels were not found in the base of the allantois until 4-somite pairs had formed in the fetus (approximately 8.25 days postcoitum), and vascular continuity with the yolk sac and fetus was not present until the 6-somite-pair stage (approximately 8.5 days postcoitum). Immunohistochemical analysis demonstrated that flk-1, a molecular marker of early endothelial cells, is expressed in significantly more distal than basal core cells in the early allantois and never in mesothelium. Furthermore, synchronous grafting of donor yolk sac containing blood islands into blood islands of headfold-stage host conceptuses provided no evidence that the yolk sac contributes endothelial cells to the allantois. Finally, when removed from conceptuses and cultured in isolation, neural plate and headfold-stage allantoises formed a conspicuous vascular network that was positive for Flk-1. Hence, the vasculature of the allantois is formed intrinsically by vasculogenesis rather than extrinsically via angiogenesis from the adjacent yolk sac or fetus. Whether allantoic vasculogenesis is associated with erythropoiesis was also investigated. Benzidine-staining in situ revealed that primitive erythroid cells were not identified in the allantois until 6-somite pairs when continuity between its vasculature and that of the yolk sac was first evident. Nevertheless, a small number of allantoises removed from conceptuses at a considerably earlier stage were found to contain erythroid precursor cells following culture in isolation. To determine whether such erythroid cells could be of allantoic origin, host allantoises were made chimeric with lacZ-expressing donor allantoises that were additionally labeled with [3H]methyl thymidine. Following culture and autoradiography, many lacZ-expressing benzidine-stained cells were observed in donor allantoises, but none contained silver grains above background. Moreover, no cells of donor allantoic origin were found in the fetus or yolk sac. Hence, vasculogenesis seems to be independent of erythropoiesis in the allantois and to involve a distal-to-proximal gradient in differentiation of allantoic mesoderm into the endothelial cell lineage. Furthermore, this gradient is established earlier than reported previously, being present at the neural plate stage.

Allantois↗
PubMed2004

Assembly and remodeling of the fibrillar fibronectin extracellular matrix during gastrulation and neurulation in Xenopus laevis.

Fibronectin, a major component of the extracellular matrix is critical for processes of cell traction and cell motility. Whole-mount confocal imaging of the three-dimensional architecture of the extracellular matrix is used to describe dynamic assembly and remodeling of fibronectin fibrils during gastrulation and neurulation in the early frog embryo. As previously reported, fibrils first appear under the prospective ectoderm. We describe here the first evidence for regulated assembly of fibrils along the somitic mesoderm/endoderm boundary as well as at the notochord/somitic mesoderm boundary and clearing of fibrils from the dorsal and ventral surfaces of the notochord that occurs over the course of a few hours. As gastrulation proceeds, fibrils are restored to the dorsal surface of the notochord, where the notochord contacts the prospective floor plate. As the neural folds form, fibrils are again remodeled as deep neural plate cells move medially. The process of neural tube closure leaves a region of the ectoderm overlying the neural crest transiently bare of fibrils. Fibrils are assembled surrounding the dorsal surface of the neural tube as the neural tube lumen is restored.

Animals↗
PubMed2005

An essential role of Xenopus Foxi1a for ventral specification of the cephalic ectoderm during gastrulation.

During gastrulation in Xenopus, the head ectoderm is subdivided into the central nervous system (CNS) anlage (neural plate) and the non-CNS ectoderm (i.e. epidermis, placodes and neural crest). The winged-helix transcription factor Xfoxi1a is one of the earliest markers for the preplacodal region at the mid-neurula stage. Interestingly, before the establishment of the preplacodal region, Xfoxi1a expression is detected in the entire cephalic non-neural ectoderm at the mid- and late gastrula stages. The present study focuses on the role of Xfoxi1a particularly at the gastrula stages. The early Xfoxi1a expression in the anteroventral ectoderm is dependent on Bmp signals and suppressed by Wnt signals. Inhibition of Xfoxi1a activities by injection of antisense oligonucleotides leads to suppression of non-CNS ectodermal markers (e.g. keratin) and expansion of the anterior expression domain of the CNS marker Sox2. Conversely, misexpression of Xfoxi1a suppresses Sox2 and induces keratin in the anterior neural plate. In the animal cap, Xfoxi1a overexpression antagonizes the neuralizing activity of Chordin (Chd). Studies using an inducible Xfoxi1a construct (GR-Xfoxi1a) show that the ventralizing function of Xfoxi1a is confined to the gastrula stage. Thus, Xfoxi1a is an essential regulator of ventral specification of the early head ectoderm during gastrulation.

