Ultrastructural observations on closure of the neural tube in the 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.

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.

Neural tube defects: role of lithium carbonate exposure in embryonic neural development in a murine model

BACKGROUND

Lithium carbonate (Li₂CO₃) is widely used in the treatment of clinical-affective psychosis. Exposure to Li₂CO₃ during pregnancy increases the risk of neural tube defects (NTDs) in offspring, which are severe birth defects of the central nervous system. The mechanism of Li₂CO₃-induced NTDs remains unclear.

METHODS

C57BL/6 mice were injected with different doses of Li₂CO₃ intraperitoneally on gestational day 7.5 (GD7.5), and embryos collected at GD11.5 and GD13.5. The mechanisms of Li₂CO₃ exposure-induced NTDs were determined utilizing immunohistochemistry, western blotting, EdU imaging, enzymatic method, gas chromatography-mass spectrometry (GC-MS), ELISA and HE staining.

RESULTS

The NTDs incidence was 33.7% following Li₂CO₃ exposure. Neuroepithelial cell proliferation and phosphohistone H3 level were significantly increased in NTDs embryos, compared with control group (P < 0.01), while the expressing levels of p53 and caspase-3 were significantly decreased. IMPase and GSK-3β activity was inhibited in Li₂CO₃-treated maternal and embryonic neural tissues (P < 0.01 and P < 0.05, respectively), along with decreased levels of inositol and metabolites, compared with control groups (P < 0.01).

CONCLUSIONS

Lithium-induced NTDs model in C57BL/6 mice was established. Enhanced cell proliferation and decreased apoptosis following lithium exposure were closely associated with the impairment of inositol biosynthesis, which may contribute to lithium-induced NTDs.

IMPACT

Impairment of inositol biosynthesis has an important role in lithium exposure-induced NTDs in mice model. Lithium-induced NTDs model on C57BL/6 mice was established. Based on this NTDs model, lithium-induced impairment of inositol biosynthesis resulted in the imbalance between cell proliferation and apoptosis, which may contribute to lithium-induced NTDs. Providing evidence to further understand the molecular mechanisms of lithium-induced NTDs and enhancing its primary prevention.

Hindbrain neuropore tissue geometry determines asymmetric cell-mediated closure dynamics in mouse embryos

Gap closure is a common morphogenetic process. In mammals, failure to close the embryonic hindbrain neuropore (HNP) gap causes fatal anencephaly. We observed that surface ectoderm cells surrounding the mouse HNP assemble high-tension actomyosin purse strings at their leading edge and establish the initial contacts across the embryonic midline. Fibronectin and laminin are present, and tensin 1 accumulates in focal adhesion-like puncta at this leading edge. The HNP gap closes asymmetrically, faster from its rostral than caudal end, while maintaining an elongated aspect ratio. Cell-based physical modeling identifies two closure mechanisms sufficient to account for tissue-level HNP closure dynamics: purse-string contraction and directional cell motion implemented through active crawling. Combining both closure mechanisms hastens gap closure and produces a constant rate of gap shortening. Purse-string contraction reduces, whereas crawling increases gap aspect ratio, and their combination maintains it. Closure rate asymmetry can be explained by asymmetric embryo tissue geometry, namely a narrower rostral gap apex, whereas biomechanical tension inferred from laser ablation is equivalent at the gaps' rostral and caudal closure points. At the cellular level, the physical model predicts rearrangements of cells at the HNP rostral and caudal extremes as the gap shortens. These behaviors are reproducibly live imaged in mouse embryos. Thus, mammalian embryos coordinate cellular- and tissue-level mechanics to achieve this critical gap closure event.

Loss of Grhl3 is correlated with altered cellular protrusions in the non-neural ectoderm during neural tube closure

BACKGROUND

The transcription factor Grainyhead-like 3 (GRHL3) has multiple roles in a variety of tissues during development including epithelial patterning and actin cytoskeletal regulation. During neural tube closure (NTC) in the mouse embryo, GRHL3 is expressed and functions in the non-neural ectoderm (NNE). Two important functions of GRHL3 are regulating the actin cytoskeleton during NTC and regulating the boundary between the NNE and neural ectoderm. However, an open question that remains is whether these functions explain the caudally restricted neural tube defect (NTD) of spina bifida observed in Grhl3 mutants.

