Topographical changes along the neural fold associated with neurulation in the hamster and mouse

A Lifeact-EGFP quail for studying actin dynamics in vivo

Here, we report the generation of a transgenic Lifeact-EGFP quail line for the investigation of actin organization and dynamics during morphogenesis in vivo. This transgenic avian line allows for the high-resolution visualization of actin structures within the living embryo, from the subcellular filaments that guide cell shape to the supracellular assemblies that coordinate movements across tissues. The unique suitability of avian embryos to live imaging facilitates the investigation of previously intractable processes during embryogenesis. Using high-resolution live imaging approaches, we present the dynamic behaviors and morphologies of cellular protrusions in different tissue contexts. Furthermore, through the integration of live imaging with computational segmentation, we visualize cells undergoing apical constriction and large-scale actin structures such as multicellular rosettes within the neuroepithelium. These findings not only enhance our understanding of tissue morphogenesis but also demonstrate the utility of the Lifeact-EGFP transgenic quail as a new model system for live in vivo investigations of the actin cytoskeleton.

Claudin-3 in the non-neural ectoderm is essential for neural fold fusion in chicken embryos

The neural tube, the embryonic precursor to the brain and spinal cord, begins as a flat sheet of epithelial cells, divided into non-neural and neural ectoderm. Proper neural tube closure requires that the edges of the neural ectoderm, the neural folds, to elevate upwards and fuse along the dorsal midline of the embryo. We have previously shown that members of the claudin protein family are required for the early phases of chick neural tube closure. Claudins are transmembrane proteins, localized in apical tight junctions within epithelial cells where they are essential for regulation of paracellular permeability, strongly involved in apical-basal polarity, cell-cell adhesion, and bridging the tight junction to cytoplasmic proteins. Here we explored the role of Claudin-3 (Cldn3), which is specifically expressed in the non-neural ectoderm. We discovered that depletion of Cldn3 causes folic acid-insensitive primarily spinal neural tube defects due to a failure in neural fold fusion. Apical cell surface morphology of Cldn3-depleted non-neural ectodermal cells exhibited increased membrane blebbing and smaller apical surfaces. Although apical-basal polarity was retained, we observed altered Par3 and Pals1 protein localization patterns within the apical domain of the non-neural ectodermal cells in Cldn3-depleted embryos. Furthermore, F-actin signal was reduced at apical junctions. Our data presents a model of spina bifida, and the role that Cldn3 is playing in regulating essential apical cell processes in the non-neural ectoderm required for neural fold fusion.

From signalling to form: the coordination of neural tube patterning

The development of the vertebrate spinal cord involves the formation of the neural tube and the generation of multiple distinct cell types. The process starts during gastrulation, combining axial elongation with specification of neural cells and the formation of the neuroepithelium. Tissue movements produce the neural tube which is then exposed to signals that provide patterning information to neural progenitors. The intracellular response to these signals, via a gene regulatory network, governs the spatial and temporal differentiation of progenitors into specific cell types, facilitating the assembly of functional neuronal circuits. The interplay between the gene regulatory network, cell movement, and tissue mechanics generates the conserved neural tube pattern observed across species. In this review we offer an overview of the molecular and cellular processes governing the formation and patterning of the neural tube, highlighting how the remarkable complexity and precision of vertebrate nervous system arises. We argue that a multidisciplinary and multiscale understanding of the neural tube development, paired with the study of species-specific strategies, will be crucial to tackle the open questions.

Stepwise modulation of apical orientational cell adhesions for vertebrate neurulation

Neurulation transforms the neuroectoderm into the neural tube. This transformation relies on reorganising the configurational relationships between the orientations of intrinsic polarities of neighbouring cells. These orientational intercellular relationships are established, maintained, and modulated by orientational cell adhesions (OCAs). Here, using zebrafish (Danio rerio) neurulation as a major model, we propose a new perspective on how OCAs contribute to the parallel, antiparallel, and opposing intercellular relationships that underlie the neural plate-keel-rod-tube transformation, a stepwise process of cell aggregation followed by cord hollowing. We also discuss how OCAs in neurulation may be regulated by various adhesion molecules, including cadherins, Eph/Ephrins, Claudins, Occludins, Crumbs, Na+ /K+ -ATPase, and integrins. By comparing neurulation among species, we reveal that antiparallel OCAs represent a conserved mechanism for the fusion of the neural tube. Throughout, we highlight some outstanding questions regarding OCAs in neurulation. Answers to these questions will help us understand better the mechanisms of tubulogenesis of many tissues.

The Tbx6 Transcription Factor Dorsocross Mediates Dpp Signaling to Regulate Drosophila Thorax Closure

Movement and fusion of separate cell populations are critical for several developmental processes, such as neural tube closure in vertebrates or embryonic dorsal closure and pupal thorax closure in Drosophila. Fusion failure results in an opening or groove on the body surface. Drosophila pupal thorax closure is an established model to investigate the mechanism of tissue closure. Here, we report the identification of T-box transcription factor genes Dorsocross (Doc) as Decapentaplegic (Dpp) targets in the leading edge cells of the notum in the late third instar larval and early pupal stages. Reduction of Doc in the notum region results in a thorax closure defect, similar to that in dpp loss-of-function flies. Nine genes are identified as potential downstream targets of Doc in regulating thorax closure by molecular and genetic screens. Our results reveal a novel function of Doc in Drosophila development. The candidate target genes provide new clues for unravelling the mechanism of collective cell movement.

Hallmarks of primary neurulation are conserved in the zebrafish forebrain

Primary neurulation is the process by which the neural tube, the central nervous system precursor, is formed from the neural plate. Incomplete neural tube closure occurs frequently, yet underlying causes remain poorly understood. Developmental studies in amniotes and amphibians have identified hingepoint and neural fold formation as key morphogenetic events and hallmarks of primary neurulation, the disruption of which causes neural tube defects. In contrast, the mode of neurulation in teleosts has remained highly debated. Teleosts are thought to have evolved a unique mode of neurulation, whereby the neural plate infolds in absence of hingepoints and neural folds, at least in the hindbrain/trunk where it has been studied. Using high-resolution imaging and time-lapse microscopy, we show here the presence of these morphological landmarks in the zebrafish anterior neural plate. These results reveal similarities between neurulation in teleosts and other vertebrates and hence the suitability of zebrafish to understand human neurulation.

Jonathan Werner, Maraki Negesse et al. visualize zebrafish neurulation during development to determine whether hallmarks of neural tube formation in other vertebrates also apply to zebrafish. They find that neural tube formation in the forebrain shares features such as hingepoints and neural folds with other vertebrates, demonstrating the strength of the zebrafish model for understanding human neurulation.

Embryology of Spinal Dysraphism and its Relationship to Surgical Treatment

Spinal dysraphism is an umbrella term that encompasses a number of congenital malformations that affect the central nervous system. The etiology of these conditions can be traced back to a specific defect in embryological development, with the more disabling malformations occurring at an earlier gestational age. A thorough understanding of the relevant neuroembryology is imperative for clinicians to select the correct treatment and prevent complications associated with spinal dysraphism. This paper will review the neuroembryology associated with the various forms of spinal dysraphism and provide a clinical-pathological correlation for these congenital malformations.