Amino Acid Sequence↗
PubMed1987

Neural cell adhesion molecule expression in Xenopus embryos.

The spatiotemporal pattern of expression of the neural cell adhesion molecule NCAM was mapped immunohistochemically in embryos of the frog Xenopus, from blastula to early swimming stages, using a polyclonal antibody that recognizes Xenopus NCAM. The neural plate stage was the earliest at which NCAM could be detected. The initial sites of NCAM immunoreactivity were neural ectoderm, somitic mesoderm, and chordamesoderm. During formation of the neural tube, NCAM immunoreactivity became restricted to the neuroectoderm and its derivatives. During closure of the neural tube and for 2-4 hr thereafter, NCAM was expressed in a distinctive radial pattern in coronal sections of the neural tube. NCAM was observed in neural crest cells before migration and after formation of cranial and spinal ganglia. During the period of initial neurite outgrowth, NCAM became concentrated in the developing central nerve fiber pathways. NCAM was seen on peripheral nerves from the time of their initial outgrowth and it was strongly expressed at neuromuscular junctions during the period of their formation. These results show that NCAM is expressed after neural induction and functions during morphogenesis of the neural plate and tube, some neural crest derivatives, development of nerve fiber tracts, and formation of neuromuscular connections.

Animals↗
PubMed1995

Early stages of notochord and floor plate development in the chick embryo defined by normal and induced expression of HNF-3 beta.

We have cloned a cDNA encoding the chick HNF-3 beta gene and have used RNA and antibody probes that detect HNF-3 beta to monitor the normal and induced expression of the gene in early embryos. HNF-3 beta expressed in Koller's sickle, at the onset of primitive streak formation, and later in Hensen's node. At neural plate and neural tube stages, HNF-3 beta is expressed transiently in the notochord and is then expressed by floor plate cells. Prospective floor plate cells that are located in the epiblast immediately anterior to Hensen's node prior to its regression do not express HNF-3 beta, providing evidence that floor plate fate is normally determined only after these cells populate the midline of the neural plate and overlie the notochord. Removal of the notochord in vivo prevents floor plate development and in this condition HNF-3 beta is not expressed by cells at the ventral midline of the neural tube. Notochord grafts induce ectopic floor plate development and ectopic neural expression of HNF-3 beta. In vitro, neural plate explants are induced to express HNF-3 beta by notochord cells in a contact-dependent but cycloheximide-resistant manner, providing evidence that expression of HNF-3 beta is a direct response of neural plate cells to notochord-derived inducing signals.

Amino Acid Sequence↗
PubMedDate not supplied

Molecular analysis of neural crest formation.

Neural crest cells arise within the ectoderm during neurulation and give rise to most of the peripheral nervous system. Following neural tube closure, they come to lie within the dorsal neural tube from which they emerge and subsequently migrate extensively to numerous and characteristic sites. There, they differentiate into neurons and glia of the peripheral nervous system, cartilage and bone of the face, melanocytes and various other cell types. Fate mapping experiments have demonstrated that the neural crest arises at the juncture between presumptive epidermis and neural plate. However, injection of lineage tracer into individual cells reveals that single neural fold cells are not committed to a neural crest fate; rather these cells can form all ectodermal derivatives (epidermis, neural tube, neural crest). Inductive interactions between the neural and non-neural ectoderm can generate neural crest cells, suggesting that signals travel through the epidermis to generate neural crest cells prior to neural tube closure. Induction of the neural crest appears to be a multiphasic process and involves a combination of an early Wnt signal together with later functions of BMP signaling pathways. We have used a variety of molecular screens to isolate molecular constituents involved in neural crest formation. We have identified a secreted factor, Noelin-1, which is expressed in the prospective avian neural plate and may play a role making the neural tube competent to form neural crest. Noelin-1 mRNA is expressed in a graded pattern in the closing neural tube, with highest expression in the neural folds and no detectable expression at the ventral midline. Its expression precedes that of Slug, a zinc finger transcription factor that represents the earliest known neural crest marker gene. Over-expression of Noelin-1 using recombinant retroviruses causes an excess of neural crest emigration and prolongs the time that the neural tube is competent to generate and regenerate neural crest cells. These results support an important role for Noelin-1 in rendering the neural tube competent to respond to inductive cues to generate neural crest.