RESULTS

Using scanning electron microscopy and immunofluorescence based imaging on Grhl3 mutants and wildtype controls, we show that GRHL3 is dispensable for NNE identity or epithelial maintenance in the caudal NNE but is needed for regulation of cellular protrusions during NTC. Grhl3 mutants have decreased lamellipodia relative to wildtype embryos during caudal NTC, first observed at the onset of delays when lamellipodia become prominent in wildtype embryos. At the axial level of NTD, half of the mutants show increased and disorganized filopodia and half lack cellular protrusions.

CONCLUSION

These data suggest that altered cellular protrusions during NTC contribute to the etiology of NTD in Grhl3 mutants.

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.

miR-322 treatment rescues cell apoptosis and neural tube defect formation through silencing NADPH oxidase 4

AIMS

Failure of neural tube closure resulting from excessive apoptosis leads to neural tube defects (NTDs). NADPH oxidase 4 (NOX4) is a critical mediator of cell growth and death, yet its role in NTDs has never been characterized. NOX4 is a potential target of miR-322, and we have previously demonstrated that miR-322 was involved in high glucose-induced NTDs. In this study, we investigated the effect of NOX4 on the embryonic neuroepithelium in NTDs and reveal a new regulatory mechanism for miR-322 that disrupts neurulation by ameliorating cell apoptosis.

METHODS

All-trans-retinoic acid (ATRA)-induced mouse model was utilized to study NTDs. RNA pull-down and dual-luciferase reporter assays were used to confirm the interaction between NOX4 and miR-322. In mouse neural stem cells and whole-embryo culture, Western blot and TUNEL were carried out to investigate the effects of miR-322 and NOX4 on neuroepithelium apoptosis in NTD formation.

RESULTS

NOX4, as a novel target of miR-322, was upregulated in ATRA-induced mouse model of NTDs. In mouse neural stem cells, the expression of NOX4 was inhibited by miR-322; still further, NOX4-triggered apoptosis was also suppressed by miR-322. Moreover, in whole-embryo culture, injection of the miR-322 mimic into the amniotic cavity attenuated cell apoptosis in NTD formation by silencing NOX4.

CONCLUSION

miR-322/NOX4 plays a crucial role in apoptosis-induced NTD formation, which may provide a new understanding of the mechanism of embryonic NTDs and a basis for potential therapeutic target against NTDs.

Mouse Models of Neural Tube Defects

During embryonic development, the central nervous system forms as the neural plate and then rolls into a tube in a complex morphogenetic process known as neurulation. Neural tube defects (NTDs) occur when neurulation fails and are among the most common structural birth defects in humans. The frequency of NTDs varies greatly anywhere from 0.5 to 10 in 1000 live births, depending on the genetic background of the population, as well as a variety of environmental factors. The prognosis varies depending on the size and placement of the lesion and ranges from death to severe or moderate disability, and some NTDs are asymptomatic. This chapter reviews how mouse models have contributed to the elucidation of the genetic, molecular, and cellular basis of neural tube closure, as well as to our understanding of the causes and prevention of this devastating birth defect.

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.

Possibilities and limitations of three-dimensional reconstruction and simulation techniques to identify patterns, rhythms and functions of apoptosis in the early developing neural tube

The now classical idea that programmed cell death (apoptosis) contributes to a plethora of developmental processes still has lost nothing of its impact. It is, therefore, important to establish effective three-dimensional (3D) reconstruction as well as simulation techniques to decipher the exact patterns and functions of such apoptotic events. The present study focuses on the question whether and how apoptosis promotes neurulation-associated processes in the spinal cord of Tupaia belangeri (Tupaiidae, Scandentia, Mammalia). Our 3D reconstructions demonstrate that at least two craniocaudal waves of apoptosis consecutively pass through the dorsal spinal cord. The first wave appears to be involved in neural fold fusion and/or in selection processes among premigratory neural crest cells. The second one seems to assist in establishing the dorsal signaling center known as the roof plate. In the hindbrain, in contrast, apoptosis among premigratory neural crest cells progresses craniocaudally but discontinuously, in a segment-specific manner. Unlike apoptosis in the spinal cord, these segment-specific apoptotic events, however, precede later ones that seemingly support neural fold fusion and/or postfusion remodeling. Arguing with Whitehead that biological patterns and rhythms differ in that biological rhythms depend "upon the differences involved in each exhibition of the pattern" (Whitehead in An enquiry concerning the principles of natural knowledge. Cambridge University Press, London, 1919, p. 198) we show that 3D reconstruction and simulation techniques can contribute to distinguish between (static) patterns and (dynamic) rhythms of apoptosis. By deciphering novel patterns and rhythms of developmental apoptosis, our reconstructions help to reconcile seemingly inconsistent earlier findings in chick and mouse embryos, and to create rules for computer simulations.

