Neural Tube Fusion Research Articles

Extended spinal dysraphism: Rachischisis or myeloschisis

Myeloschisis (rachischisis) is a rare congenital malformation linked to incomplete development of the spine and characterized by posterior opening of the vertebrae [1,2]. It is spread over more than one floor or even the entire spine and incompatible with life. Its diagnosis is clinical, confirmed by ultrasound and cross-sectional imaging [2]. This anomaly in its thoracic location is rare. There are two types: Segmentation and development defects [4]. Developmental defects include hemi vertebrae and failure of neural tube fusion which in turn includes the simple cleft of the posterior arch of the vertebrae (spina bifida) [3]. It can involve one or more vertebral arches, determining extensive spinal dysraphism. Certain factors favor their appearance. According to the literature, only one article published in 1972 by Levy et al, which reported an incidence of 0.01% of thoracic spina bifida [3]. We report the case of a 33-day-old female infant born at term in breech presentation, with a pregnancy that was not well followed. There was a notion of taking fenugreek during pregnancy. Hospitalized for meningitis, with a dysmorphic face, clinically presented a dorsolumbar spina bifida (Figure 1). She had received antibiotic therapy and regular dressings since her admission. A cerebral-medullary scan was performed and showed a progressive opening of the spinous processes from the cervical and dorsal spine, more marked at the height of the spinous process of the fourth dorsal vertebra to distality, with issue of meningeal structures and spinal cord. Conflicts of interest: The authors declare that they have no conflicts of interest.

Axial Mesodermal Dysplasia Complex with a Unique Abnormal Course of Vestibulocochlear Nerve

Axial mesodermal dysplasia complex (AMDC) is a combination of multiple congenital malformations arising due to the mesodermal cell migration, neural tube fusion, and rhombencephalon segmentation. Here, we present the imaging findings of a 15-year-old boy with AMDC who has bilateral inner ear malformations associated with a vestibulocochlear nerve extending to Meckel cave, cystic lesion in prepontine cisterna, cervical vertebral segmentation anomalies, and maxillar bone anomalies.

Can We Further Improve the Quality of Nephro-Urological Care in Children with Myelomeningocele?

Myelomeningocele (MMC) results from a failure of normal neural tube fusion in early fetal development. Retrospective, observational study of medical data of 54 children treated in Pediatric Nephrology and Urology Clinics for five years was performed. The following data were analyzed: serum creatinine, eGFR, urine analysis, renal scintigraphy (RS), renal ultrasound, and urodynamics. Mean age of studied population: 12.3 years, median of eGFR at the beginning and at the end of survey was 110.25 and 116.5 mL/min/1.73 m2 accordingly. Median of frequency of urinary tract infections (fUTI): 1.2 episodes/year. In 24 children: low-pressure, in 30 children: high-pressure bladder was noted. Vesicouretral reflux (VUR) was noted in 23 children (42.6%). fUTI were more common in high-grade VUR group. High-grade VURs were more common in group of patients with severe renal damage. At the end of the survey 11.1% children were qualified to higher stages of chronic kidney disease. Renal parenchyma damage progression in RS was noted in 22.2% children. Positive VUR history, febrile recurrent UTIs, bladder wall trabeculation, and older age of the patients constitute risk factors of abnormal renal scans. More than 2.0 febrile, symptomatic UTIs annually increase by 5.6-fold the risk of severe renal parenchyma damage after five years.

Secondary Myelitis in Dermal Sinus Causing Paraplegia in a Child with Previously Normal Neurological Function.

Neural tube defects result from failure of neural tube fusion during early embryogenesis, the fourth week after conception. The spectrum of severity is not uniform across the various forms of this congenital anomaly as certain presentations are not compatible with extrauterine life (anencephaly) while, on the other hand, other defects may remain undiagnosed as they are entirely asymptomatic (occult spina bifida). We report a child with previously normal neurological development, a devastating clinical course following superinfection of a subtle spina bifida defect which resulted in a flaccid paralysis below the level of the lesion and permanent neurological deficits following resolution of the acute infection and a back closure surgery.