Non-neural surface ectodermal rosette formation and F-actin dynamics drive mammalian neural tube closure

The mechanisms underlying mammalian neural tube closure remain poorly understood. We report a unique cellular process involving multicellular rosette formation, convergent cellular protrusions, and F-actin cable network of the non-neural surface ectodermal cells encircling the closure site of the posterior neuropore, which are demonstrated by scanning electron microscopy and genetic fate mapping analyses during mouse spinal neurulation. These unique cellular structures are severely disrupted in the surface ectodermal transcription factor Grhl3 mutants that exhibit fully penetrant spina bifida. We propose a novel model of mammalian neural tube closure driven by surface ectodermal dynamics, which is computationally visualized.

Insights into the Etiology of Mammalian Neural Tube Closure Defects from Developmental, Genetic and Evolutionary Studies

The human neural tube defects (NTD), anencephaly, spina bifida and craniorachischisis, originate from a failure of the embryonic neural tube to close. Human NTD are relatively common and both complex and heterogeneous in genetic origin, but the genetic variants and developmental mechanisms are largely unknown. Here we review the numerous studies, mainly in mice, of normal neural tube closure, the mechanisms of failure caused by specific gene mutations, and the evolution of the vertebrate cranial neural tube and its genetic processes, seeking insights into the etiology of human NTD. We find evidence of many regions along the anterior–posterior axis each differing in some aspect of neural tube closure—morphology, cell behavior, specific genes required—and conclude that the etiology of NTD is likely to be partly specific to the anterior–posterior location of the defect and also genetically heterogeneous. We revisit the hypotheses explaining the excess of females among cranial NTD cases in mice and humans and new developments in understanding the role of the folate pathway in NTD. Finally, we demonstrate that evidence from mouse mutants strongly supports the search for digenic or oligogenic etiology in human NTD of all types.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

Spina Bifida: Pathogenesis, Mechanisms, and Genes in Mice and Humans

Spina bifida is among the phenotypes of the larger condition known as neural tube defects (NTDs). It is the most common central nervous system malformation compatible with life and the second leading cause of birth defects after congenital heart defects. In this review paper, we define spina bifida and discuss the phenotypes seen in humans as described by both surgeons and embryologists in order to compare and ultimately contrast it to the leading animal model, the mouse. Our understanding of spina bifida is currently limited to the observations we make in mouse models, which reflect complete or targeted knockouts of genes, which perturb the whole gene(s) without taking into account the issue of haploinsufficiency, which is most prominent in the human spina bifida condition. We thus conclude that the need to study spina bifida in all its forms, both aperta and occulta, is more indicative of the spina bifida in surviving humans and that the measure of deterioration arising from caudal neural tube defects, more commonly known as spina bifida, must be determined by the level of the lesion both in mouse and in man.

Rho GTPases in mammalian spinal neural tube closure

Neural tube closure is an important morphogenetic event that involves dramatic reshaping of both neural and non-neural tissues. Rho GTPases are key cytoskeletal regulators involved in cell motility and in several developmental processes, and are thus expected to play pivotal roles in neurulation. Here, we discuss 2 recent studies that shed light on the roles of distinct Rho GTPases in different tissues during neurulation. RhoA plays an essential role in regulating actomyosin dynamics in the neural epithelium of the elevating neural folds, while Rac1 is required for the formation of cell protrusions in the non-neural surface ectoderm during neural fold fusion.

Dynamic behaviors of the non-neural ectoderm during mammalian cranial neural tube closure

The embryonic brain and spinal cord initially form through the process of neural tube closure (NTC). NTC is thought to be highly similar between rodents and humans, and studies of mouse genetic mutants have greatly increased our understanding of the molecular basis of NTC with relevance for human neural tube defects. In addition, studies using amphibian and chick embryos have shed light into the cellular and tissue dynamics underlying NTC. However, the dynamics of mammalian NTC has been difficult to study due to in utero development until recently when advances in mouse embryo ex vivo culture techniques along with confocal microscopy have allowed for imaging of mouse NTC in real time. Here, we have performed live imaging of mouse embryos with a particular focus on the non-neural ectoderm (NNE). Previous studies in multiple model systems have found that the NNE is important for proper NTC, but little is known about the behavior of these cells during mammalian NTC. Here we utilized a NNE-specific genetic labeling system to assess NNE dynamics during murine NTC and identified different NNE cell behaviors as the cranial region undergoes NTC. These results bring valuable new insight into regional differences in cellular behavior during NTC that may be driven by different molecular regulators and which may underlie the various positional disruptions of NTC observed in humans with neural tube defects.

Regulation of cell protrusions by small GTPases during fusion of the neural folds

Epithelial fusion is a crucial process in embryonic development, and its failure underlies several clinically important birth defects. For example, failure of neural fold fusion during neurulation leads to open neural tube defects including spina bifida. Using mouse embryos, we show that cell protrusions emanating from the apposed neural fold tips, at the interface between the neuroepithelium and the surface ectoderm, are required for completion of neural tube closure. By genetically ablating the cytoskeletal regulators Rac1 or Cdc42 in the dorsal neuroepithelium, or in the surface ectoderm, we show that these protrusions originate from surface ectodermal cells and that Rac1 is necessary for the formation of membrane ruffles which typify late closure stages, whereas Cdc42 is required for the predominance of filopodia in early neurulation. This study provides evidence for the essential role and molecular regulation of membrane protrusions prior to fusion of a key organ primordium in mammalian development.

DOI:http://dx.doi.org/10.7554/eLife.13273.001

The neural tube is an embryonic structure that gives rise to the brain and spinal cord. It originates from a flat sheet of cells – the neural plate – that rolls up and fuses to form a tube during development. If this closure fails, it leads to birth defects such as spina bifida, a condition that causes severe disability because babies are born with an exposed and damaged spinal cord.

As the edges of the neural plate meet, they need to fuse together to produce a closed tube. It was known that cells at these edges extend protrusions. However, it was unclear how these protrusions are regulated, whether they arise from neural or non-neural cells and whether or not they are required for the neural tube to close fully.

By studying mutant mouse embryos, Rolo et al. found that cellular protrusions are indeed required for the neural tube to close completely. These protrusions proved to be regulated by proteins called Rac1 and Cdc42, which control the filaments inside the cell that are responsible for cell shape and movement. Rolo et al. also found that the cells that give rise to the protrusions are not part of the neural plate itself. Instead, these cells are neighboring cells from the layer that later forms the epidermis of the skin (the surface ectoderm).

Future studies will need to investigate which signals instruct those precise cells to make protrusions and to discover what happens to the protrusions after contact is made with cells on the opposite side. It will also be important to determine whether spina bifida may arise in humans if the protrusions are defective or absent.

DOI:http://dx.doi.org/10.7554/eLife.13273.002

Fate Specification of Neural Plate Border by Canonical Wnt Signaling and Grhl3 is Crucial for Neural Tube Closure

During primary neurulation, the separation of a single-layered ectodermal sheet into the surface ectoderm (SE) and neural tube specifies SE and neural ectoderm (NE) cell fates. The mechanisms underlying fate specification in conjunction with neural tube closure are poorly understood. Here, by comparing expression profiles between SE and NE lineages, we observed that uncommitted progenitor cells, expressing stem cell markers, are present in the neural plate border/neural fold prior to neural tube closure. Our results also demonstrated that canonical Wnt and its antagonists, DKK1/KREMEN1, progressively specify these progenitors into SE or NE fates in accord with the progress of neural tube closure. Additionally, SE specification of the neural plate border via canonical Wnt signaling is directed by the grainyhead-like 3 (Grhl3) transcription factor. Thus, we propose that the fate specification of uncommitted progenitors in the neural plate border by canonical Wnt signaling and its downstream effector Grhl3 is crucial for neural tube closure. This study implicates that failure in critical genetic factors controlling fate specification of progenitor cells in the neural plate border/neural fold coordinated with neural tube closure may be potential causes of human neural tube defects.