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.

Caspases and matrix metalloproteases facilitate collective behavior of non-neural ectoderm after hindbrain neuropore closure

Background

Mammalian brain is formed through neural tube closure (NTC), wherein both ridges of opposing neural folds are fused in the midline and remodeled in the roof plate of the neural tube and overlying non-neural ectodermal layer. Apoptosis is widely observed from the beginning of NTC at the neural ridges and is crucial for the proper progression of NTC, but its role after the closure remains less clear.

Results

Here, we conducted live-imaging analysis of the mid-hindbrain neuropore (MHNP) closure and revealed unexpected collective behavior of cells surrounding the MHNP. The cells first gathered to the closing point and subsequently relocated as if they were released from the point. Inhibition of caspases or matrix metalloproteases with chemical inhibitors impaired the cell relocation.

Conclusions

These lines of evidence suggest that apoptosis-mediated degradation of extracellular matrix might facilitate the final process of neuropore closure.

Electronic supplementary material

The online version of this article (10.1186/s12861-018-0175-3) contains supplementary material, which is available to authorized users.

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

Grainyhead-like 2 downstream targets act to suppress epithelial-to-mesenchymal transition during neural tube closure

The transcription factor grainyhead-like 2 (GRHL2) is expressed in non-neural ectoderm (NNE) and Grhl2 loss results in fully penetrant cranial neural tube defects (NTDs) in mice. GRHL2 activates expression of several epithelial genes; however, additional molecular targets and functional processes regulated by GRHL2 in the NNE remain to be determined, as well as the underlying cause of the NTDs in Grhl2 mutants. Here, we find that Grhl2 loss results in abnormal mesenchymal phenotypes in the NNE, including aberrant vimentin expression and increased cellular dynamics that affects the NNE and neural crest cells. The resulting loss of NNE integrity contributes to an inability of the cranial neural folds to move toward the midline and results in NTD. Further, we identified Esrp1, Sostdc1, Fermt1, Tmprss2 and Lamc2 as novel NNE-expressed genes that are downregulated in Grhl2 mutants. Our in vitro assays show that they act as suppressors of the epithelial-to-mesenchymal transition (EMT). Thus, GRHL2 promotes the epithelial nature of the NNE during the dynamic events of neural tube formation by both activating key epithelial genes and actively suppressing EMT through novel downstream EMT suppressors.

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.

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

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

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

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

Neural tube defects

Neural tube defects (NTDs), including spina bifida and anencephaly, are severe birth defects of the central nervous system that originate during embryonic development when the neural tube fails to close completely. Human NTDs are multifactorial, with contributions from both genetic and environmental factors. The genetic basis is not yet well understood, but several nongenetic risk factors have been identified as have possibilities for prevention by maternal folic acid supplementation. Mechanisms underlying neural tube closure and NTDs may be informed by experimental models, which have revealed numerous genes whose abnormal function causes NTDs and have provided details of critical cellular and morphological events whose regulation is essential for closure. Such models also provide an opportunity to investigate potential risk factors and to develop novel preventive therapies.

dNTP deficiency induced by HU via inhibiting ribonucleotide reductase affects neural tube development