Animals↗
PubMed1999

posterior end mark 3 (pem-3), an ascidian maternally expressed gene with localized mRNA encodes a protein with Caenorhabditis elegans MEX-3-like KH domains.

Maternal factors localized in the posterior-vegetal cytoplasm of an ascidian egg are essential for cell specification and pattern formation of the embryo. The molecular identification of the localized factors and the elucidation of the machinery associated with the localization are therefore key research subjects. I report here the isolation and characterization of a novel maternally expressed gene, posterior end mark 3 (pem-3). The pem-3 cDNA was obtained from a cDNA library of fertilized egg mRNAs subtracted with gastrula mRNAs of Ciona savignyi. As in the case of pem (Yoshida et al., 1996, Development 122, 2005-2012), the pem-3 maternal transcript was gradually concentrated after fertilization in the posterior-vegetal cytoplasm of the egg, and it later marked the posterior end of developing embryos. The PEM-3 protein was also detected in the posterior end of early embryos. The nucleotide sequence predicted that pem-3 encodes a probable RNA-binding protein with two KH domains that have an extensive similarity with those of Caenorhabditis elegans MEX-3. MEX-3 is also localized in nematode embryos (Draper et al., 1996, Cell 87, 205-216), suggesting that PEM-3 is a candidate homologue of MEX-3. In addition to maternal expression, a zygotic transcript of pem-3 and its gene product were detected in cells of the neural plate, mesenchyme, and epidermis of embryos after the neural-plate stage. Inhibition of zygotic expression using an antisense oligonucleotide resulted in the development of abnormal larvae without sensory pigment cells, suggesting that the zygotic PEM-3 plays a role in the differentiation of the brain of the ascidian larva.

Amino Acid Sequence↗
PubMed2005

Regulation of Slug transcription in embryonic ectoderm by beta-catenin-Lef/Tcf and BMP-Smad signaling.

Neural crest is formed at the boundary of epidermal and neural ectoderm. To understand the molecular mechanism of neural crest formation, we focused on the transcriptional regulation of the Slug gene. In the upstream sequence of the chicken Slug gene, we have identified potential binding sites for transcription factors, such as Lef/Tcf and Smad1. Transgenic mouse embryos carrying the chicken Slug promoter-reporter gene showed a crest-specific activation of the reporter, suggesting the isolated sequence included the cis-regulatory elements to receive Slug-inducing signals in the mouse neural crest. While these potential cis-regulatory elements could be recognized and activated by corresponding transcription factors, such as Lef1 and Smad1, Wnt-Lef-beta-catenin signal failed to induce endogenous Slug expression in quail neural plate tissue prepared from forebrain and midbrain levels. In contrast, Slug expression and subsequent epithelial-mesenchymal transition were effectively induced by BMP4. Consistently, while we could detect phosphorylation of Smad1 in the ectoderm including the neural plate and the neural fold region, the activation of a reporter gene for a detection of canonical Wnt signal activation was below the level of detection at the forebrain and midbrain levels. These observations indicated that in the anterior ectoderm BMP signal has a predominant role for Slug expression.

Animals↗
PubMed2000

Phenoloxidase, a marker enzyme for differentiation of the neural ectoderm and the epidermal ectoderm during embryonic development of amphioxus Branchiostoma belcheri tsingtaunese.