Grainyhead-like 2 regulates neural tube closure and adhesion molecule expression during neural fold fusion

Defects in closure of embryonic tissues such as the neural tube, body wall, face and eye lead to severe birth defects. Cell adhesion is hypothesized to contribute to closure of the neural tube and body wall; however, potential molecular regulators of this process have not been identified. Here we identify an ENU-induced mutation in mice that reveals a molecular pathway of embryonic closure. Line2F homozygous mutant embryos fail to close the neural tube, body wall, face, and optic fissure, and they also display defects in lung and heart development. Using a new technology of genomic sequence capture and high-throughput sequencing of a 2.5Mb region of the mouse genome, we discovered a mutation in the grainyhead-like 2 gene (Grhl2). Microarray analysis revealed Grhl2 affects the expression of a battery of genes involved in cell adhesion and E-cadherin protein is drastically reduced in tissues that require Grhl2 function. The tissue closure defects in Grhl2 mutants are similar to that of AP-2α null mutants and AP-2α has been shown to bind to the promoter of E-cadherin. Therefore, we tested for a possible interaction between these genes. However, we find that Grhl2 and AP-2α do not regulate each other's expression, E-cadherin expression is normal in AP-2α mutants during neural tube closure, and Grhl2;AP-2α trans-heterozygous embryos are morphologically normal. Taken together, our studies point to a complex regulation of neural tube fusion and highlight the importance of comparisons between these two models to understand more fully the molecular pathways of embryonic tissue closure.

Cephalothoracopagus, janiceps, disymmetros, monoomphalian conjoined twins undiagnosed until early second trimester

Cephalothoracopagus is the less common type of conjoined twins (CTs) with an incidence estimated at one in three million births or one in 58 conjoined twins. Maternal gene Vg1, a member of the TGF-beta family of cell-signalling molecules which are implicated in dorsoanterior development, and specific actions of Hox and Pax genes that are implicated in very early embryogenesis may be identified as aetiologic factors. A prenatal diagnosis of cephalothoracopagus CTs diagnosed at 17 weeks in a woman undergoing amniocentesis for advanced maternal age is reported. Although first-trimester diagnosis of CTs is feasible and has been reported as early as 8 weeks's gestation, CTs may be misdiagnosed with monoamniotic twins, lymphangioma, teratoma, and/or neoplasm and may be undiagnosed until early second trimester. Three-dimensional and color Doppler ultrasound enabled precise prenatal visualization of the fusion site. Ultrafast MR imaging should be considered an adjunct to ultrasound for antenatal characterization of structural anomalies and for planning surgical separation in selected cases. Echocardiography is mandatory in all cases of CTs as congenital heart defects are seen in 20-30% and polyhydramnios in 50-75%. Neural tube and midline fusion defects, diaphragmatic hernia, and imperforate anus are the frequently associated abnormalities. Prognosis among survivors is usually poor (44% die in the neonatal period) and is dependent upon the type of conjunction, degree of involvement of the shared organs, and presence or absence of associated anomalies, with the worst prognosis in case of twins sharing liver and heart.

Physical Blocking Neural Tube Closure Affects Radial Intercalation and Neural Crest Midline-directed Migration in<I>Xenopus</I>Dorsal Explants

Neural tube defects (NTDs) are severe congenital malformation diseases, which occur in 1 out of 1000 births in human. In Xenopus , several tissue movements are involved in the neural tube closure process. Immediately after the neural tube fusion, the neural crest cells get monopolar protrusion toward dorsal midline and migrate to form the roof of the neural tube. At the same time, radial intercalation takes place from the ventral neural tube and forces it to be single-layered. Here, we physically block the neural tube closure to test the cell movements and the following patterning in Xenopus laevis explants. The results show that the single-layered neural tube fails to form and the neural crest cells remain at the lateral regions in the explants with NTDs. However, the patterning of the neural tube is not affected as indicated by the normal expression of the preneural genes. These results indicate a requirement of the neural tube fusion for the radial intercalation and the dorsal midline directed neural crest migration, but not for the dorsal-ventral patterning of the neural tube.