Exposure to environmental toxic chemicals in utero during the neural tube development period can cause developmental disorders. To evaluate the disruption of neural tube development programming, the murine neural tube defects (NTDs) model was induced by interrupting folate metabolism using methotrexate in our previous study. The present study aimed to examine the effects of dNTP deficiency induced by hydroxyurea (HU), a specific ribonucleotide reductase (RNR) inhibitor, during murine neural tube development. Pregnant C57BL/6J mice were intraperitoneally injected with various doses of HU on gestation day (GD) 7.5, and the embryos were checked on GD 11.5. RNR activity and deoxynucleoside triphosphate (dNTP) levels were measured in the optimal dose. Additionally, DNA damage was examined by comet analysis and terminal deoxynucleotidyl transferase mediated dUTP nick end-labeling (TUNEL) assay. Cellular behaviors in NTDs embryos were evaluated with phosphorylation of histone H3 (PH-3) and caspase-3 using immunohistochemistry and western blot analysis. The results showed that NTDs were observed mostly with HU treatment at an optimal dose of 225 mg/kg b/w. RNR activity was inhibited and dNTP levels were decreased in HU-treated embryos with NTDs. Additionally, increased DNA damage, decreased proliferation, and increased caspase-3 were significant in NTDs embryos compared to the controls. Results indicated that HU induced murine NTDs model by disturbing dNTP metabolism and further led to the abnormal cell balance between proliferation and apoptosis.

Midline craniofacial malformations with a lipomatous cephalocele are associated with insufficient closure of the neural tube in the tuft mouse

BACKGROUND

Genetic variations affecting neural tube closure along the head result in malformations to the face and brain, posing a significant impact on health care costs and the quality of life.

METHODS

We have established a mouse line from a mutation that arose spontaneously in our wild-type colony that we called tuft. Tuft mice have heritable midline craniofacial defects featuring an anterior lipomatous cephalocele.

RESULTS

Whole-mount skeletal stains indicated that affected newborns had a broader interfrontal suture where the cephalocele emerged between the frontal bones. Mice with a cephalocele positioned near the rostrum also presented craniofacial malformations such as ocular hypertelorism and midfacial cleft of the nose. Gross and histological examination revealed that the lipomatous cephalocele originated as a fluid filled cyst no earlier than E14.5 while embryos with a midfacial cleft was evident during craniofacial development at E11.5. Histological sections of embryos with a midfacial cleft revealed the cephalic neuroectoderm remained proximal or fused to the frontonasal ectoderm about the closure site of the anterior neuropore, indicating a defect to neural tube closure. We found the neural folds along the rostrum of E9 to E10.5 embryos curled inward and failed to close as well as embryos with exencephaly and anencephaly at later stages. Whole-mount in situ hybridization of anterior markers Fgf8 and Sonic hedgehog indicated closure of the rostral site was compromised in severe cases.

CONCLUSION

We present a model demonstrating how anterior cranial cephaloceles are generated following a defect to neural tube closure and relevance to subsequent craniofacial morphogenesis in the tuft mouse.

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.

Mechanisms of tissue fusion during development

Tissue fusion events during embryonic development are crucial for the correct formation and function of many organs and tissues, including the heart, neural tube, eyes, face and body wall. During tissue fusion, two opposing tissue components approach one another and integrate to form a continuous tissue; disruption of this process leads to a variety of human birth defects. Genetic studies, together with recent advances in the ability to culture developing tissues, have greatly enriched our knowledge of the mechanisms involved in tissue fusion. This review aims to bring together what is currently known about tissue fusion in several developing mammalian organs and highlights some of the questions that remain to be addressed.

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.

Development of the vertebrate central nervous system: formation of the neural tube

The developmental process of neurulation involves a series of coordinated morphological events, which result in conversion of the flat neural plate into the neural tube, the primordium of the entire central nervous system (CNS). Failure of neurulation results in neural tube defects (NTDs), severe abnormalities of the CNS, which are among the commonest of congenital malformations in humans. In order to gain insight into the embryological basis of NTDs, such as spina bifida and anencephaly, it is necessary to understand the morphogenetic processes and molecular mechanisms underlying neural tube closure. The mouse is the most extensively studied mammalian experimental model for studies of neurulation, while considerable insight into underlying developmental mechanisms has also arisen from studies in other model systems, particularly birds and amphibians. We describe the process of neural tube formation, discuss the cellular mechanisms involved and highlight recent findings that provide links between molecular signaling pathways and morphogenetic tissue movements.