•

Neural plate border/neural fold possesses stem cell-like characters during primary neurulation.

•

Canonical Wnt and its antagonists progressively specify progenitors into surface or neural fates upon neural tube closure.

•

Fate specification into surface ectoderm in the neural fold is directed by the Grhl3 transcription factor.

•

Fate specification of uncommitted progenitors in the neural plate border is intimately coupled to neural tube closure.

Developmental time rather than local environment regulates the schedule of epithelial polarization in the zebrafish neural rod

Background

Morphogenesis requires developmental processes to occur both at the right time and in the right place. During neural tube formation in the zebrafish embryo, the generation of the apical specializations of the lumen must occur in the center of the neural rod after the neural cells have undergone convergence, invagination and interdigitation across the midline. How this coordination is achieved is uncertain. One possibility is that environmental signaling at the midline of the neural rod controls the schedule of apical polarization. Alternatively, polarization could be regulated by a timing mechanism and then independent morphogenetic processes ensure the cells are in the correct spatial location.

Results

Ectopic transplantation demonstrates the local environment of the neural midline is not required for neural cell polarization. Neural cells can self-organize into epithelial cysts in ectopic locations in the embryo and also in three-dimensional gel cultures. Heterochronic transplants demonstrate that the schedule of polarization and the specialized cell divisions characteristic of the neural rod are more strongly regulated by time than local environmental signals. The cells’ schedule for polarization is set prior to gastrulation, is stable through several rounds of cell division and appears independent of the morphogenetic movements of gastrulation and neurulation.

Conclusions

Time rather than local environment regulates the schedule of epithelial polarization in zebrafish neural rod.

How to form and close the brain: insight into the mechanism of cranial neural tube closure in mammals

The development of the embryonic brain critically depends on successfully completing cranial neural tube closure (NTC). Failure to properly close the neural tube results in significant and potentially lethal neural tube defects (NTDs). We believe these malformations are caused by disruptions in normal developmental programs such as those involved in neural plate morphogenesis and patterning, tissue fusion, and coordinated cell behaviors. Cranial NTDs include anencephaly and craniorachischisis, both lethal human birth defects. Newly emerging methods for molecular and cellular analysis offer a deeper understanding of not only the developmental NTC program itself but also mechanical and kinetic aspects of closure that may contribute to cranial NTDs. Clarifying the underlying mechanisms involved in NTC and how they relate to the onset of specific NTDs in various experimental models may help us develop novel intervention strategies to prevent NTDs.

Epithelial fusion during neural tube morphogenesis

Adhesion and fusion of epithelial sheets marks the completion of many morphogenetic events during embryogenesis. Neural tube closure involves an epithelial fusion sequence in which the apposing neural folds adhere initially via cellular protrusions, proceed to a more stable union, and subsequently undergo remodeling of the epithelial structures to yield a separate neural tube roof plate and overlying nonneural ectoderm. Cellular protrusions comprise lamellipodia and filopodia, and studies in several different systems emphasize the critical role of RhoGTPases in their regulation. How epithelia establish initial adhesion is poorly understood but, in neurulation, may involve interactions between EphA receptors and their ephrinA ligands. Epithelial remodeling is spatially and temporally correlated with apoptosis in the dorsal neural tube midline, but experimental inhibition of this cell death does not prevent fusion and remodeling. A variety of molecular signaling systems have been implicated in the late events of morphogenesis, but genetic redundancy, for example among the integrins and laminins, makes identification of the critical players challenging. An improved understanding of epithelial fusion can provide insights into normal developmental processes and may also indicate the mode of origin of clinically important birth defects.

Local protease signaling contributes to neural tube closure in the mouse embryo

We report an unexpected role for protease signaling in neural tube closure and the formation of the central nervous system. Mouse embryos lacking protease-activated receptors 1 and 2 showed defective hindbrain and posterior neuropore closure and developed exencephaly and spina bifida, important human congenital anomalies. Par1 and Par2 were expressed in surface ectoderm, and Par2 was expressed selectively along the line of closure. Ablation of G(i/z) and Rac1 function in these Par2-expressing cells disrupted neural tube closure, further implicating G protein-coupled receptors and identifying a likely effector pathway. Cluster analysis of protease and Par2 expression patterns revealed a group of membrane-tethered proteases often coexpressed with Par2. Among these, matriptase activated Par2 with picomolar potency, and hepsin and prostasin activated matriptase. Together, our results suggest a role for protease-activated receptor signaling in neural tube closure and identify a local protease network that may trigger Par2 signaling and monitor and regulate epithelial integrity in this context.

Acute ethanol administration causes malformations but does not affect cranial morphometry in neonatal mice

Ethanol is a known teratogen and has been implicated in the etiology of human fetal alcohol syndrome (FAS), which is characterized by distinct craniofacial abnormalities such as microcephaly, agnathia, and ocular aberrations. Attempts at quantifying the craniofacial anomalies arising from ethanol exposure have largely been limited to radiographic evaluation in postnatal rats. Such studies discount the role of the cranial soft tissue in the morphology of FAS. We present a study whose aim was to conduct measurements of the entire head including soft and hard tissue in full-term fetuses of mice by means of a digital analyzer, while at the same time comparing stained skeletal tissues in treated and untreated animals. Thirteen pregnant C57BL/6J mice were fed with 25% ethanol (vol/vol) on gestation days (E) 6, 7, and 8, whereas 10 pregnant mice received water only. Fetuses were retrieved from the animals just before delivery on E18, digitally photographed, measured, and assessed for abnormalities. Ethanol-exposed mice showed a number of abnormalities such as anophthalmia and agnathia, but these were not significantly increased over those from nontreated fetuses (P=.5). Birth weight (P=.5), crown-rump length (P=.8), and mandibular length (P=.9) were also not significantly reduced compared to control fetuses. However, defects in some cranial bones and degrees of ossification that trailed same-stage controls were observed in treated animals, at a nonstatistically significant level (P=.14). Acute maternal ingestion of alcohol in mice during pregnancy may not cause a significant increase in craniofacial or skeletal defects when evaluated at term. However, these effects may be latent, manifesting postnatally. The postnatal ability of mice for recovery from alcohol-induced birth defects deserves further investigation.