CBP/p300 and associated transcriptional co-activators exhibit distinct expression patterns during murine craniofacial and neural tube development.

Mutations in each of the transcriptional co-activator genes - CBP, p300, Cited2, Cart1 and Carm1 - result in neural tube defects in mice. The present study thus furnishes a complete and comparative temporal and spatial expression map of CBP/p300 and associated transcriptional co-activators, Cited2, Cart1 and Carm1 during the period of murine neural tube development (embryonic days 8.5 to 10.5). Each co-activator except Cart1 was expressed in the dorsal neural folds on E8.5. Although CBP and p300 are functionally interchangeable in vitro, their respective expression patterns diverge during embryogenesis before neural fold fusion is complete. CBP gene expression was lost from the neural folds by E8.75 and was thereafter weakly expressed in the maxillary region and limb buds, while p300 exhibited strong expression in the first branchial arch, limb bud and telencephalic regions on E9.5. Cart1 exhibited strong expression in the forebrain mesenchyme from E9.0 through E10.5. Although CBP, p300, Carm1 and Cited2 share temporal expression on E8.5, these co-activators have different spatial expression in mesenchyme and/or the neuroepithelium. Nevertheless, co-localization to the dorsal neural folds on E8.5 suggests a functional role in elevation and/or fusion of the neural folds. Target genes, and pathways that promote cranial neural tube fusion that are activated by CBP/p300/Carm1/Cited2/Cart1-containing transcriptional complexes await elucidation.

EphrinA-EphA receptor interactions in mouse spinal neurulation: implications for neural fold fusion

The molecular basis of neural fold adhesion and fusion is a poorly understood aspect of neurulation. Cell surface glycosyl phosphatidylinositol (GPI)-anchored proteins have been implicated in neural fold adhesion, with ephrinAs particularly attractive candidates in view of the cranial neural tube defects observed in mice lacking ephrinA5 or the EphA7 receptor. Here, we demonstrate that ephrinsA1, A3 and A4, as well as several EphA receptors, are expressed in the closing mouse spinal neural tube. Most ephrinAs and EphA receptors were found to be expressed in multiple tissues in the caudal region, whereas EphA2 receptor was expressed specifically at the apices of the neural folds just prior to onset of neural tube fusion. Using mouse whole embryo culture, we found that cleavage of GPI-anchored molecules from the embryonic cell surface resulted in delay of spinal neural tube closure. Injection of EphA1 and EphA3 fusion proteins intra-amniotically into cultured embryos was used to specifically disrupt ephrinA-EphA receptor interactions, and led to inhibition of spinal neural tube closure, without adverse effects on growth or developmental progression. These treatments did not disturb neural plate bending or neural fold elevation, both of which are critical for spinal neural tube closure. Our findings demonstrate that ephrinA-EphA receptor interactions are required for closure of the mouse spinal neural tube, and support the hypothesis that ephrinA-EphA receptor interactions may participate in the molecular recognition events that culminate in adhesion and fusion of the tips of the neural folds during spinal neurulation.

Heparan sulfate Ndst1 gene function variably regulates multiple signaling pathways during mouse development

Disruption of heparan sulfate (HS) synthesis in vertebrate development causes malformations that are composites of those caused by mutations of multiple HS binding growth factors and morphogens. We previously reported severe developmental defects of the forebrain and the skull in mutant mice bearing a targeted disruption of the heparan sulfate-generating enzyme GlcNAc N-deacetylase/GlcN N-sulfotransferase 1 (Ndst1). Here, we further characterize the molecular mechanisms leading to frontonasal dysplasia in Ndst1 mutant embryos and describe additional malformations, including impaired spinal and cranial neural tube fusion and skeletal abnormalities. Of the numerous proteins that bind HS, we show that impaired fibroblast growth factor, Hedgehog, and Wnt function may contribute to some of these phenotypes. Our findings, therefore, suggest that defects in HS synthesis may contribute to multifactor types of congenital developmental defects in humans, including neural tube defects.

