Segmental costovertebral malformations: association with neural tube defects. Report of 3 cases and review of the literature

Patients with spondylocostal dysostosis (SCD) have vertebral abnormalities and numerical or structural rib anomalies that produce thoracic asymmetry. Rib anomalies and dysmorphism are the typical features that differentiate this syndrome from spondylothoracic dysostosis (STD). Jarcho-Levin syndrome is a severe form with involvement of the whole vertebral column. Other associated findings such as congenital heart defects, abdominal wall malformations, genitourinary malformations and upper limb anomalies may be found; in addition, neural tube defects (NTDs) have been associated with this malformation. SCD is transmitted both in a recessive form and as a dominant defect. We report on 3 children with SCD; 2 also had NTDs. All of them were studied with X-rays and spinal magnetic resonance (MR), and over the same period they underwent multidisciplinary clinical functional evaluation. One of our cases with NTD also presented polythelia, which has not previously been described in patients with SCD. The common association of segmental costovertebral malformations with NTDs could be related to an early gastrulation genomic defect, or one after gastrulation, when there are two independent somitic columns. The latter sometimes progresses and then involves primary and secondary neurulation. Also, the association of SCD with NTDs could be related to the interaction of different genes, resulting in this complex phenotype. Therefore, additional genetical and embryological studies are necessary to provide evidence of an etiological link between SCD and NTD.

Pax2/5 and Pax6 subdivide the early neural tube into three domains

The nested expression patterns of the paired-box containing transcription factors Pax2/5 and Pax6 demarcate the midbrain and forebrain primordium at the neural plate stage. We demonstrate that, in Pax2/5 deficient mice, the mesencephalon/metencephalon primordium is completely missing, resulting in a fusion of the forebrain to the hindbrain. Morphologically, in the alar plate the deletion is characterized by the substitution of the tectum (dorsal midbrain) and cerebellum (dorsal metencephalon) by the caudal diencephalon and in the basal plate by the replacement of the midbrain tegmentum by the ventral metencephalon (pons). Molecularly, the loss of the tectum is demonstrated by an expanded expression of Pax6, (the molecular determinant of posterior commissure), and a rostral shift of the territory of expression of Gbx2 and Otp (markers for the pons), towards the caudal diencephalon. Our results suggest that an intact territory of expression of Pax2/5 in the neural plate, nested between the rostral and caudal territories of expression of Pax6, is necessary for defining the midbrain vesicle.

Telomere shortening in mTR-/- embryos is associated with failure to close the neural tube

Mice genetically deficient for the telomerase RNA (mTR) can be propagated for only a limited number of generations. In particular, mTR-/- mice of a mixed C57BL6/129Sv genetic background are infertile at the sixth generation and show serious hematopoietic defects. Here, we show that a percentage of mTR-/- embryos do not develop normally and fail to close the neural tube, preferentially at the forebrain and midbrain. The penetrance of this defect increases with the generation number, with 30% of the mTR-/- embryos from the fifth generation showing the phenotype. Moreover, mTR-/- kindreds in a pure C57BL6 background are only viable up to the fourth generation and also show defects in the closing of the neural tube. Cells derived from mTR-/- embryos that fail to close the neural tube have significantly shorter telomeres and decreased viability than their mTR-/- littermates with a closed neural tube, suggesting that the neural tube defect is a consequence of the loss of telomere function. The fact that the main defect detected in mTR-/- embryos is in the closing of the neural tube, suggests that this developmental process is among the most sensitive to telomere loss and chromosomal instability.

The biological resolution of malformations of the central nervous system

It has been known for years that the assembly of the nervous system is under genetic control. During the last 10 years, the genes that direct the formation of the brain and spinal cord have begun to be discovered at an amazing pace. Mutations in the fruit fly and advances in molecular genetics have led the way. Gene mutations that cause many of the malformations of the human brain and spinal cord are now known. This has many physician-scientists hoping that an understanding of cause might lead to cure.