Neural crest and the origin of ectomesenchyme: neural fold heterogeneity suggests an alternative hypothesis

The striking similarity between mesodermally derived fibroblasts and ectomesenchyme cells, which are thought to be derivatives of the neural crest, has long been a source of interest and controversy. In mice, the gene encoding the alpha subunit of the platelet-derived growth factor receptor (PDGFRalpha) is expressed both by mesodermally derived mesenchymal cells and by ectomesenchyme. Whole-mount immunostaining previously revealed that PDGFRalpha is present in the cephalic neural fold epithelium of early murine embryos (Takakura et al. [1997] J Histochem Cytochem 45:883-893). We now show that, within the neural fold, a sharp boundary exists between E-cadherin-expressing non-neural epithelium and the neural epithelium of the dorsal ridge. In addition, we found that cells coexpressing E-cadherin and PDGFRalpha are present in the non-neural epithelium of the neural folds. These observations raise the possibility that at least some PDGFRalpha(+) ectomesenchyme originates from the lateral non-neural domain of neural fold epithelium. This inference is consistent with previous reports (Nichols [ 1981] J Embryol Exp Morphol 64:105-120; Nichols [ 1986] Am J Anat 176:221-231) that mesenchymal cells emerge precociously from an epithelial neural fold domain resembling the primitive streak in the early embryonic epiblast. Therefore, we propose the name "metablast" for this non-neural epithelial domain to indicate that it is the site of a delayed local delamination of mesenchyme similar to involution of mesoderm during gastrulation. We further propose the testable hypothesis that neural crest and ectomesenchyme are developmentally distinct progenitor populations and that at least some ectomesenchyme is metablast-derived rather than neural crest-derived tissue. Developmental Dynamics 229:118-130, 2004.

Lumbar spine imaging. Normal variants, imaging pitfalls, and artifacts

Accurate recognition and reporting of spine abnormalities on MRI requires knowledge of normal anatomy and its variants. This article deals with common normal variants, points out pitfalls which may be sources of errors in interpretation and describes imaging artifacts which are essential to be recognized and not mistaken for true pathologies.

Exencephaly and cleft cerebellum in SELH/Bc mouse embryos are alternative developmental consequences of the same underlying genetic defect

SELH/Bc inbred mice have ataxia in 5-10% of young adults and exencephaly in 10-20% of newborns. SELH/Bc mice also have a high rate of spontaneous mutation and therefore it could not be assumed that these two abnormalities share the same genetic cause. Previously, we have shown that the liability to exencephaly in SELH/Bc mice is multifactorial, involving two to three loci, and that all the ataxics have a midline cleft cerebellum. The purpose of the present study was to resolve the genetic relationship between liability to exencephaly and liability to cleft cerebellum. We tested whether these traits were transmitted together by segregating F2 males; cotransmission would indicate that both traits are probably caused by the same genes. Approximately 100 embryos from each of 25 F2 sires from a cross between SELH/Bc and the normal LM/Bc strain were scored for exencephaly and the non-exencephalic embryos were scored for cleft cerebellum. The range of exencephaly production by these 25 F2 sires was 0% to 16%; the sires had been selected to represent the extremes of the range of exencephaly production. We found that the 10 sires that produced no exencephaly also produced no cleft cerebellum and 12 of the 15 sires that produced some exencephaly also produced some cleft cerebellum. This indicated strongly that the two traits are transmitted together (Fisher's exact test, P < 0.0002). Furthermore, within exencephaly-producing sires, the specific frequencies of the two traits were significantly positively correlated (Spearman rs = 0.58; P < 0.05), indicating that the same multifactorial risk factors influence both traits. All SELH/Bc embryos omit one normal initiation site of cranial neural tube closure, Closure 2. In a previous study, absence of the Closure 2 initiation site of cranial neural tube closure has been shown to be genetically correlated with liability to exencephaly. In the second part of the present study, the same Closure 2 data from eight of the F2 sires were observed to be significantly positively correlated with liability to cleft cerebellum (Spearman rs = 0.83; P < 0.05). The results of this genetic approach have supported the hypothesis, based on observation of embryos, that one basic multifactorial genetic defect in SELH mice leads to an abnormal cranial neural tube closure mechanism, to exencephaly to cleft cerebellum, and to ataxia.

Prenatal craniofacial development: new insights on normal and abnormal mechanisms

Technical advances are radically altering our concepts of normal prenatal craniofacial development. These include concepts of germ layer formation, the establishment of the initial head plan in the neural plate, and the manner in which head segmentation is controlled by regulatory (homeobox) gene activity in neuromeres and their derived neural crest cells. There is also a much better appreciation of ways in which new cell associations are established. For example, the associations are achieved by neural crest cells primarily through cell migration and subsequent cell interactions that regulate induction, growth, programmed cell death, etc. These interactions are mediated primarily by two groups of regulatory molecules: "growth factors" (e.g., FGF and TGF alpha) and the so-called steroid/thyroid/retinoic acid superfamily. Considerable advances have been made with respect to our understanding of the mechanisms involved in primary and secondary palate formation, such as growth, morphogenetic movements, and the fusion/merging phenomenon. Much progress has been made on the mechanisms involved in the final differentiation of skeletal tissues. Molecular genetics and animal models for human malformations are providing many insights into abnormal development. A mouse model for the fetal alcohol syndrome (FAS), a mild form of holoprosencephaly, demonstrates a mid-line anterior neural plate deficiency which leads to olfactory placodes being positioned too close to the mid-line, and other secondary changes. Work on animal models for the retinoic acid syndrome (RAS) shows that there is major involvement of neural crest cells. There is also major crest cell involvement in similar syndromes, apparently including hemifacial microsomia. Later administration of retinoic acid prematurely and excessively kills ganglionic placodal cells and leads to a malformation complex virtually identical to the Treacher Collins syndrome. Most clefts of the lip and/or palate appear to have a multifactorial etiology. Genetic variations in TGF alpha s, RAR alpha s, NADH dehydrogenase, an enzyme involved in oxidative metabolism, and cytochrome P-450, a detoxifying enzyme, have been implicated as contributing genetic factors. Cigarette smoking, with the attendant hypoxia, is a probable contributing environmental factor. It seems likely that few clefts involve single major genes. In most cases, the pathogenesis appears to involve inadequate contact and/or fusion of the facial prominences or palatal shelves. Specific mutations in genes for different FGF receptor molecules have been identified for achondroplasia and Crouzon's syndrome, and in a regulatory gene (Msx2) for one type of craniosynostosis. Poorly co-ordinated control of form and size of structures, or groups of structures (e.g., teeth and jaws), by regulatory genes should do much to explain the very frequent "mismatches" found in malocclusions and other dentofacial "deformities". Future directions for research, including possibilities for prevention, are discussed.

Prenatal craniofacial development: new insights on normal and abnormal mechanisms

Technical advances are radically altering our concepts of normal prenatal craniofacial development. These include concepts of germ layer formation, the establishment of the initial head plan in the neural plate, and the manner in which head segmentation is controlled by regulatory (homeobox) gene activity in neuromeres and their derived neural crest cells. There is also a much better appreciation of ways in which new cell associations are established. For example, the associations are achieved by neural crest cells primarily through cell migration and subsequent cell interactions that regulate induction, growth, programmed cell death, etc. These interactions are mediated primarily by two groups of regulatory molecules: "growth factors" (e.g., FGF and TGFalpha) and the so-called steroid/thyroid/retinoic acid superfamily. Considerable advances have been made with respect to our understanding of mechanisms involved in primary and secondary palate formation, such as growth, morphogenetic movements, and the fusion/merging phenomenon. Much progress has been made on the mechanisms involved in the final differentiation of skeletal tissues. Molecular genetics and animal models for human malformations are providing many insights into abnormal development. A mouse model for the fetal alcohol syndrome(FAS), a mild form of holoprosencephaly, demonstrates a mid-line anterior neural plate deficiency which leads to olfactory placodes being positioned too close to the mid-line, and other secondary changes. Work on animal models for the retinoic acid syndrome (RAS) shows that there is major involvement of neural crest cells. There is also major crest cell involvement in similar syndromes, apparently including hemifacial microsomia. Later administration of retinoic acid prematurely and excessively kills ganglionic placodal cells and leads to a malformation complex virtually identical to the Treacher Collins syndrome. Most clefts of the lip and/or palate appear to have a multifactorial etiology. Genetic variations in TGF alpha s, RAR alpha s, NADH dehydrogenase, an enzyme involved in oxidative metabolism, and cytochrome P-450, a detoxifying enzyme, have been implicated as contributing genetic factors. Cigarette smoking, with the attendant hypoxia, is a probable contributing environmental factor. It seems likely that few clefts involve single major genes. In most cases, the pathogenesis appears to involve inadequate contact and/or fusion of the facial prominences or palatal shelves. Specific mutations in genes for different FGF receptor molecules have been identified for achondroplasia and Crouzon's syndrome, and in a regulatory gene (Msx2) for one type of craniosynostosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for multi-site closure of the neural tube in humans

