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PubMed1989

Shaping and bending of the avian neural plate as analysed with a fluorescent-histochemical marker.

Shaping and bending of the neural plate are cardinal events of neurulation. These processes are initiated in avian embryos shortly after the onset of gastrulation and concluded concomitantly with the completion of gastrulation. The epiblast undergoes extensive morphogenetic movements during gastrulation and neurulation, but the directions, distances, rates, mechanisms and roles of such rearrangements are largely unknown. To begin to understand these morphogenetic movements, we have mapped regional displacements of the epiblast by injecting a fluorescent-histochemical marker into selected prenodal, nodal and postnodal levels of the blastoderm. Lateral epiblast regions (600 microns lateral to the midline and consisting primarily of surface epithelium) are displaced craniomedially, medial regions (300 microns lateral to the midline and consisting of neural plate and preingressed mesoderm) predominantly medially, and midline regions (consisting of neural plate and primitive streak) predominantly caudally. Displacements within the avian neural plate parallel those previously described for the amphibian neural plate. Furthermore, similar tissue displacements occur within the prenodal and postnodal levels of the avian epiblast despite the fact that neurulation is occurring in the former and gastrulation in the latter. Finally, our results show that ectodermal rudiments contained within a single cross-sectional level of the embryo are a composite of cells derived from multiple craniocaudal and mediolateral levels. Thus, regional tissue displacements are important events to consider in the analysis of the early morphogenesis of axial and paraxial organ rudiments derived from the epiblast.

Animals↗

PubMed1989

Immunohistochemical localisation of chondroitin sulphate proteoglycans and the effects of chondroitinase ABC in 9- to 11-day rat embryos.

Studies on cell behaviour in vitro have indicated that the chondroitin sulphate proteoglycan (CSPG) family of molecules can participate in the control of cell proliferation, differentiation and adhesion, but its morphogenetic functions had not been investigated in intact embryos. Chondroitin/chondroitin sulphates have been identified in rat embryos at low levels at the start of neurulation (day 9) and at much higher levels on day 10. In this study we have sought evidence for the morphogenetic functions of CSPGs in rat embryos during the period of neurulation and neural crest cell migration by a combination of two approaches: immunocytochemical localization of CSPG by means of an antibody, CS-56, to the chondroitin sulphate component of CSPG, and exposure of embryos to the enzyme chondroitinase ABC. Staining of the CS-56 epitope was poor at the beginning of cranial neurulation; bright staining was at first confined to the primary mesenchyme under the convex neural folds late on day 9. In day 10 embryos, all mesenchyme cells were stained, but at different levels of intensity, so that primary mesenchyme, neural crest and sclerotomal cells could be distinguished from each other. Basement membranes were also stained, particularly bright staining being present where two epithelial were basally apposed, e.g., neural/surface ectoderms, dorsal aorta/neural tube, prior to migration of a population of cells between them. Staining within the neural epithelium was first confined to the dorsolateral edge region, and associated with the onset of neural crest cell emigration; after neural tube closure, neuroepithelial staining was more general. Neural crest cells were stained during migration, but the reaction was absent in areas associated with migration end-points (trigeminal ganglion anlagen, frontonasal mesenchyme). Embryos exposed to chondroitinase ABC in culture showed no abnormalities until early day 10, when cranial neural crest cell emigration from the neural epithelium was inhibited and neural tube closure was retarded. Sclerotomal cells failed to take their normal pathway between the dorsal aorta and neural tube. Correlation of the results of these two methods suggests: (1) that by decreasing adhesiveness within the neural epithelium at specific stages, CSPG facilitates the emigration of neural crest cells and the migratory movement of neuroblasts, and may also provide increased flexibility during the generation of epithelial curvatures; (2) that by decreasing the adhesiveness of fibronectin-containing extracellular matrices, CSPG facilitates the migration of neural crest and sclerotomal cells. This second function is particularly important when migrating cells take pathways between previously apposed tissues.

Animals↗

PubMed1990

Isolation of the mouse Hox-2.9 gene; analysis of embryonic expression suggests that positional information along the anterior-posterior axis is specified by mesoderm.