A simple molecular model of neurulation

A molecular model for the morphogenesis of the central nervous system is built and solved by computer. The formalism rests on molecular-biological data gathered from insects and vertebrates during neural differentiation and neuronal fate specification. Two genetic, hierarchically organized switches are introduced, one associated with f1p4al tissue formation, and the other with neuronal specification. The model switches evolve in time, setting up very similar "prepatterns" of genetic activity in both insects and vertebrates, as observed experimentally. We introduce the hypothesis that cell adhesion and motion are regulated by the switches. If cell motion is turned on by the neural switch, the whole neural tissue (neural plate) thickens, buckles, and folds, ultimately creating a closed neural tube (primary neurulation). When mitoses are more frequent in neural plate tissue, ingression of a neural cell mass takes place instead (secondary neurulation). If cell motions are controlled by the neuronal switch, rather than by the neural one, the differentiation of isolated neuroblasts is observed, which delaminate individually (as in insect neural cord formation). The model thus displays the three major known patterns of neurogenesis; the transition between the vertebrate and insect cases is predicted to result from changes in genetic regulation downstream of the switch genes, and affecting cell adhesion and motility properties. Little is known experimentally about the concerned pathways: their importance as a fruitful area for future investigation is emphasized by our theoretical results.

Embryonic folate metabolism and mouse neural tube defects

Folic acid prevents 70 percent of human neural tube defects (NTDs) but its mode of action is unclear. The deoxyuridine suppression test detects disturbance of folate metabolism in homozygous splotch (Pax3) mouse embryos that are developing NTDs in vitro. Excessive incorporation of [3H]thymidine in splotch embryos indicates a metabolic deficiency in the supply of folate for the biosynthesis of pyrimidine. Exogenous folic acid and thymidine both correct the biosynthetic defect and prevent some NTDs in splotch homozygotes, whereas methionine has an exacerbating effect. These data support a direct normalization of neurulation by folic acid in humans and suggest a metabolic basis for folate action.

[Complex biological systems as experimental and prenatal toxicology models]

Experimental biomedical sciences have emphasised the use of model systems in the study and understanding of physiological and pathological processes. Developmental biology, genetics and prenatal toxicology, dealing with cellular differentiation, organogenesis and dismorphogenic alterations, have shown that system models, even those far removed from mammals and humans in the zoological scale (e.g. C. elegans, Hydra, Drosophila), can be useful tools for understanding pathogen mechanisms in developmental toxicology. In particular the study of neural tube and its alterations shows how a biologically complex model (such as the laboratory rodent) necessitates, in turn, a series of models to thoroughly analyse and clarify the limits and levels of research.

Abnormalities of floor plate, notochord and somite differentiation in the loop-tail (Lp) mouse: a model of severe neural tube defects

Mouse embryos homozygous for the loop-tail (Lp) mutation fail to initiate neural tube closure at E8.5, leading to a severe malformation in which the neural tube remains open from midbrain to tail. During initiation of closure, the normal mouse neural plate bends sharply in the midline, at the site of the future floor plate. In contrast, Lp/Lp embryos exhibit a broad region of flat neural plate in the midline, displacing the sites of neuroepithelial bending to more lateral positions. Sonic hedgehog (Shh) and Netrin1 are expressed in abnormally broad domains in the ventral midline of the E9.5 Lp/Lp neural tube, suggesting over-abundant differentiation of the floor plate. The notochord is also abnormally broad in Lp/Lp embryos with enlarged domains of Shh and Brachyury expression. The paraxial mesoderm shows evidence of ventralisation, with increased expression of the sclerotomal marker Pax1, and diminished expression of the dermomyotomal marker Pax3. While the expression domain of Pax3 does not differ markedly from wild-type, there is a dorsal shift in the domain of Pax6 expression in the neural tube at caudal levels of Lp/Lp embryos. We suggest that the Lp mutation causes excessive differentiation of floor-plate and notochord, with over-production of Shh from these midline structures causing ventralisation of the paraxial mesoderm and, to a lesser extent, the neural tube. Comparison with other mouse mutants suggests that the enlarged floor plate may be responsible for the failure of neural tube closure in Lp/Lp embryos.