Four separate initiation sites for neural tube (NT) fusion have been demonstrated recently in mice and other experimental animals. We evaluated the question of whether the multisite model vs. the traditional single-site model of NT closure provided the best explanation for neural tube defects (NTDs) in humans. Evidence for segmental vs. continuous NT closure was obtained by review of our recent clinical cases of NTDs and previous medical literature. With the multi-site NT closure model, we find that the majority of NTDs can be explained by failure of fusion of one of the closures or their contiguous neuropores. We hypothesize that: Anencephaly results from failure of closure 2 for meroacranium and closures 2 and 4 for holoacranium. Spina-bifida cystica results from failure of rostral and/or caudal closure 1 fusion. Craniorachischisis results from failure of closures 2, 4, and 1. Closure 3 non-fusion is rare, presenting as a midfacial cleft extending from the upper lip through the frontal area ("facioschisis"). Frontal and parietal cephaloceles occur at the sites of the junctions of the cranial closures 3-2 and 2-4 (the prosencephalic and mesencephalic neuropores). Occipital cephaloceles result from incomplete membrane fusion of closure 4. In humans, the most caudal NT may have a 5th closure site involving L2 to S2. Closure below S2 is by secondary neurulation. Evidence for multi-site NT closure is apparent in clinical cases of NTDs, as well as in previous epidemiological studies, empiric recurrence risk studies, and pathological studies. Genetic variations of NT closures sites occur in mice and are evident in humans, e.g., familial NTDs with Sikh heritage (closure 4 and rostral 1), Meckel-Gruber syndrome (closure 4), and Walker-Warburg syndrome (2-4 neuropore, closure 4). Environmental and teratogenic exposures frequently affect specific closure sites, e.g., folate deficiency (closures 2, 4, and caudal 1) and valproic acid (closure 5 and canalization). Classification of NTDs by closure site is recommended for all studies of NTDs in humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucose causes lengthening of the microvilli of the neural plate of the rat embryo and produces a helical pattern on their surface

Prominent microvilli have been observed on the surface of neural plates in the embryos of many species. Since glucose is the main source of energy for embryos before neural tube closure and the onset of vascular circulation, it was of interest to study the relationship between these microvilli and glucose utilization in the neural plate. By applying microdrops of amniotic fluid to chemstrips, which colorimetrically measure glucose by glucose oxidase reaction, we determined that day 10 rat amniotic fluid glucose level was 31.6 +/- 1.6 mg/dl. On day 10 and within about 20 min from removal of the decidual sites, no glucose was found in the amniotic fluid. By use of a scanning electron microscope, the microvilli of the day 10 neural plate were found to have a 10-fold increase in length during a 40-min exposure to Hanks' solution at 21-23 degrees C. Similarly exposed embryos in Hanks' without glucose did not have microvillus elongation. However, under whole embryo culture conditions at 38 degrees C no extension of the microvilli was found. In the closed neural tube of the day 10 embryo, the microvilli were stubby and did not elongate with glucose exposure. Similarly, day 11 and 14 embryos had short microvilli which did not elongate with direct exposure to glucose at 21-23 degrees C. The short microvilli on the surface of the closed neural tube on day 11, 14, and 16 were associated with low glucose concentrations in the neural tube fluids. By use of a field emission scanning electron microscope, the surfaces of the microvilli in the extended position were seen to be covered by a right-handed helical array of globular objects the size of large molecules. The findings support the hypothesis that microvillar length may modulate glucose uptake. Shortening is associated with low concentrations of glucose in closed neural tubes, and lengthening occurs at glucose exposures of 100 mg/dl.

Comparison of staging systems for the gastrulation and early neurulation period in rodents: a proposed new system

Because there is no standard developmental staging system for the early postimplantation period of rodent embryos, investigators must now choose between a variety of systems that differ significantly. We have reviewed many of these staging systems and have summarized the ambiguities within them and the inconsistencies among them. In order to compare systems, we first obtained a consensus of the order of developmental events from the literature, and then attempted to fit existing systems into this order taking into account inconsistencies in terminology and blurred borderlines between stages. We were able to do this for most systems but not all because some were too divergent. We found that inconsistencies in definition of some terms, such as "primitive streak stage" and those used to describe the early neurulation process (neural plate, neural groove, neural folds, and head fold) cause much confusion. In order to develop an unambiguous system which can be used by all investigators, we propose to modify Theiler's system, which is one of the most commonly used systems but is not defined precisely during the early postimplantation period. We suggest making subdivisions of the original stages as follows: 1) stage 8 into 8a and 8b, by the degree of extension of the proamniotic cavity into the extraembryonic region; 2) stage 10 into 10a and 10b, by the completion of amnion formation; 3) stage 11 into 11a, 11b, and 11c, by the appearance of neural folds and foregut pocket. After Stage 12, the number of somite pairs can be used to precisely stage embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Supra-neuroectodermal cells and fibers on the primary nasal cavity and in the fourth ventricle of mouse and human embryos: scanning and transmission electron microscopic studies

Neuroectoderm-derived epithelia of the primary nasal cavity and the fourth ventricular floor and roof were observed by scanning (SEM) and transmission electron microscopy (TEM) and SEM-TEM correlative views in mouse embryos of 9th to 13th days of gestation, and in 38 externally normal human embryos ranging at Carnegie stages from 13 to 18 (about 5 to 7 weeks of gestation). Smooth-surfaced spindle-shaped cells with one or more cytoplasmic processes and cord-like cytoplasmic structures were observed by SEM on the wall of the primary nasal cavity of both species. They had morphological features similar to those of neuronal type 1 supraependymal (SE) cells and SE fibers on the floor and roof of the fourth ventricle in both species. Type 1 SE cells, SE fibers, and corresponding structures in the primary nasal cavity were localized in relation to the underlying developing nerve and vascular systems. Furthermore, their processes and fibers ran roughly parallel to these underlying structures and they penetrated the epithelial layer at the ends, suggesting a connection with underlying structures. From TEM and SEM-TEM correlative observations, SE fibers in the fourth ventricle and cord-like structures in the primary nasal cavity, both with a larger diameter, were deduced as single axon-like processes or bundles of processes. Those fibers and cord-like structures of smaller diameters were interpreted as elongated telophase bridges; both contained parallel packed microtubules and connected distant cells. Since these processes and fibers were generally longer and became fewer at later developmental stages, they appeared to be transient neuronal structures. They may play a development-related role in such morphogenetic cell movements as in the developing nerve and vascular systems in the epithelial and/or subepithelial layers, but not as direct rudiments of adult nerve tissues.