Apoptosis is not required for mammalian neural tube closure

Apoptotic cell death occurs in many tissues during embryonic development and appears to be essential for processes including digit formation and cardiac outflow tract remodeling. Studies in the chick suggest a requirement for apoptosis during neurulation, because inhibition of caspase activity was found to prevent neural tube closure. In mice, excessive apoptosis occurs in association with failure of neural tube closure in several genetic mutants, but whether regulated apoptosis is also necessary for neural tube closure in mammals is unknown. Here we investigate the possible role of apoptotic cell death during mouse neural tube closure. We confirm the presence of apoptosis in the neural tube before and during closure, and identify a correlation with 3 main events: bending and fusion of the neural folds, postfusion remodeling of the dorsal neural tube and surface ectoderm, and emigration of neural crest cells. Both Casp3 and Apaf1 null embryos exhibit severely reduced apoptosis, yet neurulation proceeds normally in the forebrain and spine. In contrast, the mutant embryos fail to complete neural tube closure in the midbrain and hindbrain. Application of the apoptosis inhibitors z-Vad-fmk and pifithrin-alpha to neurulation-stage embryos in culture suppresses apoptosis but does not prevent initiation or progression of neural tube closure along the entire neuraxis, including the midbrain and hindbrain. Remodeling of the surface ectoderm to cover the closed tube, as well as delamination and migration of neural crest cells, also appear to be normal in the apoptosis-suppressed embryos. We conclude that apoptosis is not required for neural tube closure in the mouse embryo.

Quantitative trait loci affecting phenotypic variation in the vacuolated lens mouse mutant, a multigenic mouse model of neural tube defects

The vacuolated lens (vl) mouse mutant arose spontaneously on the C3H/HeSn background and exhibits neural tube defects (NTDs), congenital cataract, and occasionally a white belly spot. We previously reported that 1) the vl phenotypes are due to a mutation in an orphan G protein-coupled receptor (GPCR), Gpr161; 2) the penetrance of the vl NTD and cataract phenotypes are affected by genetic background, allowing three unlinked quantitative trait loci (QTL) to be mapped (modifiers of vacuolated lens, Modvl1-3); and 3) phenotype-based bioinformatics followed by genetic and molecular analysis identified a lens-specific transcription factor that contributes to the cataract-modifying effect of Modvl3. We now extend this analysis in three ways. First, using the Gpr161 mutation to unequivocally identify mutant adults and embryos, we determined that approximately 50% of vl/vl NTD-affected embryos die during development. Second, the MOLF/Ei genetic background suppresses this embryonic lethality but increases the incidence of the adult belly spot phenotype. Additional QTL analysis was performed, and two novel modifiers were mapped [Modvl4, logarithm of odds ratio (LOD) 4.4; Modvl5, LOD 5.0]. Third, phenotype-based bioinformatics identified candidate genes for these modifiers including two GPCRs that cause NTD or skin/pigmentation defects (Modvl4: Frizzled homolog 6; Modvl5: Melanocortin 5 receptor). Because GPCRs form oligomeric complexes, these genes were resequenced and nonsynonymous coding variants were identified. Bioinformatics and protein modeling suggest that these variants may be functional. Our studies further establish vl as a multigenic mouse model for NTDs and identify additional QTL that interact with Gpr161 to regulate neurulation.

The orphan G protein-coupled receptor, Gpr161, encodes the vacuolated lens locus and controls neurulation and lens development

The vacuolated lens (vl) mouse mutant causes congenital cataracts and neural tube defects (NTDs), with the NTDs being caused by abnormal neural fold apposition and fusion. Our positional cloning of vl indicates these phenotypes result from a deletion mutation in an uncharacterized orphan G protein-coupled receptor (GPCR), Gpr161. Gpr161 displays restricted expression to the lateral neural folds, developing lens, retina, limb, and CNS. Characterization of the vl mutation indicates that C-terminal tail of Gpr161 is truncated, leading to multiple effects on the protein, including reduced receptor-mediated endocytosis. We have also mapped three modifier quantitative trait loci (QTL) that affect the incidence of either the vl cataract or NTD phenotypes. Bioinformatic, sequence, genetic, and functional data have determined that Foxe3, a key regulator of lens development, is a gene responsible for the vl cataract-modifying phenotype. These studies have extended our understanding of the vl locus in three significant ways. One, the cloning of the vl locus has identified a previously uncharacterized GPCR-ligand pathway necessary for neural fold fusion and lens development, providing insight into the molecular regulation of these developmental processes. Two, our QTL analysis has established vl as a mouse model for studying the multigenic basis of NTDs and cataracts. Three, we have identified Foxe3 as a genetic modifier that interacts with Gpr161 to regulate lens development.