Neural tube and craniofacial defects with special emphasis on folate pathway genes

Neural tube and orofacial defects are common congenital malformations in humans. While etiologically heterogeneous, they are for the most part multifactorial in their pathogenesis, having both genetic and environmental components in their development. In recent years, there has been a great deal of epidemiologic evidence demonstrating that women who received multivitamins containing folic acid periconceptionally had significantly reduced occurrence and recurrence risks for producing infants with such malformations. This risk reduction is not observed in all populations, further suggestive of a genetic regulation of this phenomenon. Unfortunately, the mechanisms underlying the beneficial effects of folic acid are not well-understood. In this article, we review the relevant epidemiologic data on both neural tube defects and orofacial malformations, the fundamental embryological processes involved in closing the neural tube, and the development of the craniofacies, and propose a working hypothesis for susceptibility to these malformations. This hypothesis is based on the interworkings of cellular folate transport, focusing on the key elements involved in potocytosis. We propose that infants with mutations in the folate receptor alpha gene might be at increased risk for congenital anomalies due to a reduced binding affinity for 5-methyltetrahydrofolate, the physiologic form of folic acid. Various experimental approaches to test the working hypothesis are considered.

The role of vitamin A in the development of the central nervous system

We describe here the defects that arise in the central nervous system (CNS) of quail embryos when they develop in the absence of vitamin A. It has been assumed that because of the effects of excess vitamin A and its metabolites, particularly retinoic acid (RA), on the CNS they are involved in various aspects of CNS development. We show that this is indeed the case, because these deficient quail embryos have three defects in their CNS. First, the posterior hindbrain fails to develop because the cells fated to form this part of the CNS in the very early embryo die by apoptosis. Second, the neural tube fails to extend neurites into the periphery both in vivo and in vitro. Third, the neural crest cells throughout the embryo die by apoptosis. These results demonstrate a crucial requirement for vitamin A in CNS development.

Vinculin knockout results in heart and brain defects during embryonic development

The vinculin gene codes for a cytoskeletal protein, found in focal adhesion plaques and in cell-cell adherens junctions. Vinculin was inactivated by homologous recombination using a targeting vector in embryonic stem (ES) cells. The heterozygous ES cells were introduced into mice by established procedures to produce heterozygous animals that were normal and fertile. No homozygous vinculin−/− embryos were born and analyses during the gestational period showed that the vinculin null embryos were small and abnormal from day E8 but some survived until E10. The most prominent defect was lack of midline fusion of the rostral neural tube, producing a cranial bilobular appearance and attenuation of cranial and spinal nerve development. Heart development was curtailed at E9.5, with severely reduced and akinetic myocardial and endocardial structures. Mutant embryos were 30–40% smaller, somites and limbs were retarded and ectodermal tissues were sparse and fragile. Fibroblasts (MEF) isolated from mutant embryos were shown to have reduced adhesion to fibronectin, vitronectin, laminin and collagen compared to wild-type levels. In addition, migration rates over these substrata were two-fold higher and the level of focal adhesion kinase (FAK) activity was three-fold higher. We conclude that vinculin is necessary for normal embryonic development, probably because of its role in the regulation of cell adhesion and locomotion, cell behaviors essential for normal embryonic morphogenesis, although specific roles in neural and cardiac development cannot be ruled out.