Studies on wound healing in the neuroepithelium of the chick embryo

Wound healing has been studied by light microscopy, SEM, and TEM in the neuroepithelium of the early neurula (stages 6 and 8) and advanced neurula (stages 10 and 12) chick embryos. Healing involves two major events: (1) apposition of the wound edges and (2) restitution of the neuroepithelium at the wound site (i.e., restoration of the epithelial integrity of neuroepithelium). Apposition of the wound edges occurs within the first 15 minutes of re-incubation and involves the entire length of the wound. The main event during restoration is a change in the shapes of the rounded cells to elongated forms (i.e., spindle, wedge, and inverted wedge shapes). Wounds of younger embryos heal faster than those of older ones.

Embryonic development of the house shrew (Suncus murinus). I. Embryos at stages 9 and 10 with 1 to 12 pairs of somites

The embryonic development during the period from 1 to 12 pairs of somites was observed in an insectivore species, the house shrew (Suncus murinus), which has been bred within a closed colony. Embryos were staged by the number of somite pairs. Each stage was punctuated at every addition of three pairs of somites and numbered after the Carnegie system. The first somite became apparent between 8 and 9.0 days after fertilization, and the 12th somite appeared between 9.5 and 10.0 days. The rate of somite formation was one pair in every 3-4 h on average. The embryonic events during this period were as follows: 1. From the beginning of stage 9, the embryonic body consistently displayed a kyphosis, and as development progressed, the caudal portion of the embryo spiralled clockwise. 2. The first and second pharyngeal arches formed; their development was precocious among mammalian embryos in relation to somitic count. 3. The segmental pattern of the neural fold was similar to that of laboratory rodents and primates. The first fusion of the cranial neural folds took place in the occipital somite region, the second fusion in the diencephalic region, and the third at the end of the neural plate, thus leaving two neuropores in the cephalic region. 4. The timing of appearance of the optic sulcus was similar to that of human embryos but was delayed in comparison with that of laboratory rodents. 5. The heart always showed a more advanced state than that of other mammalian embryos. From the beginning of stage 9, an unpaired endocardial tube was seen in the bulbo-ventricular region, and deflection from a symmetrical appearance soon took place. 6. The differentiation of foregut was also precocious, and the thyroid and respiratory primordia appeared earlier than in other mammals. The present study emphasizes that there are considerable variations in timing and manner of morphogenesis among early mammalian embryos.

The incorporation and dispersion of cells and latex beads on microinjection into the amniotic cavity of the mouse embryo at the early-somite stage

The ability of cells and latex beads to become incorporated into the cranial region of embryos after microinjection into the amniotic cavity was studied. Premigratory neural crest cells isolated from the lateral margins of the neuroepithelium, 3T3 fibroblast cells or H35 hepatoma cells were labelled with WGA-gold conjugates, and were then microinjected into the amniotic cavity of embryos with two to three somites in vitro. Latex beads were similarly microinjected into different groups of embryos. Incorporation of injected cells or latex beads was found in the neural crest of the midbrain and the hindbrain of 5-20% of the recipients 4 h after microinjection. At 6 and 12 h, increasingly more embryos (20-77%) were observed with labelled cells or latex beads in the crest region. While hepatoma cells and latex beads were restricted to the crest region, injected neural crest cells and fibroblasts were also found in the lateral mesenchyme, bounded laterally by the surface ectoderm and medially by the closing neural tube. By 24 h after microinjection, the injected cells or latex beads were found in 50-80% of the recipients. Neural crest cells and fibroblasts, which showed similar patterns of distribution in the embryos, were located on the dorsal aspect of the neural tube, the lateral mesenchyme, the pharyngeal arches and the regions for ganglia. Hepatoma cells and latex beads were limited to the dorsal regions of the neural tube. When microinjection was carried out in embryos with seven to eight somites, incorporation of cells or latex beads was found in 44-75% of embryos, but no dispersion of the incorporated cells or latex beads into the mesenchyme was found 24 h after microinjection. Incorporation and dispersion of cells and latex beads were not observed when embryos with 18-20 somites were used as recipients. The present study showed that neural crest or fibroblast cells when injected into the amniotic cavity could be incorporated into the neural crest, and then undergo migration along the neural crest pathways, whereas hepatoma cells and latex beads could only be incorporated. The incorporation and migration of the exogenous tissues are related to the formation and the accessibility of the neural crest in the recipients.

Neural tube and neural crest: a new view with time-lapse high-definition photomicroscopy

The dynamic process of neural tube formation and neural crest migration in live, unstained cultured avian embryos at Hamburger-Hamilton (H.H.) stages 8-11 was investigated by time-lapse cinematography using a high-definition microscope. These studies have demonstrated that neural tube closure in the trunk region differs from that observed in the head. The cephalic neural folds elevate slowly, then make contact rapidly. Following this initial apposition, they gradually "zip-up" in the rostrad and caudad direction. In the trunk region where the neuroepithelium bulges adjacent to the somites, the edges of the folds pulsate and forcefully touch-retract-touch in these bulging regions; the intersomitic epithelia retract, remain open even after more posterior somitic regions have apposed, and then close slowly. Epithelial blebs and N-CAM antibody were observed at the leading edges of the neuroepithelia. Between the open folds only a few bridging cells were seen; they probably represent the sites of initial cell adhesion following epithelial retraction. Focusing into the developing embryo shows that neuroepithelial fusion occurs prior to surface epithelial fusion. A meshwork of synchronously pulsating neural crest cells was identified below the surface epithelium and a preliminary investigation of their initial migration was conducted.

A morphometric and computer-assisted three-dimensional reconstruction study of neural tube formation in chick embryos

The origin of the driving forces for neural tube formation remains uncertain but is currently thought to involve the participation of microfilament bundles situated in the apical ends of neuroepithelial cells. In the work presented here, we show how morphometric measurements that map local variations in the apical geometry of neuroepithelial cells (especially apical constriction) can provide information on the distribution of motive forces within the neuroepithelium during neural tube formation. When used in combination with computer-assisted, three-dimensional reconstruction, it becomes possible to analyze the morphometric data from a dynamic, three-dimensional perspective. As an example application of this method, we have used morphometry to evaluate the effects of ionomycin on the developing neuroepithelium. Treatment of early (stages 6-8) chick embryos with 5 microM ionomycin was found to cause rapid bending of the neuroepithelium within 1 min of exposure and a dramatic acceleration of the normal sequence of neural tube formation. Electron microscopy and morphometry revealed that this acceleration was coincident with a marked increase in the local degree of apical constriction of neuroepithelial cells, presumably a consequence of enhanced contractile activity of apical microfilament bundles. This work shows that transient elevation of free calcium levels can accelerate the usual sequential phases of NT formation. The rapidity of the response (hours of normal development reduced to minutes), increased prominence of apical microfilament bundles, and the enhanced degree of apical constriction strongly support a direct causal role for apical microfilament bundles and apical constriction of neuroepithelial cells in bending of the neuroepithelium.