It is rapidly becoming accepted that the vertebrate neural tube, in particular the hindbrain, develops into a segmented structure. After segment formation, cells in the neural tube do not cross segmental boundaries, and segment-specific gene expression is observed. However, it is not known what positional cues instruct the neural tube to express genes in this restricted manner. We have cloned a murine homeobox-containing gene, Hox-2.9, whose expression in the neural tube at E9.5 is restricted to a segment of the hindbrain known as rhombomere 4. A study of its expression pattern earlier in development revealed that prior to the start of neurulation (E7.5) Hox-2.9 is expressed within a posterior to the embryonic mesoderm that will participate in hindbrain formation. With the onset of neurulation, expression then becomes detectable in the neural plate as well, but only in the part that overlies the Hox-2.9-expressing mesoderm; it is not detected in the more anterior neuroectoderm that will form the future midbrain and forebrain. On the basis of these findings, we propose that the mesoderm is providing cues that serve to instruct the overlying neuroectoderm with respect to its position along the anteroposterior axis and that Hox-2.9 participates in or reflects this process. As neurulation continues and individual segments form, a second phase of expression is detected in the neural tube in which high levels of Hox-2.9 transcripts become restricted to rhombomere 4. Hox-2.9 expression is also detected in the developing branchial arch units of the hindbrain region, in a pattern that suggests to us that here, too, mesoderm is providing a localized signal that induces Hox-2.9 expression, in this case in endoderm of the pharynx and in superficial ectoderm. In general, we interpret the expression patterns of Hox-2.9 in the hindbrain region as suggesting that the specific mechanisms of pattern formation in mammals are fundamentally similar to those of amphibians and avians - i.e. anteroposterior positional information is acquired by mesoderm, mesoderm induces positional values within (neuro-) ectoderm and endoderm, and both events occur within a restricted window of time.

Amino Acid Sequence↗

PubMed1997

Patterning of the embryo along the anterior-posterior axis: the role of the caudal genes.

Patterning along the anterior-posterior axis takes place during gastrulation and early neurulation. Homeobox genes like Otx-2 and members of the Hox family have been implicated in this process. The caudal genes in Drosophila and C. elegans have been shown to determine posterior fates. In vertebrates, the caudal genes begin their expression during gastrulation and they take up a posterior position. By injecting sense and antisense RNA of the Xenopus caudal gene Xcad-2, we have studied a number of regulatory interactions among homeobox genes along the anterior-posterior axis. Initially, the Xcad-2 and Otx-2 genes are mutually repressed and, by late gastrulation, they mark the posterior- or anterior-most domains of the embryo, respectively. During late gastrulation and neurulation, Xcad-2 plays an additional regulatory function in relation to the Hox genes. Hox genes normally expressed anteriorly are repressed by Xcad-2 overexpression while those normally expressed posteriorly exhibit more anterior expression. The results show that the caudal genes are part of a posterior determining network which during early gastrulation functions in the subdivision of the embryo into anterior head and trunk domains. Later in gastrulation and neurulation these genes play a role in the patterning of the trunk region.

Animals↗

PubMed1998

The role of F-cadherin in localizing cells during neural tube formation in Xenopus embryos.

The cell adhesion molecule F-cadherin is expressed in Xenopus embryos at boundaries that subdivide the neural tube into different regions, including one, the sulcus limitans, which partitions the caudal neural tube into a dorsal and ventral half (alar and basal plate, respectively). Here we examine the role of F-cadherin in positioning cells along the caudal neuraxis during neurulation. First, we show that ectopic expression of F-cadherin restricts passive cell mixing within the ectodermal epithelium. Second, we show that F-cadherin is first expressed at the sulcus limitans prior to the extensive cell movements that accompany neural tube formation, suggesting that it might serve to position cells at the sulcus limitans by counteracting their tendency to disperse during neurulation. We test this idea using an assay that measures changes in cell movements during neurulation in response to differential cell adhesion. Using this assay, we show that cells expressing F-cadherin localize preferentially to the sulcus limitans, but still disperse when located away from the sulcus limitans. In addition, inhibiting cadherin function prevents cells from localizing precisely at the sulcus limitans. These results indicate that positioning of cells at the sulcus limitans is mediated in part by the differential expression of F-cadherin.

Animals↗

PubMed1987

Differential deposition of basement membrane components during formation of the caudal neural tube in the mouse embryo.