Nap1-regulated neuronal cytoskeletal dynamics is essential for the final differentiation of neurons in cerebral cortex

The cytoskeletal regulators that mediate the change in the neuronal cytoskeletal machinery from one that promotes oriented motility to one that facilitates differentiation at the appropriate locations in the developing neocortex remain unknown. We found that Nck-associated protein 1 (Nap1), an adaptor protein thought to modulate actin nucleation, is selectively expressed in the developing cortical plate, where neurons terminate their migration and initiate laminar-specific differentiation. Loss of Nap1 function disrupts neuronal differentiation. Premature expression of Nap1 in migrating neurons retards migration and promotes postmigratory differentiation. Nap1 gene mutation in mice leads to neural tube and neuronal differentiation defects. Disruption of Nap1 retards the ability to localize key actin cytoskeletal regulators such as WAVE1 to the protrusive edges where they are needed to elaborate process outgrowth. Thus, Nap1 plays an essential role in facilitating neuronal cytoskeletal changes underlying the postmigratory differentiation of cortical neurons, a critical step in functional wiring of the cortex.

Neural tube closure and neural tube defects: studies in animal models reveal known knowns and known unknowns

The vertebrate central nervous system is a hollow structure that develops first as a flat sheet of cells and subsequently rolls into a tube during embryogenesis. Failure of this rolling process, called neural tube closure, results in a class of common human birth defects called neural tube defects. The cellular and molecular mechanisms governing neural tube closure have been studied extensively in animal models, but much remains to be elucidated. In this review, I will highlight recent progress in understanding neural tube closure mechanisms and how these studies can inform our search for the genes that underlie human neural tube defects. Supplementary material for this article can be found on the American Journal of Medical Genetics (Part C) website (http://www.mrw.interscience.wiley.com/suppmat/1552-4868/suppmat/2005/135/v135.1.wallingford.html)

The genetic basis of mammalian neurulation

More than 80 mutant mouse genes disrupt neurulation and allow an in-depth analysis of the underlying developmental mechanisms. Although many of the genetic mutants have been studied in only rudimentary detail, several molecular pathways can already be identified as crucial for normal neurulation. These include the planar cell-polarity pathway, which is required for the initiation of neural tube closure, and the sonic hedgehog signalling pathway that regulates neural plate bending. Mutant mice also offer an opportunity to unravel the mechanisms by which folic acid prevents neural tube defects, and to develop new therapies for folate-resistant defects.

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.

Cytoskeleton gradients in three dimensions during neurulation in the rabbit

Morphogenetic movements leading to the formation of the neural tube and cellular differentiation leading to neuronal and glial cell lineages are both part of early development of the vertebrate nervous system. In order to analyze the degree of overlap between these processes, cellular differentiation during the shaping of the neural plate is investigated immunohistochemically by using monoclonal intermediate filament protein antibodies and the 7.5-8.0-day-old rabbit embryo as a model. Western blotting is used to confirm the specificity of the antibodies, which include a new monoclonal vimentin antibody suitable for double-labeling in combination with monoclonal cytokeratin (and fibronectin) antibodies. Starting in the early somite embryo and concomitant with neural plate folding, a gradual loss of cytokeratin 8 (and 18) expression in the neuroepithelium is mirrored by a gain in vimentin expression with partial coexpression of both proteins. At the prospective rhombencephalic and spino-caudal levels, vimentin expression, in particular, changes (i.e., increases) along gradients in three dimensions: along the longitudinal axis of each neuroepithelial cell from basal to apical, in the transverse plane of the embryo from dorsolateral to ventromedial and along the craniocaudal axis from prospective rhombencephalic toward spino-caudal levels of the neural plate. At the prospective mes- and prosencephalic levels, the expression change also proceeds from basal to apical within each neuroepithelial cell, but along the other axes described here, the progress in expression change is more complex. Although the functional meaning of these highly ordered expression changes is at present unclear, the gradients suggest a novel pattern of neuroepithelial differentiation which may be functionally related to the process of interkinetic nuclear migration (Sauer [1935] J. Comp. Neurol. 62:377-402) and which partially coincides with the morphogenetic movements involved in the shaping of the neural plate.