The development of phenoloxidase during amphioxus embryogenesis was spectrophotometrically and histochemically studied for the first time in the present study. It was found that (1) PO activity initially appeared in the general ectoderm including the neural ectoderm and the epidermal ectoderm at the early neurula stage but not in the mesoderm or the endoderm, and (2) PO activity disappeared in the neural plate cells but remained unchanged in the epidermal cells when the neural plate was morphologically quite distinct from the rest of the ectoderm. It is apparent that PO could serve as a marker enzyme for differentiation of the neural ectoderm from the epidermal ectoderm during embryonic development of amphioxus.

Animals↗
PubMed2001

Towards a cellular and molecular understanding of neurulation.

Neurulation occurs during the early embryogenesis of chordates, and it results in the formation of the neural tube, a dorsal hollow nerve cord that constitutes the rudiment of the entire adult central nervous system. The goal of studies on neurulation is to understand its tissue, cellular and molecular basis, as well as how neurulation is perturbed during the formation of neural tube defects. The tissue basis of neurulation consists of a series of coordinated morphogenetic movements within the primitive streak (e.g., regression of Hensen's node) and nascent primary germ layers formed during gastrulation. Signaling occurs between Hensen's node and the nascent ectoderm, initiating neurulation by inducing the neural plate (i.e., actually, by suppressing development of the epidermal ectoderm). Tissue movements subsequently result in shaping and bending of the neural plate and closure of the neural groove. The cellular basis of the tissue movements of neurulation consists of changes in the behavior of the constituent cells; namely, changes in cell number, position, shape, size and adhesion. Neurulation, like any morphogenetic event, occurs within the milieu of generic biophysical determinants of form present in all living tissues. Such forces govern and to some degree control morphogenesis in a tissue-autonomous manner. The molecular basis of neurulation remains largely unknown, but we suggest that neurulation genes have evolved to work in concert with such determinants, so that appropriate changes occur in the behaviors of the correct populations of cells at the correct time, maximizing the efficiency of neurulation and leading to heritable species- and axial-differences in this process. In this article, we review the tissue and cellular basis of neurulation and provide strategies to determine its molecular basis. We expect that such strategies will lead to the identification in the near future of critical neurulation genes, genes that when mutated perturb neurulation in a highly specific and predictable fashion and cause neurulation defects, thereby contributing to the formation of neural tube defects.

Animals↗
PubMed2000

Expression of chick BMP-1/Tolloid during patterning of the neural tube and somites.

The expression pattern described here is that of the chick BMP-1/Tolloid family of secreted metalloproteinases during early stages of development. BMP-1/Tolloid transcripts are expressed in the blastoderm, at gastrulation stages and as the neural plate forms and neural tube folds, BMP-1/Tolloid is found at the neural plate/ectodermal transition. Expression is maintained in the premigratory neural crest, and transiently in the migrating cephalic neural crest cells. BMP-1/Tolloid is also expressed in the caudal, but not in the anterior notochord, and in the ventral neural tube at the time of dorso-ventral patterning. Further sites of BMP-1/Tolloid expression are the lateral plate mesoderm and the dermotome and the myotome of the somites.

Animals↗
PubMed1985

The ultrastructure of early cephalic neural crest cell migration in the mouse.

A study of the ultrastructural changes associated with the detachment of the presumptive neural crest cells from the neuroepithelium in the midbrain region in mouse embryos at 9 and 9 1/2 days of gestation was carried out. The first sign of neural crest cell formation occurred in this region before fusion of the neuroepithelium had occurred. Neural crest cells arose from both the neural plate and the adjoining surface ectoderm. Initially, the cells of the neural plate and the surface ectoderm were attached to each other by zonula occludens and zonula adherans at their apical surfaces however, these junctions disappeared just prior to the beginning of the migration of the crest cells. The first sign of migration of the crest cells was the disappearance of the basal lamina in the region of the presumptive crest cells. Once the basal lamina was lost, cell junctions were formed between the epithelial cells and the underlying mesenchymal cells. Once the crest cells had migrated into the underlying mesenchyme, they tended to form clumps of closely related, irregularly shaped cells. Phagosomes and accumulations of glycogen particles were found within some crest cells when they were still within 50 to 100 microns of the epithelium.