The distribution of basement membrane and extracellular matrix components laminin, fibronectin, type IV collagen and heparan sulphate proteoglycan was examined during posterior neuropore closure and secondary neurulation in the mouse embryo. During posterior neuropore closure, these components were densely deposited in basement membranes of neuroepithelium, blood vessels, gut and notochord; although deposition was sparse in the midline of the regressing primitive streak. During secondary neurulation, mesenchymal cells formed an initial aggregate near the dorsal surface, which canalized and merged with the anterior neuroepithelium. With aggregation, fibronectin and heparan sulphate proteoglycan were first detected at the base of a 3- to 4-layer zone of radially organized cells. With formation of a lumen within the aggregate, laminin and type IV collagen were also deposited in the forming basement membrane. During both posterior neuropore closure and secondary neurulation, fibronectin and heparan sulphate proteoglycan were associated with the most caudal portion of the neuroepithelium, the region where newly formed epithelium merges with the consolidated neuroepithelium. In regions of neural crest migration, the deposition of basement membrane components was altered, lacking laminin and type IV collagen, with increased deposition of fibronectin and heparan sulphate proteoglycan.

Animals↗

PubMed1987

Fluidity of the neural epithelium during forebrain formation in rat embryos.

During neurulation in rat embryos, the forebrain grows more rapidly than can be accounted for by intrinsic cell division alone, while the adjacent midbrain/rostral hindbrain maintains a constant cell number despite a high mitotic index (the cell cycle time is 6 h throughout the neural epithelium). We have proposed that neuroepithelial cells flow in a rostral direction within the midbrain/rostral hindbrain region, towards and into the rapidly expanding forebrain. Evidence in support of this hypothesis is provided by cell-labelling studies: labelled neuroepithelial cells were injected into specific sites in the midbrain or rostral hindbrain neuroepithelium of unlabelled embryos; after culture of these embryos for 24 h, the labelled cells were found in positions rostral to the injection sites. In the midbrain/rostral hindbrain region, mitotic spindles were found to be predominantly orientated parallel with the long axis of the embryo; transversely orientated spindles were more frequently observed close to the lateral edges than more medially. Neural crest cells emigrate from the lateral edges during neurulation. These observations suggest that mitotic spindle orientation reflects the direction of cell movement: in the lateral region movement towards the lateral edge would maintain cell number in the transverse plane as neural crest cells emigrate; elsewhere, cells are moving mainly in the longitudinal plane, towards the forebrain. The possible causal relationship between cell movement and mitotic spindle orientation is discussed. Cell flow within the intact neural epithelium is compared with cell flow in the intestinal villus epithelium. Other types of epithelial cell movement observed in the cranial neural epithelium during neurulation include expansion and shrinkage of the epithelial surface associated with change of cell shape and microfilament-mediated curvature. Cell rearrangement involving exchange of neighbours and cell movement towards and into a site of epithelio-mesenchymal conversion are also implicated.

Animals↗

PubMed2001

Lineage allocation during early embryogenesis. Mapping of the neural primordia and application to the analysis of mouse mutants.

The methods outlined in this chapter discuss a range of techniques that have been employed for lineage analysis studies of the neural primordia from the onset of gastrulation and during neurulation. As the mouse has been extensively mapped, lineage analysis during normal morphogenesis is well understood. Attention is now focused on the tissue interactions that are essential for gastrulation and neurulation to proceed normally. The key to understanding these tissue interactions lies in the study of mutant embryos where abnormal development of specific tissue types affects the processes of gastrulation and neurulation. Lineage analysis and tissue potency experiments on particular mutant embryos will provide insight into these essential tissue interactions. As the first step toward undertaking such analysis of the neural derivatives, we have outlined the mutant strains available and detailed a protocol for the introgression of the lacZ transgene onto the mutant background.

Animals↗

PubMed1995

Development of methionine synthase, cystathionine-beta-synthase and S-adenosyl-homocysteine hydrolase during gestation in rats.

The developmental onset of three homocysteine metabolizing enzymes in the rat conceptus was investigated. Cystathionine-beta-synthase and methionine synthase were assayed from day 10 to day 20 of gestation in decidual and placental tissue, from day 10 to day 12 of gestation in embryonic tissue, from day 14 to day 20 of gestation in fetal liver and from day 14 to day 20 of gestation in fetal tissue without liver. On each day, material was obtained from at least four conceptuses from two dams. S-adenosylhomocysteine hydrolase was assayed in neurulating conceptuses in decidual tissue, parietal yolksac plus ectoplacental cone, visceral yolksac plus amnion and embryo proper. Conceptuses were pooled from seven (day 9.5 of gestation) or three (days 10.5 and 11.5 of gestation) dams. In embryonic and fetal tissue cystathionine-beta-synthase first occurred in fetal liver. During the organogenic phase it was present only in decidual tissue. Methionine synthase was present in all tissues from the first gestational day investigated and S-adenosylhomocysteine hydrolase was present in all tissues throughout the neurulating period. Our results indicate that the homocysteine-methionine cycle, which is crucial to transmethylation reactions, is functional during the neurulating period in embryonic tissue. Owing to the absence of cystathionine-beta-synthase at this stage of development in embryonic tissue, the homocysteinyl moiety is conserved in the homocysteine-methionine cycle.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗

PubMed2003

In vitro high-field magnetic resonance imaging-documented anatomy of a fetal myelomeningocele at 20 weeks' gestation. A contribution to the rationale of intrauterine surgical repair of spina bifida.

OBJECT: It remains uncertain if closure of a myelomeningocele at midgestation changes the neurological condition at birth in an infant born with spina bifida. The authors conducted a study to provide a detailed analysis of the morphology of the spinal cord with the myelomeningocele at the time fetal surgery usually is performed. METHODS: The myelomeningocele of a 20-week-gestation-age fetus was examined, and data were compared with those obtained in a neurologically intact specimen of the same age. In vitro high-field 9.4-tesla magnetic resonance (MR) microscopy was used to examine the fetal material. High-field MR spectroscopy provided images in the three orthogonal planes with a resolution comparable with low-power optical microscopy. The authors observed that the fetal cord of the myelomeningocele specimen was tapered and tethered at S3-4 while the conus medullaris in the normal fetus reaches L-4. No neurulation defects were noted. The axial MR images clearly revealed the nonfusion of the mesodermal structures. The absence of neurulation defects suggests that at least in some cases of spina bifida the spinal cord initially is well developed but is damaged later on chemically and mechanically. This might be an argument in favor of intrauterine myelomeningocele repair. By 20 weeks' gestation, however, the deformation of the cord inside the myelomeningocele is severe. An optimization of the preoperative assessment by means of MR imaging therefore might be considered a valuable contribution to intrauterine surgery. The in vitro high-field MR microscopic findings of this study could be used as references for clinical intrauterine MR imaging. CONCLUSIONS: The detailed in vitro high-field MR analysis of a 20-week-gestation-age fetus with spina bifida demonstrated that an improvement of the preoperative intrauterine imaging should be pursued to detect those cases without neurulation defects and with minimal deformation of the spinal cord.

Arnold-Chiari Malformation↗

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↗

PubMed1995

Variations in the formation of the human caudal spinal cord.

Collection of 15 human embryos between 4-8 developmental weeks was used to histologically investigate variations in the development of the caudal part of the spinal cord and the neighboring axial organs (notochord and vertebral column). In the 4-week embryo, two types of neurulation were parallelly observed along the anteroposterior body axis: primary in the areas cranial to the neuroporus caudalis and secondary in the more caudal tail regions. In the 5-week embryos, both parts of the neural tube fused, forming only one continuous lumen in the developing spinal cord. In the three examined embryos we found anomalous pattern of spinal cord formation. Caudal parts of these spinal cords displayed division of their central canal into two or three separate lumina, each surrounded by neuroepithelial layer. In the caudal area of the spinal cord, derived by secondary neurulation, formation of separate lumina was neither connected to any anomalous notochord or vertebral column formation, nor the appearance of any major axial disturbances. We suggest that development of the caudal part of the spinal cord differs from its cranial region not only in the type of neurulation, but also in the destiny of its derivatives and possible modes of abnormality formation.

Abortion, Legal↗

PubMed1995

Xenopus laevis: a model system for the study of embryonic retinoid metabolism. II. Embryonic metabolism of all-trans-3,4-didehydroretinol to all-trans-3,4-didehydroretinoic acid.