Normal mouse strains differ in the site of initiation of closure of the cranial neural tube

The scanning electron microscopic study of day 9 embryos reported here documents differences among normal mouse strains in morphology of cranial neural tube closure. The site of initiation of contact and fusion of the cranial neural folds, previously defined as Closure 2 (Macdonald et al., '89), is located in the region of the junction between the forebrain (prosencephalon) and midbrain (mesencephalon) in three normal strains: LM/Bc, AEJ/RkBc, and ICR/Bc. However in a fourth normal strain, SWV/Bc, Closure 2 is initiated much further rostral, in the prosencephalic region. In addition, the anterior neuropore, rostral to Closure 2, closes late in ICR/Bc embryos, relative to the posterior progress of development of the Closure 2 seam. Initiation of closure from the most rostral end of the neural tube (Closure 3) appears to be relatively delayed in ICR/Bc embryos. We hypothesize that the observed genetic polymorphism in location of the first site of fusion between the cranial neural folds in normal mouse embryos may be one basis for differences among normal strains in liability to exencephaly induced by teratogens.

Abnormalities of neural tube formation in pre-spina bifida splotch-delayed mouse embryos

The splotch-delayed homozygous mutant (Spd/Spd) develops spina bifida with or without exencephaly, has spinal ganglia abnormalities, and delays in posterior neuropore closure and neural crest cell emigration. The heterozygote (Spd/+) has a pigmentation defect, and occasionally neural tube defects. To investigate the underlying mechanisms, we compared the neuroepithelium in the posterior neuropore region of cytogenetically identified 15-18 somite pair Spd/Spd, Spd/+, and +/+ mouse embryos by transmission electron and light microscopy. The notochordal area and cell number in the non-fused neuroepithelium region of Spd/Spd and Spd/+ embryos were significantly reduced compared to those of normal (+/+) embryos, which suggests an abnormality in notochord elongation. In the mesoderm, the mean cell number and mean ratio of cell number to area in the non-fused region were significantly lower in the Spd/Spd compared with +/+ embryos. The distance of exposed neuroepithelium above the mesoderm in the just-fused region was significantly lower in the Spd/Spd versus +/+ embryos, which may indicate an insufficient force exerted by the mesoderm during neural tube closure. Within the neuroepithelium, significantly more intercellular space was found in Spd/Spd than in +/+ embryos indicating disorganization. The basal lamina was poorly organized and the formation delayed around the neural tube in Spd/Spd and Spd/+ embryos. All together, these results suggest an early abnormality in interactions among the neuroepithelium, mesoderm, and notochord, which may lead to the delay or inhibition of neural tube closure observed in Spd/Spd mutants.

Methionine and neural tube closure in cultured rat embryos: morphological and biochemical analyses

When headfold-stage rat embryos were cultured on cow serum, their neural tubes failed to close unless the serum was supplemented with methionine. Methionine deficiency did not appear to affect the ability of the neural epithelium to fuse as a type of fusion was observed between anterior and posterior regions of the open neural tube in methionine-deficient embryos. Although methionine deficiency reduced the cell density and mitotic indices of cranial mesenchyme and neural epithelial cells, this did not appear to be a factor in failure of the neural tube to close. For example, embryos cultured on diluted cow serum also had fewer mesenchymal cells yet could complete neural tube closure if provided with methionine. Examination of the tips of the neural folds suggested that microfilament contraction could be involved; in the absence of methionine the neural folds failed to turn in. This possibility was supported by the reductions in neurite extension of isolated neural tubes cultured without methionine and by the reductions in microfilament associated methylated amino acids contained in embryo neural tube proteins.

Neural tube formation in the mouse: a morphometric and computerized three-dimensional reconstruction study of the relationship between apical constriction of neuroepithelial cells and the shape of the neuroepithelium

Morphometry and computerized three-dimensional reconstruction were used to study the relationship between apical constriction of neuroepithelial cells and the pattern of bending of the neuroepithelium in the developing neural tube of the 12-somite mouse embryo. The neuroepithelium of the mouse exhibits prominent regional variations in size and shape along the embryo axis. The complex shape of most of the cephalic neural tube (e.g., forebrain and midbrain) is due to the coexistence of concave and convex bending sites whereas more caudal regions (e.g., hindbrain and spinal cord) generally lack sites of convex bending and have a relatively simple shape. The apical morphology of neuroepithelial cells was found to be correlated more closely with the local status of bending of the neuroepithelium than with the specific region of the neural tube in which they are located. In areas of enhanced apical constriction, microfilament bundles were particularly prominent. Morphometry revealed that patterns of bending of the neuroepithelium were correlated almost exactly with those of apical constriction throughout the forming neural tube. These findings support the idea that apical constriction of neuroepithelial cells, resulting from tension generated by microfilament bundles, plays a major role in bending of the neuroepithelium during neural tube formation in the mouse.

Ultrastructural defects in the apical neural folds in mutant embryos with spina bifida

Ultrastructural pathology in the apical neural folds was analyzed by means of tannic acid (TA) and ruthenium red (RR) cytochemistry in abnormal (vl/vl) mutant mouse embryos ranging in age from 17-35 somites. At lumbosacral levels of the spinal cord where closure fails to occur, as well as at more cranial levels where closure occurs but results in dorsal midline abnormalities, normal deposition of TA-positive and RR-positive material occurred in the space that develops between the overlying surface ectoderm (SE) and neuroepithelium (NE). However, in lumbosacral regions, pleomorphic excrescences projected abnormally from the apices of the transitional zone cells between SE and NE cells of the open neural folds. These abnormal projections consisted of enlarged cytoplasmic blebs, as well as entire cells. The cells were not necrotic nor did they show evidence of incipient degeneration. However, it is possible that they represent aberrant putative neural crest cells, as indicated by their location in the transitional zone and by the filopodia and lamellipodia projecting from their luminal surfaces.

Bidirectional closure of the rostral neuropore in the human embryo

The length of each neuropore was measured in 23 human embryos of stages 10-12 (about 22-26 days), and the closure of the lips of the rostral neuropore was studied in 24 embryos of stage 11 (about 24 days), with particular reference to the terminal lip. Graphic reconstructions were prepared from two particularly suitable examples, and mitotic figures were plotted for one of these. The lengths of the rostral and caudal neuropores are basically similar, but the rostral opening closes 1 day earlier and more abruptly (within a few hours) than the caudal (which takes a day). Closure of the rostral neuropore in the human embryo is bidirectional, proceeding simultaneously from 1) midbrain and diencephalon 2 and 2) the telencephalic region adjacent to the chiasmatic plate. Species differences are emphasized. Closure at the terminal lip of the neuropore is by fusion of right and left neural folds, as occurs elsewhere during primary neurulation. The rostral end of the neural plate in the median plane is, in the human embryo, at the rostral limit of the chiasmatic plate. Histological differences, however, exist between closure at the terminal lip and that at the dorsal lip: the surface epithelium plays a more significant role at the terminal lip, and the seam is more visible and presumably stronger. In future anencephaly it has been found that fusion at the terminal lip may occur, although that at the dorsal lip is deficient.