[Congenital defects of the spinal part of the neural tube]

The abnormal closure of the neural tube results in defects of the nervous system development, which are referred to as dysraphism. Considering successive steps of the development of the human foetus, it can be estimated that spinal cord malformations arise from pathologies of early foetal development between 17th and 28th day gestational age. This time period comprises a development of the neural plate and subsequently neural tube. The development is completed with a closure of a posterior aperture of the neural tube (caudal neuropore). Congenital malformations are often caused by defective closure of the caudal neuropore. The neural plate develops about 17th day gestational age, the cerebral vesicle appears about 21st day and the neural tube forms between 17th and 20th day. The rostral neuropore closes on the day 25th as does the caudal neuropore on the day 28th. The embryo is 2.5 mm long at the time. Noxious factors acting during that period can affect normal closure of the caudal neuropore and distort the process of spinal cord canalization. The resultant defect is called spina bifida. This is the most severe form of dysraphism-rachischisis. Meninges are also affected in this defect. They cannot cover the neural canal and on the margins of the lesion are replaced by epithelium. In milder types of the defect lack of a complete bony framework is concealed by soft tissues, forming a sac of variable size over the lesion. Spina bifida can be subdivided according to the sac structure: meningocoele, meningocysticoele, meningomyelocoele, meningomyelocystocoele. Central canal pathologies constitute another form of spinal cord malformations presenting as hydromyelia. Abnormal closure of the neural tube may affect development of the vertebral column and spinal cord along their entire length or only at a certain portion. Malformations are seen most frequently in the lumbo-sacral and then cervical regions.

Neurulation abnormalities secondary to altered gene expression in neural tube defect susceptible Splotch embryos

The murine mutant Splotch (Sp) is a well-established model for studying neural tube closure defects. In the current investigation, the progression through neural tube closure (NTC) as well as the expression patterns of 12 developmentally regulated genes were examined in the neural tissue of wildtype (+/+), Splotch heterozygous (Sp/+), and Splotch homozygous (Sp/Sp) embryos during neurulation. The overall growth of the embryos, as measured by the number of somite pairs, did not differ significantly between the three genotypes at any of the collection time-points. There was, however, a significant delay in the progression through NTC for both the Sp/+ and Sp/Sp embryos. A univariate analysis on the expression of the 12 candidate genes (bcl-2, FBP-2, Hmx-2, Msx-3, N-cam, N-cad, noggin, p53, Pax-3, Shh, Wee-1, wnt-1) revealed that although 11 were statistically altered, across time or by genotype, there were no significant interactions between gestation age and genotype for any of these genes during NTC. However, a multivariate statistical analysis on the simultaneous expression of these genes revealed interactions at both gestation day (GD) 8:12 (day:hour) and 9:00 among Pax-3, N-cam, N-cad, bcl-2, p53, and Wee-1 that could potentially explain the aberrant NTC. The data from these studies suggest that a disruption in the genes that govern the cell cycle or extracellular matrices of the developing neural tube might play a critical role in the occurrence of the NTDs observed in Splotch embryos.

First-trimester prenatal sonographic findings associated with OEIS (omphalocele-exstrophy-imperforate anus-spinal defects) complex: a case and review of the literature

First-trimester sonographic findings associated with omphalocele-exstrophy-imperforate anus-spinal defects (OEIS) complex and review of the literature regarding this rare congenital anomaly are presented.

No turning, a mouse mutation causing left-right and axial patterning defects

Patterning along the left/right axes helps establish the orientation of visceral organ asymmetries, a process which is of fundamental importance to the viability of an organism. A linkage between left/right and axial patterning is indicated by the finding that a number of genes involved in left/right patterning also play a role in anteroposterior and dorsoventral patterning. We have recovered a spontaneous mouse mutation causing left/right patterning defects together with defects in anteroposterior and dorsoventral patterning. This mutation is recessive lethal and was named no turning (nt) because the mutant embryos fail to undergo embryonic turning. nt embryos exhibit cranial neural tube closure defects and malformed somites and are caudally truncated. Development of the heart arrests at the looped heart tube stage, with cardiovascular defects indicated by ballooning of the pericardial sac and the pooling of blood in various regions of the embryo. Interestingly, in nt embryos, the direction of heart looping was randomized. Nodal and lefty, two genes that are normally expressed only in the left lateral plate mesoderm, show expression in the right and left lateral plate mesoderm. Lefty, which is normally also expressed in the floorplate, is not found in the prospective floor plate of nt embryos. This suggests the possibility of notochordal defects. This was confirmed by histological analysis and the examination of sonic hedgehog, Brachyury, and HNF-3 beta gene expression. These studies showed that the notochord is present in the early nt embryo, but degenerates as development progresses. Overall, these findings support the hypothesis that the notochord plays an active role in left/right patterning. Our results suggest that nt may participate in this process by modulating the notochordal expression of HNF-3 beta.