This study demonstrates early embryonic metabolism of exogenous all-trans-3,4-didehydroretinol (vitamin A2) to all-trans-3,4-didehydroretinal and to all-trans-3,4-didehydroretinoic acid in Xenopus embryos during neurulation. The latter metabolite was recently shown to bind with high affinity and to activate various retinoic acid receptors. Embryos treated with all-trans-3,4-didehydroretinol during early or late gastrulation exhibited abnormalities along the anteroposterior axis. The abnormalities were primarily in the posterior regions of the embryo, with only minor defects anteriorally. Eye malformations, typical for early exposure to 9-cis- and all-trans-retinols and retinals (companion paper), were not observed. We also present evidence that all-trans-3,4-didehydroretinoic acid is present endogenously during early neurulation and is evenly distributed along the anteroposterior axis. After treatment with all-trans-3,4-didehydroretinol, embryonic levels of all-trans-3,4-didehydroretinoic acid exceeded endogenous levels of this metabolite during early and late neurulation. We hypothesize that the dysmorphogenic effects produced by treatment of Xenopus embryos with the alcohol precursor, all-trans-3,4-didehydroretinol, are the result of its embryonic conversion to its corresponding acid ligand.

Abnormalities, Drug-Induced↗

PubMed1997

Encephalocele: experimental model. Morphogenesis, pathogenesis and clinical correlations discussion.

RESEARCH OBJECT: This study intends to consider an encephalocele experimental model, obtained in embryonate eggs, treated in a post-neurulation phase with chemical teratogens. This study intends to point out possibility that pathogenic process, which have determined a malformation, is referable to an original defect of embryonic cranial coatings development and that neurulation defect is secondary. EXPERIMENTAL PLAN: Chick and duck embryonate eggs have been used. They have been inoculated in their development phase with known chemical teratogens, as Dintoina and Blue Trypan. A controlling group has been inoculated with physiological solution and, then, it has been followed till the hatching. The experimental group has been undergone to artificial hatching, according to prearranged conditions and it has been analysed during different phases of its development. MEASURES: Four cranioencephalic malformations have been obtained: three cases of encephalocele and one case of exencephalia. The individual pathological compounds have been studied under the morphological and histopathological profile. The skull base dimensions have been taken and then compared with the controlling group ones. CONCLUSIONS: In the light of experimental data, some considerations have been undertaken considering the pathogenetic hypothesis findable in medical literature, stressing the possibility that encephalocele may be arranged as a post-neurulation defect and that, for taxonomic aims, it has to fit in a different group within cranial disraphims.

Animals↗

PubMed1999

Apparent lability of neural tube closure in laboratory animals and humans.

Neural tube defects (NTDs), a set of structural abnormalities affecting the brain, spinal cord, and the skeletal and connective tissues that protect them, are common malformations among humans and laboratory animals. The embryogenesis of the neural tube is presented to convey the complexity of the phenomenon, the multiplicity of requisite cellular and subcellular processes, and the precise timing of events that must occur for successful neural tube development. Interruption, even transitory, of any of these intricate processes or disruption of an embryo's developmental schedule can lead to an NTD. The population distribution of human NTDs demonstrates that genetic predisposition functions in susceptibility to NTDs. Data from animal studies support these concepts. NTDs are common outcomes in developmental toxicity safety assessments, occurring among control and treated groups. Numerous agents have caused increased levels of NTDs in laboratory animals, and species with shorter gestational periods appear more prone to toxicant-induced NTDs than those with longer gestations. Data from post-implantation whole embryo culture, although not predictive of human risk, are useful in studying neurulation mechanisms and in demonstrating the importance of maintaining embryonic schedules of development. We conclude that the concept that NTDs are produced by only a few toxicants that selectively target the developing nervous system is untenable. Rather, the combination of the time in gestation that an agent is applied, its dose, and its ability to disrupt critical processes in neurulation leads to NTDs. We further conclude that, because of both the relatively high prevalence and the multifactorial nature of NTDs, the mere occurrence of an NTD is insufficient for inferring that the defect was caused by an exogenous agent.

Animals↗

PubMed1996

Expression of an Msx homeobox gene in ascidians: insights into the archetypal chordate expression pattern.