Genetically determined absence of an initiation site of cranial neural tube closure is causally related to exencephaly in SELH/Bc mouse embryos

The SELH/Bc mouse strain (SELH) has a high frequency of the lethal neural tube closure defect, exencephaly, in newborns and embryos. Previous work has shown that all SELH mouse embryos have an abnormal mechanism of rostral neural tube closure. They lack initiation of contact and fusion of the cranial neural tube at the prosencephalon/mesencephalon boundary [Closure 2), and undergo closure by extension of a more rostral site of fusion. This process fails in 10-20% of embryos, where the mesencephalic folds remain unelevated, resulting in exencephaly. Previous work has also shown that the cause of liability to exencephaly in SELH mice is multigenic, involving a small number of loci. The purpose of the present study was to test the hypothesis that the genes causing the lack of Closure 2 also cause the liability to exencephaly in SELH, by observation of their joint transmission from genetically segregating animals. A concurrent mapping study provided the necessary genetic material, a segregating F2 generation from a cross of SELH with the normal LM/Bc strain. The genetic liability to exencephaly transmitted by individual F2 sires had been measured by the frequencies of exencephalic day 14 embryos they produced in test-crosses with SELH females. A selected subset of 13 of these test-crossed F2 sires was bred with a second set of SELH females, and the embryos were examined earlier, during the period of neural tube closure, on days 8 and 9 of gestation, to determine the presence of Closure 2. Six F2 sires were among the highest exencephaly producers (6-11%), six were among the lowest (0%), and one was intermediate (5%). Among embryos at the appropriate stage for scoring, the presence of Closure 2 was observed to be inversely correlated with the later risk of exencephaly, being present in 93% (71/76) from the low-risk sires and 35% (36/103) from the high-risk sires. In each case, the remaining embryos had a closure mechanism like that of SELH embryos. Among the individual intermediate- and high-risk sires, there was also a clear correlation between the frequency of exencephaly in older embryos and the frequency of lack of Closure 2 in early embryos (rs = 0.88; P < 0.05). This study demonstrates that high liability to exencephaly and absence of Closure 2 are genetically transmitted together. That is, the cause of the lack of Closure 2 in SELH mice is shown to be also the probable cause of the high liability to exencephaly.

Apical cell escape from the neuroepithelium and cell transformation during terminal lip fusion in the house shrew embryo

The house shrew embryo has many cells in the ventricular lumen and on the luminal surface of the fusing terminal lip of the cephalic neural tube. The origin and fate of these cells were studied by means of light and electron microscopy, and by DiI labeling in a whole-embryo culture system. The cells appeared at stage 11A and persisted until stage 12A. Most of the cells seemed to originate from the neuroepithelium, as shown by frequent observations of epithelial cell escape and DiI labeling analysis. The cells on the luminal surface sometimes showed apoptotic features, but were not subjected to phagocytosis. Some of the escaping cells seemed to migrate to the ventral part of the prosencephalic neuropore and insert themselves into it. Others separated from the luminal surface and floated into the lumen. It seems likely that the floating cells either become autolyzed, or else change into macrophage-like cells, the latter alternative being supported by the results of DiI labeling. The macrophage-like cells actively phagocytosed the other degenerating cells and apoptotic bodies. These observations suggest that the apical escape of cells may play an important role in the remodeling of the neural fold during the terminal lip fusion, and that early neuroepithelial cells may have the potential to become cells with vigorous phagocytic activity, like macrophages.

Patterns of epithelial cell death during early development of the human inner ear

A light microscopic study of cell death in a developmental series of otic primordia from 23 human embryos (Carnegie stages 9 to 14) was completed. Degenerated cells were noted predominantly in the placode (stages 9 and 10), cup (stages 11 and 12), and otocyst (stages 13 and 14). A systematic camera lucida study of the appearance and topography of degenerating epithelial cells showed four different areas of cell death in the otic primordia that related to 1) invagination and detachment of the otic anlage, 2) early histogenesis of the statoacoustic ganglion, and 3) development of the endolymphatic duct. The possible role of cell death in the morphogenesis of the inner ear related to morphogenetic movements is discussed.

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.