The formal pathogenesis of isolated common carotid or innominate arteries: the concept of malseptation of the aortic sac

Deficient connections (= isolation) of the innominate artery or the common carotid artery to the aorta are rare congenital anomalies of the human aortic arch complex that are usually associated with a patent vascular connection between the isolated artery and a pulmonary artery. In the present study we demonstrate chick fetuses with a corresponding anomaly, the isolation of the brachiocephalic artery. In our chick fetuses the left brachiocephalic artery did not arise from the aortic arch, but was connected to the pulmonary trunk proximal (upstream) to the patent left and right ductus arteriosus. These cases are of interest because the presence of a congenital pulmonary-systemic arterial connection proximal (upstream) to the ductus arteriosus cannot be explained by the traditional concept of the morphogenesis of the aortic arch complex. The development of the normal and abnormal branching patterns of the aortic arch arteries is traditionally explained by transformation of the primitive embryonic pharyngeal arch arterial system due to obliteration of some of its vascular segments. Based on this concept, the isolation of an aortic arch artery can be explained by obliteration of vascular segments proximal and distal to this artery, whereas its connection to a pulmonary artery can be explained only by deficient obliteration (persistence) of the distal portion of the right or left sixth pharyngeal arch artery. The connecting "vascular segment" between an isolated aortic arch artery and the pulmonary circulation, therefore, is traditionally interpreted as a patent ductus arteriosus. The formal pathogenesis of congenital pulmonary-systemic arterial connections proximal (upstream) to the ductus arteriosus is discussed. The presented cases of isolation of the brachiocephalic artery are explained by disturbances in the partition of the embryonic aortic sac, possibly due to abnormal development of the "cardiac" neural crest.

Development of a lethal congenital heart defect in the splotch (Pax3) mutant mouse

OBJECTIVE

The splotch (Sp2h) mutation disrupts the Pax3 gene and is lethal in homozygotes. The aim of the present study was to investigate the cause of lethality.

METHODS AND RESULTS

Using the splotch (Sp2H) mouse mutant, we demonstrated that approximately 60% of Sp2H homozygotes die in utero at 13.5-14.5 days of gestation. All these embryos have cardiac malformations involving partial or complete failure of septation of the outflow tract. Although the cause of death in utero is unknown, the dying embryos are edematous, their superior caval veins are over-expanded, and the fetal liver is enlarged and engorged with blood, all signs of cardiac failure. The remaining Sp2H homozygotes die around the time of birth, and these embryos have grossly normal hearts. All Sp2H homozygotes have neural tube defects, either spina bifida, exencephaly, or both. Although these defects clearly do not cause death in utero, they are very likely responsible for the perinatal death of homozygotes that survive to late gestation. There is no correlation between the presence or absence of a cardiac defect and the type of neural tube defect. On the other hand, there is a striking correlation between presence of a cardiac defect and reduction or absence of dorsal root ganglia, which are derivatives of the neural crest.

CONCLUSIONS

In this paper, we show that the lethality has a biphasic pattern, and the data strongly suggests that mid-gestation lethality is due to cardiac defects and not the associated neural tube defects. This finding supports the idea that 'conotruncal' cardiac defects involving the ventricular outflow tracts develop as a result of failure of the 'cardiac' neural crest to colonise the developing heart in the mid-gestation embryo, and that the resulting heart defects are solely responsible for the observed mortality.