The Msx homeobox genes are expressed in complex patterns during vertebrate development in conjunction with inductive tissue interactions. As a means of understanding the archetypal role of Msx genes in chordates, we have isolated and characterized an Msx gene in ascidians, protochordates with a relatively simple body plan. The Mocu Msx-a and McMsx-a genes, isolated from the ascidians Molgula oculata and Molgula citrina, respectively, have homeodomains that place them in the msh-like subclass of Msx genes. Therefore, the Molgula Msx-a genes are most closely related to the msh genes previously identified in a number of invertebrates. Southern blot analysis suggests that there are one or two copies of the Msx-a gene in the Molgula genome. Northern blot and RNase protection analysis indicate that Msx-a transcripts are restricted to the developmental stages of the life cycle. In situ hybridization showed that Msx-a mRNA first appears just before gastrulation in the mesoderm (presumptive notochord and muscle) and ectoderm (neural plate) cells. Transcript levels decline in mesoderm cells after the completion of gastrulation, but are enhanced in the folding neural plate during neurulation. Later, Msx-a mRNA is also expressed in the posterior ectoderm and in a subset of the tail muscle cells. The ectoderm and mesoderm cells that express Msx-a are undergoing morphogenetic movements during gastrulation, neurulation, and tail formation. Msx-a expression ceases after these cells stop migrating. The ascidian M. citrina, in which adult tissues and organs begin to develop precociously in the larva, was used to study Msx-a expression during adult development. Msx-a transcripts are expressed in the heart primordium and the rudiments of the ampullae, epidermal protrusions with diverse functions in the juvenile. The heart and ampullae develop in regions where mesenchyme cells interact with endodermal or epidermal epithelia. A comparison of the expression patterns of the Molgula genes with those of their vertebrate congeners suggests that the archetypal roles of the Msx genes may be in morphogenetic movements during embryogenesis and in mesenchymal-epithelial interactions during organogenesis.

Amino Acid Sequence↗

PubMed1996

Expression of the cCdx-B homeobox gene in chick embryo suggests its participation in rostrocaudal axial patterning.

cCdx-B (formerly cHox-cad 2) is a chick homeobox-containing gene related to the Drosophila caudal. Compared with other caudal homologues, its similarity is highest with the murine Cdx-4. In the present study, we characterize the localization of cCdx-B transcripts to the caudal region of the embryo by using reverse transcription-polymerase chain reaction (RT-PCR) and, in detail, by using in situ hybridization. Chick embryos from gastrulation to early organogenesis were hybridized with digoxigenin-labeled riboprobes, and the pattern of expression of cCdx-B mRNA was analyzed in wholemount embryos and in tissue sections. In the early gastrula, transcripts were localized in a gradient through the caudal half of the embryo, in the epiblast and the mesoderm cells, but not including Hensen's node. During neurulation, cCdx-B transcripts were found more rostrally, with high levels localized in Hensen's node and the posterior neural plate. Expression was also high in paraxial mesoderm, with a rostral limit in the most recently formed somite. There was no expression in definitive endoderm. During late neurulation and tail bud formation, cCdx-B mRNA expression regressed posteriorly and was finally confined to the tail bud region. This pattern of expression of cCdx-B, regulated in time and space, is different from that of the other known chick caudal homologue, cCdx-A. Both genes may play a coordinated role in the posterior axial patterning of the chick embryo, whereas cCdx-B may specify further the identity of the tail region.

Amino Acid Sequence↗

PubMed1997

Epidermal ectoderm is required for full elevation and for convergence during bending of the avian neural plate.

Previous studies suggest that bending of the neural plate requires the juxtaposition of neural plate and non-neuroepithelial tissues. The current study examines the role of one of these tissues, the epidermal ectoderm, in bending. Chick blastoderms were harvested from fertile eggs incubated for 24 hr and cultured dorsal-side-up on agar-albumen substrates. In one experiment, a rectangular flap of epidermal ectoderm on one side of each blastoderm was separated from underlying layers and gently reflected onto the area opaca; a fragment of tungsten wire was placed on top of the flap to hold it down and to prevent healing. Embryos were then allowed to develop in a humidified incubator for 2-18 hr. Asymmetric neurulation was observed between the operated and control sides as early as 2 hr after surgery. The amount of asymmetry was quantified in serial transverse sections from embryos collected 8 hr after surgery. Elevation of the lateral edge of the neural plate on the operated side averaged one half to two thirds of that on the control side, and convergence of the operated side around the dorsolateral hinge point toward the dorsal midline did not occur. These results demonstrate that epidermal ectoderm is required for full elevation and for convergence during bending. In another experiment, lateral epidermal ectoderm was removed, leaving only a medial strip consisting of both the epidermal component of the future neural fold and flanking future epidermis. This experiment revealed that although epidermal ectoderm is necessary for full elevation and for convergence of the neural folds, a medial strip of epidermal ectoderm is sufficient to drive bending. Collectively, these results further support the idea that neurulation is a multifactorial process driven by both intrinsic and extrinsic factors acting in concert.

Animals↗