Roles of JUMONJI in mouse embryonic development

Cardiac development is a complex biological process requiring the integration of cell specification, differentiation, migration, proliferation, and morphogenesis. Although significant progress has been made recently in understanding the molecular basis of cardiac development, mechanisms of transcriptional control of cardiac development remain largely unknown. In search for the developmentally important genes, the jumonji gene (jmj) was identified by gene trap technology and characterized as a critical nuclear factor for mouse embryonic development. Jmj has been shown to play important roles in cardiovascular development, neural tube fusion process, hematopoiesis, and liver development in mouse embryos. The amino acid sequence of the JUMONJI protein (JMJ) reveals that JMJ belongs to the AT-rich interaction domain transcription factor family and more recently has been described as a member of the JMJ transcription factor family. Here, we review the roles of jmj in multiple organ development with a focus on cardiovascular development in mice.

Forced expression of platelet-derived growth factor B in the mouse cerebellar primordium changes cell migration during midline fusion and causes cerebellar ectopia

The platelet-derived growth factor (PDGF) and receptors are expressed in the developing central nervous system and in brain tumors. To investigate the role of PDGF during normal cerebellar development, we created transgenic mice where PDGF-B was introduced into the endogenous Engrailed1 locus ( En1). These mice expressed PDGF-B in all types of cells that constitute the developing cerebellum, with localized high expression in the ventral midline of the cerebellar anlage. This affected cell migration in the midline during fusion of the cerebellar anlage and caused misplacement of midline structures. PDGFR-α- and laminin α1-positive meningeal cells migrated inwards, attracted by the ectopic transgene expression in the ventral neuroepithelium. Other cells followed the meningeal cells and in the adult mouse, cells from all cortical cell layers were found misplaced in the midline. Moreover, the transgene caused an enhancement of capillary vessels. The findings indicate that normal PDGF signaling is important for proper neural tube fusion. It also illustrates that meningeal structures can influence the process.

Prenatal alcohol exposure retards the migration and development of serotonin neurons in fetal C57BL mice

Incomplete neural tube fusion (iNTF), induced by alcohol, in midline floor and roof plates was found in our recent study. In this study, serotonin (5-HT) neurons, known to be born entirely in the midline raphe at brainstem, were examined during their development with fetal alcohol exposure. Weight-matched C57BL mice pregnant dams were divided into three groups on E8: one received ethanol via a chocolate Sustacal liquid diet providing 20% ethanol-derived calories as the sole source of nutrients (ALC); the second received an isocaloric Sustacal liquid diet and was pair-fed to individual dams in the ethanol-fed group (PF); the third was fed ad lib rat chow (Chow). Fetal brains were obtained on E15 and were processed for immunostaining of 5-HT and its trophic factor, S100β. The ascending 5-HT neurons, in normal development, appear bilaterally near midline on E12, and by E15, as seen in chow and PF groups, migrate from the midline germinal zone laterally and dorsally to their final position with rich fibers. In contrast, in the E15 ALC group, many 5-HT-im neurons were found remaining in the midline germinal region or had migrated, but with under-differentiated, sparse fibers. There were 20–30% fewer 5-HT-im neurons in ALC as compared to PF and Chow. In addition, the number of S100β cells was less in ALC as compared with PF and Chow groups. No difference was found between PF and Chow in number of 5-HT-im or S100β-im cells. The 5-HT neurons found compromised in migration and differentiation may, in part, stem from failure of access to floor plate or midline tissue induction and the insufficient support by S100β. As 5-HT neurons have been implicated for signaling brain maturation, fewer 5-HT neurons may have lasting effects on the development of brain or, if persistent in the adult, profoundly affect adult brain function.