Functional opening of the fourth ventricular outlet in C57BL/6J and delayed splotch mouse embryos

To compare the functional development of the fourth ventricular outlet in the myeloschisis-Chiari malformation complex with that of a normal brain, the chronological development of the outlet in C57BL/6J non-neural-tube defect mouse embryos was examined as the first step. Then we compared the results with those of homozygotic delayed splotch (Spd) mouse embryos which had neural-tube defects (NTDs). Ferrous chloride (Prussian blue) solution was injected into the lateral or mesencephalic ventricle on gestation days 13-16 in the case of control C57BL/6J mouse embryos and on gestation days 14-16 in the case of homozygotic Spd mouse embryos which had open spinal NTDs and hindbrain anomalies comparable to human Chiari malformation. At 30 min after the injection, acid fixative was infused through the heart to set off the Prussian blue reaction, which makes the dye visible by the precipitation of ferric chloride. According to the present method, more than 75% of C57BL/6J mouse (non-NTD control) embryos showed the evidence of function of the fourth ventricular (4V) outlet from gestation day 15. It was difficult to apply the same method to Spd mouse embryos with NTDs due to the small size of ventricles. Only 4 injections were successful, of which 3 showed the functioning evidence of the 4V outlet. Though the number of mouse embryos with NTDs studied was small, the results suggest that the chronological progress of functional opening of the fourth ventricle in mouse embryos with NTDs is similar to that of control non-NTD embryos.

[The main etiopathogenic mechanisms of neurocutaneous diseases]

Neurocutaneous syndromes constitute a large and complex group of diseases in which recent medical advances, particularly in the field of molecular biology and genetics, have afforded a deeper understanding of the way in which these diseases originate. In this article, we review the advances concerning pathogenic mechanisms. First, we discuss the malformations disorders of the central nervous system associated with skin disorders, which range from spinal and/or cranial dysraphism with skin lesions to fustrated forms of malformations of the neural tube, such us membranous aplasia cutis. Neurocutaneous vascular disorders can be due to malformational disease, such as in Sturge-Weber syndrome, as well as to autoimmune diseases. The analysis of mutations affecting the capacity for migration and differentiation of melanocyte precursors enables us to gain a better understanding of disorders of the cells of the neural crest, such as piebaldism and Waardenburg's syndrome. Mutations in tumor suppressor genes play an important part in the development of hamartomatous and neoplastic lesions in neurofibromatosis and tuberous sclerosis. Genetic mosaicism, both of the functional and the genomic kind, accounts for the great diversity of phenotypes and the distribution of neurocutaneous diseases. Lastly, neurocutaneous syndromes such as the paracrinopathies form an attractive hypothesis, which is as yet to be confirmed.

Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene

Transgenic mice were generated containing a cytomegaloviral promoter driven construct (CMV43) expressing the gap junction polylpeptide connexin 43. RNA and protein analysis confirmed that the transgene was being expressed. In situ hybridization analysis of embryo sections revealed that transgene expression was targeted to the dorsal neural tube and in subpopulations of neural crest cells. This expression pattern was identical to that seen in transgenic mice harboring other constructs driven by the cytomegaloviral promoter (Kothary, R., Barton, S. C., Franz, T., Norris, M. L., Hettle, S. and Surani, M. A. H. (1991) Mech. Develop. 35, 25–31; Koedood, M., Fitchel, A., Meier, P. and Mitchell, P. (1995) J. Virol. 69, 2194–2207), and corresponded to a subset of the endogenous Cx43 expression domains. Significantly, dye injection studies showed that transgene expression resulted in an increase in gap junctional communication. Though viable and fertile, these transgenic mice exhibited reduced postnatal viability. Examination of embryos at various stages of development revealed developmental perturbations consisting of cranial neural tube defects (NTD) and heart malformations. Interestingly, breeding of the CMV43 transgene into the Cx43 knockout mice extended postnatal viability of mice homozygote for the Cx43 knockout allele, indicating that the CMV43 trangsene may partially complement the Cx43 deletion. Both the Cx43 knockout and the CMV43 transgenic mice exhibit heart defects associated with malformations in the conotruncus, a region of the heart in which neural crest derivatives are known to have important roles during development. Together with our results indicating neural-crest-specific expression of the transgene in our CMV-based constructs, these observations strongly suggest a role for Cx43-mediated gap junctional communication in neural crest development. Furthermore, these observations indicate that the precise level of Cx43 function may be of critical importance in downstream events involving these migratory cell populations. As such, the CMV43 mouse may represent a powerful new model system for examining the role of extracardiac cell populations in cardiac morphogenesis and other developmental processes.