Animals↗
PubMed1999

Characterization of a subfamily of related winged helix genes, XFD-12/12'/12" (XFLIP), during Xenopus embryogenesis.

The fork head domain family of genes defines a growing group of proteins that serve important regulatory functions in pattern-forming events of both invertebrates and vertebrates. Here we add three closely related, novel members to this family in Xenopus laevis, termed XFD-12, XFD-12' and XFD-12". All three genes reveal indistinguishable expression patterns during Xenopus embryogenesis. During gastrulation, XFD-12 type transcripts are detected exclusively in the superficial layer of cells within the Spemann organizer territory. In the open neural plate, XFD-12 type expression defines a row of cells located along the dorsal midline and destined to become the floor plate of the neural tube. After closure of the neural tube, XFD-12 type encoding mRNAs are only detected in the tailtip and a small area located at the midbrain/hindbrain boundary. Within the Spemann organizer and in the floor plate area, expression of XFD-12 type genes is only partially overlapping with XFD-1 expression.

Amino Acid Sequence↗
PubMed2000

Identification and developmental expression of par-6 gene in Xenopus laevis.

The par genes (partitioning defective) are required to establish polarity in the Caenorhabditis elegans embryo. We have identified the Xenopus homologue of C. elegans PAR-6 (XPAR-6). XPAR-6 is a protein of 377 amino acids with one PDZ domain which is involved in mediating protein-protein interactions. It shares 59% and 58% amino acid identity with the mouse and Drosophila PAR-6, respectively, and 54% overall identity with C. elegans PAR-6. Xpar-6 is expressed both maternally and zygotically. Xpar-6 is first detected in the animal half of the egg, and this pattern of expression persists into the cleavage and blastula stages. At the gastrula stage, the message is detected in animal pole area and in a broad domain of ventral region, but is excluded from dorsal region. With the onset of neurulation, the localized expression of Xpar-6 becomes more obvious, leading to it being enriched in the dorsolateral region along the lateral edges of neural plate and anterior presumptive head region surrounding the anterior border of neural plate. At late tailbud stage, Xpar-6 transcripts show localized expression throughout the head, labeling the branchial arches, eyes, otic vesicles and brain, while more posteriorly Xpar-6 labels the somites, pronephros, tail tip and proctodeum. Therefore, this analysis suggests that Xpar-6 has a regionalized pattern of expression during Xenopus early embryogenesis.

Amino Acid Sequence↗
PubMed1995

Dorsalization of the neural tube by the non-neural ectoderm.

The patterning of cell types along the dorsoventral axis of the spinal cord requires a complex set of inductive signals. While the chordamesoderm is a well-known source of ventralizing signals, relatively little is known about the cues that induce dorsal cell types, including neural crest. Here, we demonstrate that juxtaposition of the non-neural and neural ectoderm is sufficient to induce the expression of dorsal markers, Wnt-1, Wnt-3a and Slug, as well as the formation of neural crest cells. In addition, the competence of neural plate to express Wnt-1 and Wnt-3a appears to be stage dependent, occurring only when neural tissue is taken from stage 8-10 embryos but not from stage 4 embryos, regardless of the age of the non-neural ectoderm. In contrast to the induction of Wnt gene expression, neural crest cell formation and Slug expression can be induced when either stage 4 or stage 8-10 neural plates are placed in contact with the non-neural ectoderm. These data suggest that the non-neural ectoderm provides a signal (or signals) that specifies dorsal cell types within the neural tube, and that the response is dependent on the competence of the neural tissue.

Animals↗
PubMed2003

In vivo analysis of the cyclin D1 promoter during early embryogenesis in Xenopus.