Tumor Necrosis Factor Receptor-Associated Factor 6 (TRAF6) Deficiency Results in Exencephaly and Is Required for Apoptosis within the Developing CNS

Tumor necrosis factor receptor-associated factors (TRAFs) are adaptor proteins important in mediating intracellular signaling. We report here that targeted deletion of traf6 greatly increases the frequency of failure of neural tube closure and exencephaly in traf6 (-/-) mice. The penetrance of this defect is influenced by genetic background. Neural tube fusion requires the coordination of several biological processes, including cell migration invoked by contact-dependent signaling, cell proliferation, and programmed cell death (PCD). To gain greater insight into the role of TRAF6 in these processes, neural development and migration within the CNS of traf6 (-/-) mice and controls were assessed through temporal examination of a number of immunohistochemical markers. In addition, relative levels of cellular proliferation and PCD were examined throughout embryonic development using bromodeoxyuridine (BrdU) and in situ terminal deoxynucleotidyl transferase-mediated dUTP biotinylated nick end labeling (TUNEL), respectively. The data suggest that loss of TRAF6 does not significantly alter the level of cellular proliferation or the pattern of neural differentiation per se, but rather regulates the level of PCD within specific regions of the developing CNS. Substantial reductions in TUNEL were observed within the ventral diencephalon and mesencephalon in exencephalic traf6 (-/-) embryos. Our results demonstrate a novel and prominent role for TRAF6 in the regional control of PCD within the developing CNS.

Nutrition in adolescence.

1. Giuseppina DiMeglio, MD* 1. 2. *The University of Rochester Medical Center Rochester, NY 1. Nutritional Requirements During Adolescence . Story M. In: McAnarney E, Kreipe REK, Orr DP, Comerci GD, eds. Textbook of Adolescent Medicine . 1992 WB Saunders Co Philadelphia, Penn 2. http://www.nal.usda.gov/fnic/Dietary/drv.html 3. http://www.nal.usda.gov/fnic/Dietary/Chartls.gif 4. Food Intakes of United States Children and Adolescents Compared With Recommendations. Munoz KA, Krebs-Smith SM, Ballard-Barbash R, Cleveland LE. Pediatrics . 1997;100:323-329 [OpenUrl][1][Abstract/FREE Full Text][2] 5. Nutritional Supplements. Fact vs Fiction. Johnson WA, Landry GL. State of the Art Reviews: Adolescent Medicine . 1998;9:501-514 Adolescence is a time of remarkable growth. During this time, 20% of final adult height and 50% of adult weight are attained. Bone mass increases by 45%, and dramatic bone remodeling occurs. Soft tissues, organs, and even red blood cell mass increase in size. As a result, nutritional requirements peak in adolescence. Deficits in macronutrients or micronutrients can impair growth and delay sexual maturation. Requirements for the individual are impossible to estimate because of considerable variation in the rate and amount of growth. Population-based estimates include the reference daily intake (RDI), which estimates average requirements, and the recommended dietary allowance (RDA), which estimates the intake that meets the needs of most of the population. Eating a varied diet, with foods chosen from all the food groups in the proportion recommended on the United States Department of Agriculture’s (USDA) Food Pyramid virtually insures meeting micronutrient requirements. Adolescents who meet these recommendations do not require nutritional supplements. Adolescents often have chaotic eating patterns that do not conform to dietary recommendations. Fewer than 2% of adolescents consume adequate amounts of all the food groups, and almost 20% of females and 7% of males do not consume an adequate number of portions of any of the food groups. The vast majority of adolescents eat too much fat and saturated fat and too few vegetables and fruits. Fried potato is the number one vegetable they eat. The USDA recommends that no more than 30% of a person’s calories come from fat and that cholesterol intake be limited to 300 mg/d. Growth parameters provide a good estimate of the … [1]: {openurl}?query=rft.jtitle%253DPediatrics%26rft.stitle%253DPediatrics%26rft.issn%253D0031-4005%26rft.aulast%253DMunoz%26rft.auinit1%253DK.%2BA.%26rft.volume%253D100%26rft.issue%253D3%26rft.spage%253D323%26rft.epage%253D329%26rft.atitle%253DFood%2BIntakes%2Bof%2BUS%2BChildren%2Band%2BAdolescents%2BCompared%2BWith%2BRecommendations%26rft_id%253Dinfo%253Adoi%252F10.1542%252Fpeds.100.3.323%26rft_id%253Dinfo%253Apmid%252F9282700%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [2]: /lookup/ijlink?linkType=ABST&journalCode=pediatrics&resid=100/3/323&atom=%2Fpedsinreview%2F21%2F1%2F32.atom