Developmental study of neural tube closure in a mouse stock with a high incidence of exencephaly

About 17% of embryos and fetuses in the SELH/Bc mouse stock have the anterior neural tube defect, exencephaly. No other malformations are seen. The genetic liability to exencephaly was shown to be probably genetically fixed in the SELH/Bc stock. This means that SELH/Bc embryos with successful neural tube closure are genetically the same as exencephalics. Females were significantly more likely to be affected than males (66% females). The pattern of morphological developmental events during anterior neural tube closure on days 8 and 9 of gestation was compared among 322 ICR/Bc (normal), 304 SWV/Bc (normal), and 265 SELH/Bc embryos. Anterior neural tube closure was found to follow a strikingly different pattern in almost all SELH/Bc embryos than in either of the normal strains or in previous published studies. SELH/Bc embryos lack the initial contact between the anterior folds in the posterior prosencephalon/anterior mesencephalon region (Closure 2). In spite of this, all but 17% manage to close the anterior neural tube by extending caudally the later occurring normal anterior zone of contact and fusion at the most rostral aspect of the prosencephalon (Closure 3) through the region of Closure 2 to meet the zone of closure of the rhombencephalon, Closure 4. Anterior neural tube closure was completed late, and in some SELH/Bc embryos, elevation and fusion in the mesencephalon did not occur at all. In histological sections of six- and eight-somite embryos, elevated numbers of pyknotic cells in the neuroepithelium and mesenchyme, and elevated numbers of unstained inclusions in the neuroepithelium were found; but their relationship, if any, to the abnormal pattern of neural tube closure is not clear.

Neurulation in the mouse: manner and timing of neural tube closure

The manner and timing of neural fold fusion in primary neurulation were studied in 1,575 normal ICR mouse embryos by using binocular dissecting, light, and scanning electron microscopy. The initial fusion of apposing neural folds occurred at the level of the intermediate point between the third and fourth somites (i.e., in the caudal myelencephalon) and proceeded both rostrally and caudally. A second fusion occurred at what was originally the rostral end of the neural plate and proceeded rostrodorsally. A third fusion occurred in the caudal diencephalon and proceeded both rostrally and caudally. This was followed by complete closure of the telencephalic neuropore at the midpoint of the telencephalic roof and then complete closure of the metencephalic neuropore at the rostral part of the metencephalic roof. A fourth fusion occurred at what was originally the caudal end of the neural plate and proceeded rostrally. Finally, the caudal neuropore completely closed at the level of the caudal end of the future 33rd somite.

Connecting cords and morphogenetic movements in the quail blastoderm

Connecting cords are elongated telophase bridges persisting between separating daughter cells. We have studied them with Scanning Electron Microscopy in the upper cell layer of the quail blastoderm where a high mitotic activity accompanied by interkinetic nuclear migration coincides with morphogenetic movements. The predominant orientation of the connecting cords is parallel to the direction of the morphogenetic movements.

The effect of cadmium on the development of the facial prominences: surface area measurements of day 10-8 a.m. hamster embryos

The environmental contaminant cadmium (Cd) is a proven teratogenic agent in rodents. In hamsters, it causes craniofacial dysmorphogenesis. The underlying mechanism for this damage is unknown. Early facial development in hamsters occurs during gestation days 9-11 and involves the formation and appropriate fusion of several prominences surrounding the stomodeum. The hypothesis for this study is that the occurrence of Cd-induced facial defects involves a disruption of the normal formation and/or fusion of one or more of the facial prominences. Pregnant hamsters were treated with Cd (2 mg/kg) or water intravenously on gestation day 8 (8 A.M.). On gestation day 10 (8 A.M.) surviving embryos were processed to obtain scanning electron micrographs of the frontal view of the face. Measurements of the surface areas of 15 different portions of the face were obtained using a microcomputer equipped with a digitizer. Both qualitative and quantitative differences in the faces were detected upon comparing the Cd-exposed and control embryos. The surface areas of the prominences measured were significantly smaller in the Cd-exposed embryos. However the sizes of the other regions of the Cd-exposed faces were either little affected (nasal pit areas) or markedly increased (the interval of the face between the medial nasal prominences). Two possible explanations for these data are discussed.

Neurulation in the mouse. I. The ontogenesis of neural segments and the determination of topographical regions in a central nervous system

Ontogenesis of neural segments and positional relationships between the segments and other organs during neurulation were studied in 1,423 ICR mouse embryos by binocular dissecting, light, and scanning electron microscopy. Late in the presomite stage, two transverse sulci, preotic and otic, were seen on the prospective luminal surface of the neural folds. By somite stage 19, the former subdivided into five neuromeres, and by somite stage 21, the latter subdivided into four neuromeres. From the rostral, preotic sulcus, moreover, five other neuromeres were formed by somite stage 20, and between the otic sulcus and the first somite, two neuromeres were formed by somite stage 28. In the caudal part, from the level of the first somite, a total of 39 neuromeres were formed one after another by somite stage 39, and their positions almost correlated with each corresponding somite. Furthermore, the isthmus grew in the boundary between the fifth and sixth neuromere. The most protruding zone in the preotic sulcus formed the eighth neuromere and was located adjacent to the first branchial arch and the trigeminal ganglion. The most protruding zone in the otic sulcus also formed the 11th neuromere and was located adjacent to the second branchial arch. The 12th and 13th neuromeres were situated adjacent to the otic vesicle; the 23rd to 28th neuromeres, adjacent to the forelimb bud; and the 40th to 46th neuromeres, adjacent to the hindlimb bud.

Developmental analysis of cephalic axial dysraphic disorders in arsenic-treated hamster embryos

Parenteral injection of pregnant golden hamsters with inorganic arsenic salts early in gestation results, by term, in markedly elevated embryonic-fetal mortality (approximately equal to 50%) and, in surviving fetuses, a high (approximately equal to 90%) incidence of cephalic axial dysraphic disorders ("neural tube defects"), particularly exencephaly/anencephaly and encephaloceles. The present investigation traces the day by day development of these embryopathic effects of arsenic in the hamster with an emphasis on the pathogenesis of cephalic axial dysraphic disorders. Pregnant golden hamsters were given an intraperitoneal injection of sodium arsenate (20 mg/kg) on the 8th day (08.00) of their 16 day gestation period. Matched control dams were injected with an equivalent volume of distilled water by the same route and at the same stage of gestation. Experimental and control dams were sacrificed beginning 24 h after treatment and at regular daily intervals thereafter until term. Embryos and fetuses delivered from sacrificed dams were examined for abnormalities both grossly and histologically. In embryos delivered earliest after treatment (24-48 h) the principal deleterious effect of arsenic observed was retarded growth (elevation, approximation, and fusion) of the cephalic neural folds. This growth retardation ranged in severity among embryos. In the most severely afflicted there was a site wherein the opposing cephalic neural folds had completely failed to appose and fuse ("closure"). This failure of closure of all four tissue components of the neural folds (surface ectoderm, paraxial mesoderm, neural crest cells, neuroectoderm) resulted in a persistent dorsal opening in the head, i.e., cranioschisis aperta. The extent and appearance of this opening varied from a small, ovoid aperture in the dorsal midbrain (mesencephalic) region of the head to a widely open cleft involving the fore and hindbrain regions as well as the midbrain region. In less severely afflicted early embryos, the cephalic neural folds had elevated and met in the dorsal midline but had only incompletely fused, i.e., cranioschisis occulta. Microscopic study of these latter embryos revealed that in the affected region(s), complete closure of the surface ectoderm component of the neural folds had taken place, but only partial closure of the mesoderm, neural crest and neuroectoderm components. The different types of cephalic axial dysraphic disorders presenting in arsenic-treated fetuses delivered at later gestational stages (predominantly exencephaly and encephaloceles) could all be traced back and related to one or the other of these early forms of disturbed neurulation.