Cyclin D1 plays an important role in the regulation of G1 progression of the somatic cell cycle by functioning as a regulatory subunit of cdk4 and 6. Since both cyclin D1-mRNA and -protein turn over very rapidly, the expression of cyclin D1 is mostly regulated at the transcription level. Detection of the cyclin D1-specific mRNA during early embryogenesis in Xenopus laevis revealed that expression was induced soon after the midblastula stage but not during the cleavage stage, and was mainly found in the neural plate and eye vesicles. The recently-developed transgenic frog technique enabled us to analyze the frog cyclin D1 promoter activity in vivo by using green fluorescent protein (GFP) as a marker. During early embryogenesis, a GFP signal was detected at the neural plate in the neurula of the transgenic embryos. Transgenic analysis using sequential-deletion and point mutants of the cyclin D1 promoter revealed that a single, putative TCF/LEF binding site, located approximately 3.5 kb upstream of the transcriptional initiation site, was required for neural plate-specific expression. Our results suggest that the TCF/LEF signaling pathway participates in the regulation of cyclin D1 induction during the generation of the dorsal nervous system in early frog embryogenesis.

Amino Acid Motifs↗
PubMed1997

Genetic patterning of the developing mouse tail at the time of posterior neuropore closure.

Posterior neuropore (PNP) closure coincides with the end of gastrulation, marking the end of primary neurulation and primary body axis formation. Secondary neurulation and axis formation involve differentiation of the tail bud mesenchyme. Genetic control of the primary-secondary transition is not understood. We report a detailed analysis of gene expression in the caudal region of day 10 mouse embryos during primary neuropore closure. Embryos were collected at the 27-32 somite stage, fixed, processed for whole mount in situ hybridisation, and subsequently sectioned for a more detailed analysis. Genes selected for study include those involved in the key events of gastrulation and neurulation at earlier stages and more cranial levels. Patterns of expression within the tail bud, neural plate, recently closed neural tube, notochord, hindgut, mesoderm, and surface ectoderm are illustrated and described. Specifically, we report continuity of expression of the genes Wnt5a, Wnt5b, Evx1, Fgf8, RARgamma, Brachyury, and Hoxb1 from primitive streak and node into subpopulations of the tail bud and caudal axial structures. Within the caudal notochord, developing floorplate, and hindgut, HNF3alpha, HNF3beta, Shh, and Brachyury expression domains correlate directly with known genetic roles and predicted tissue interdependence during induction and differentiation of these structures. The patterns of expression of Wnt5a, Hoxb1, Brachyury, RARgamma, and Evx1, together with observations on proliferation, reveal that the caudal mesoderm is organised at a molecular level into distinct domains delineated by longitudinal and transverse borders before histological differentiation. Expression of Wnt5a in the ventral ectodermal ridge supports previous evidence that this structure is involved in epithelial-mesenchymal interaction. These results provide a foundation for understanding the mechanisms facilitating transition from primary to secondary body axis formation, as well as the factors involved in defective spinal neurulation.

Animals↗
PubMed2006

Xenopus Zic4: conservation and diversification of expression profiles and protein function among the Xenopus Zic family.

We compared the expression and function of Xenopus Zic4 with those of the other four Xenopus laevis Zic family members (Zic1, Zic2, Zic3, and Zic5). Zic4 expression was detected mainly in the neural plate border, dorsal neural tube, and somites, and was similar to that of Zic1, which is adjacent to Zic4 on the same chromosome. Injection of wild-type or mutant Zic4 RNA caused the induction of neural crest marker gene expression, hyperplastic neural tissue, and ectopic pigment cell formation, indicating that Zic4 can induce neural and neural crest tissue, as can other Xenopus Zic genes. Deletion analysis showed that the zinc-finger domain is critical for many Zic4 functions, but the C-terminal region is differently involved in induction of two neural crest markers, Slug and Sox10. The protein function as determined by the animal cap explant assay was similar to that of Zic5, but different from those of Zic1, Zic2, and Zic3, suggesting that Xenopus Zic genes can be divided into two groups based on function. These results indicate that the five Xenopus Zic genes cooperatively regulate both neural and neural crest development, despite significantly diverged expression profiles and functions.

Amino Acid Sequence↗