Normal and abnormal neurulation in humans: Implications for the pathogenesis of neural tube defects

The closure of the neural tube (NT) in humans has generally been described as a continuous process that begins in the region of the future neck and proceeds both rostrally and caudally, Recently, four separate initiation sites for neural tube fusion have been demonstrated in mice and other experimental animals. In humans, based on the study of neural tube defects (NTD) in clinical cases, van Allen et al. proposed a multisite NT closure model which argued that five closure sites exist in the NT of human embryos.1 By direct observation of human embryos with the fusing NT (Congenital Anomaly Research Center, Kyoto University, Kyoto, Japan), it was revealed that NT closure in human embryos does initiate at multiple sites but that the mode of NT closure in humans may be different from that in rodent species. In addition to the future cervical region which is widely accepted as an initiation site of NT closure, the mesencephalic‐rhombencephalic border was found to be another initiation site. Because there may be such species difference in the mode of NT closure, we should be careful when extrapolating data from experimental animals as it may not apply to humans. It is likely that the type of NTD affects the intra‐uterine survival rate of human embryos with NTD. Almost all of the embryos with total dysraphism appear to die by 5 weeks of gestation. while those with an opening over the rhombencephalon appear to die by 6.5 weeks, and those with a defect at the frontal and parietal regions survive beyond 7 weeks.

Programmed Cell Death in Palatal and Neural Tube Fusion During Normal Development † 266

Programmed Cell Death in Palatal and Neural Tube Fusion During Normal Development † 266

Finding Neural Tube 'Zippers' May Let Geneticists Tailor Prevention of Defects

THE MYSTERY of neural tube defects is unraveling at an increasingly rapid pace. On the heels of the finding that folic acid supplements could prevent 60% to 72% of these common birth defects in North American and Northern European populations (<i>JAMA</i>. 1993;269:1257-1261) comes new evidence published this month that there is not one but five separate closures, or "zippers," involved in normal neural tube fusion (<i>Am J Med Genet.</i>1993;47:5). Until now, a specific cause could be identified in only 10% of cases, and the rest "were all just lumped together in a giant, unknown soup you couldn't do anything about because you didn't know" what environmental or genetic factors might be at fault, says Margot Van Allen, MD, clinical associate professor of medical genetics at the University of British Columbia Faculty of Medicine, Vancouver, and first author on the neural tube closure article. Rigid scientific confirmation of Van Allen's

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)

The teratogenic effects of salicylic acid on the developing nervous system in rats in vitro.

Aspirin ingestion in humans and animals has been reported to lead to a range of undesirable outcomes, including fetal death, growth retardation, and congenital abnormalities. Rat embryos were cultured for 48 h in 100-300 micrograms/ml of salicylic acid, a metabolite of aspirin, days 9.5-11.5 of gestation. When compared to growth in control embryos, a significant dose-dependent decrease in crown-rump lengths, somite numbers, and yolk sac diameters was observed. There was also a significant increase in overall dysmorphology, including eye, brachial arch, and heart anomalies, and an absence of forelimb buds. The neural tube was especially vulnerable and had frequently failed to close. Cellular and ultrastructural examination revealed extensive cell death in the neuroepithelium, with a lesser effect on the mesenchymal cells. Large condensed blebs projected into the ventricular lumen, and cell membranes as well as the basal lamina were severely disrupted, with all cytoplasmic organelles affected in dying cells. It is likely that the extensive cell necrosis and blebbing in the developing neuroepithelium at the site of neural tube fusion may be involved in failed neurulation, while necrosis at other sites in the cranial neuroepithelium may be linked with previously reported intellectual and behavioural abnormalities.