Risk of recurrence of craniospinal anomalies

The authors analyzed 1,655 situations from their Genetic Counseling Service over a 15 year period where the reason for counseling was craniospinal anomaly (neural tube defects and/or hydrocephalus) in the family. Excluding the obviously monogenically inherited cases, they investigated pregnancies undertaken after 1,285 isolated and 177 multiple forms of craniospinal abnormalities. The recurrence rate of craniospinal defects was found to be 3.66%, which is about ten times higher than the general population risk, supporting the theory of the multifactorial threshold model in the inheritance of these anomalies. The recurrence risks of neural tube defects and of hydrocephalus were 3.47% and 2.95%, respectively. The authors concluded that recurrence risk is mainly influenced by the pathoanatomic severity of the involved anomaly, the degree of relationship, and the number of affected relatives in the family. There is a positive correlation between the pathoanatomic severity of the anomaly in the proband and the offspring. At least in one-half of the cases the same type of anomaly was observed again in the offspring as in the proband. Attention is drawn to the fact that hydrocephalus (ventriculomegaly) is often manifested only in the second half of gestation. Therefore, performing ultrasound examination is strongly recommended not only at the 18th but at the 24th week of gestation, as well in pregnancies with a positive history of neural tube defects and/or hydrocephalus.

Exencephaly-anencephaly sequence: proof by ultrasound imaging and amniotic fluid cytology

We present and discuss major current theories about the developmental natural history of the anencephalic human fetus. We confirm previous observations made using transvaginal ultrasonography of exencephalic fetuses which were later imaged and/or delivered as anencephalic fetuses. We explore the possibility of proving the theory of the slowly rubbed-off exposed brain tissue by cytologic examination and special staining of aspirated cells in amniotic fluid. Three fetuses with a typical sonographic picture of exencephaly at 13-15 postmenstrual weeks underwent amniocentesis. The aspirated fluid contained pathognomonic neural cells. The same fetuses later showed the characteristic sonographic and postabortion picture of anencephaly. Our results support the theory that exencephaly is the forerunner of anencephaly.

The effect of embryonic cerebrospinal fluid pressure and morphogenetic brain expansion on wound healing in the midbrain of the chick embryo

The role of increased cerebrospinal fluid pressure and morphogenetic brain expansion on midbrain wound healing was studied in chick embryos at stages 16-22. The embryos were divided into six groups as follows: group I (stages 16/17), group II (stages 18/19), group III (stages 20-22), group IV (stages 18/19), group V (stages 20-22) and group VI (stages 18/19). The mid-brains of embryos of groups I-III were wounded and the embryos re-incubated for varying periods up to 24 h. The neuroepithelial wounds of all group-I embryos healed completely within 24 h. However, complete healing was observed in only 25% of wounds in group II and 11.4% in group III by 24 h. To reduce cerebrospinal fluid pressure and thus slow down brain expansion, longitudinal wounds (about 0.8 mm long) were made in the hindbrain roof plate of group-IV and group-V embryos, and puncture wounds (0.1 mm in diameter) also in the hindbrain roof plate of group-VI embryos. This allowed cerebrospinal fluid to escape prior to wounding the midbrain. There was a significant increase in the proportion of group-IV and group-V embryos with completely healed midbrain neuroepithelial wounds (77.3% and 28.6% respectively). However, a comparison between groups II and VI embryos yielded no statistically significant difference in healing. Thus, increasing cerebrospinal fluid pressure and brain expansion adversely affect midbrain neuroepithelial wound healing.