Cell death in suboptic necrotic centers of chick embryo diencephalon and their topographic relationship with the earliest optic fiber fascicles

The structural features of suboptic necrotic centers (SONCs) in the floor of the chick embryo diencephalon were studied. These necrotic areas were observed lateral to the prospective zone of the optic chiasm through developmental stages 14 to 24. The relationship between SONCs and the earliest optic fiber fascicles also was studied in an attempt to determine the possible significance of these cell death areas in the mechanism of optic pathway development. In SONCs, healthy neuroepithelial cells contain primary lysosomes and phagocytose fragments of dead cells. Discrete regions within the cytoplasm of some cells show electron-transparent vacuoles in contact with dense contents of ruptured lytic bodies. The cytoplasm of dying cells and dead cell fragments are notably electron dense, with numerous secondary lysosomes and electron-transparent vacuoles. These observations are interpreted on the assumption that after autophagic processes, condensation and fragmentation take place in dying cells of the SONCs. In the ventricular lumen adjacent to the SONCs, numerous more or less spherical bodies are observed that appear to be shed from the tip of the cells constituting the SONCs. Three different types of intraventricular bodies can be distinguished: loose, moderately dense, and highly dense. The first type appears to originate from apical portions of cells that undergo autolytic processes. Moderately dense fragments are interpreted as originating from dying cells in which the cytoplasm is undergoing condensation. Finally, highly dense intraventricular bodies appear to be fragments of dead cells that are shed into the ventricular lumen. SONCs separate the prospective area of the optic chiasm from lateral regions of the diencephalic floor. Extracellular spaces are poorly developed within the wall of the SONCs, whereas the neuroepithelium of the presumptive optic chiasm and regions located rostral and caudal to SONCs show abundant and extensive extracellular spaces. These are bounded by long marginal processes of neuroepithelial cells. Sagittal sections of embryonic heads at stages 22-24 reveal optic fiber fascicles penetrating the SONCs asymmetrically, as they are found only in its caudal half. These observations suggest that the SONCs function as doorways made of compact neuroepithelium, to be traversed by the earliest optic fibers before they reach the middle zone of the floor of the diencephalon through which they travel to the contralateral optic tract within large extracellular spaces.

Closure of the posterior neuropore in the vl mutant mouse

Alterations in the surface topography of cells in the apical neural folds of the posterior neuropore were analyzed by means of scanning electron microscopy in normal (+/+) and abnormal (vl/vl) embryos characterized by lumbosacral dysraphism. In early embryos (14-25 somites) surface features distinguishing the neuroepithelial cells, transitional zone cells, and surface ectoderm cells were similar in normal and abnormal embryos, as were the arrangement and configuration of filopodia and lamellipodia. However, in embryos with approximately 26-36 somites, the transitional zone of the abnormals showed a profusion of large blebs and excrescences along the entire length of the posterior neuropore. By 36 somites, the posterior neuropore was still variably open in the abnormals, in contrast to normal embryos in which no external opening could be detected. In view of the abnormalities associated with the transitional zone, it is possible that the underlying mechanism that results in lumbosacral spina bifida in this mutant may involve putative neural crest cells.

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.

The association of aplasia cutis congenita with therapy of maternal thyroid disease

Aplasia cutis congenita, the localized absence of skin at birth, usually is an isolated scalp defect. We examined an infant with aplasia cutis congenita associated with maternal Grave's disease and the use of methimazole during pregnancy. This association was reported twice before. It has certain implications with respect to therapy of pregnant hyperthyroid women.

Abnormal neural fold development in mouse trisomy 12 and trisomy 14. II. LM and TEM

LM and TEM observations of embryonic tissue during the period of cranial neurulation are described in trisomic mouse embryos known to develop exencephaly, and are compared with tissue from normal mouse embryos. The earliest regularly observed differences were visible from the late presomite stage onwards, in the extracellular matrix of the cranial region. These were local defects of the basement membrane of the neural epithelium and enlarged areas of mesenchymal extracellular matrix, with associated abnormalities of mesenchymal cell distribution, cell number and cell contacts, and deficiency of alcian blue staining. Apical neuroepithelial microfilament bundles were observed at later somite-stages in trisomic embryos c.f. = compared with controls, and development of the concave neuroepithelial curvature was correspondingly retarded. Apposition of the neural folds at the forebrain/midbrain junction was never made, even though late neural fold fusion occurred in the hindbrain and ventral forebrain. At later stages (9-20 somites) the neuroepithelial cells showed pyknotic nuclei and dense intracellular inclusions. These are interpreted as secondary effects.