Neural tube, skeletal and body wall defects in mice lacking transcription factor AP-2

The retinoic acid-inducible transcription factor AP-2 is expressed in epithelial and neural crest cell lineages during murine development. AP-2 can regulate neural and epithelial gene transcription, and is associated with overexpression of c-erbB-2 in human breast-cancer cell lines. To ascertain the importance of AP-2 for normal development, we have derived mice containing a homozygous disruption of the AP-2 gene. These AP-2-null mice have multiple congenital defects and die at birth. In particular, the AP-2 knockout mice exhibit anencephaly, craniofacial defects and thoraco-abdominoschisis. Skeletal defects occur in the head and trunk region, where many bones are deformed or absent. Analysis of these mice earlier in embryogenesis indicates a failure of cranial neural-tube closure and defects in cranial ganglia development. We have shown that AP-2 is a fundamental regulator of mammalian craniofacial development.

Neurovascular developmental interaction: a specific form of vascular maldevelopment in the malformed brain. I. An experimental study and proposal of a new teratological concept

The process of the development of the intracranial vessels was studied by means of immunohistochemical analysis of factor VIII in normal and exencephalic chick fetuses. The results revealed that the development of blood vessels in exencephalic brain was far advanced beyond the norm, with intense immunoreactivity to factor VIII on postincubation day 16 exceeding that on day 21 in normal controls. Compared with results regarding the direction of the overgrowth in the neuronal maturation process in the previous study using the chick exencephaly model, the findings of overmatured blood vessels were compatible with NSE- and somatostatin-positive elements that appeared especially in the overgrowth foci. The results of the present study suggested the pathogenic development of the "area cerebrovasculosa" in the neural placode as a phenomenon consequent upon hypervascularization in response to neuronal overgrowth, as seen in human cases of exencephaly or anencephaly. We emphasize the significance of this specific phenomenon in the development of the fetal central nervous system, namely neurovascular developmental interaction.

Vitamin A-deficient quail embryos have half a hindbrain and other neural defects

BACKGROUND

Retinoic acid (RA) is a morphogenetically active signalling molecule thought to be involved in the development of severely embryonic systems (based on its effect when applied in excess and the fact that it can be detected endogenously in embryos). Here, we adopt a novel approach and use the vitamin A-deficient (A-) quail embryo to ask what defects these embryos show when they develop in the absence of RA, with particular reference to the nervous system.

RESULTS

We have examined the anatomy, the expression domains of a variety of genes and the immunoreactivity to several antibodies in these A- embryos. In addition to the previously documented cardiovascular abnormalities, we find that the somites are smaller in A- embryos, otic vesicle development is abnormal and the somites continue up to and underneath the otic vesicle. In the central nervous system, we find that neural crest cells need RA for normal development and survival, and the neural tube fails to extend any neurites into the periphery. Using general hindbrain morphology and the expression patterns of Hoxa-2, Hoxb-1, Hoxb-4, Krox-20 and FGF-3 as markers, we conclude that segmentation in the myelencephalon (rhombomeres 4-8) is disrupted. In contrast, the dorsoventral axis of the neural tube using Shh, islet-1 and Pax-3 as markers is normal.

CONCLUSIONS

These results demonstrate at least three roles for RA in central nervous system development: neural crest survival, neurite outgrowth and hindbrain patterning.

Neural tube defects and abnormal brain development in F52-deficient mice

F52 is a myristoylated, alanine-rich substrate for protein kinase C. We have generated F52-deficient mice by the gene targeting technique. These mutant mice manifest severe neural tube defects that are not associated with other complex malformations, a phenotype reminiscent of common human neural tube defects. The neural tube defects observed include both exencephaly and spina bifida, and the phenotype exhibits partial penetrance with about 60% of homozygous embryos developing neural tube defects. Exencephaly is the prominent type of defect and leads to high prenatal lethality. Neural tube defects are observed in a smaller percentage of heterozygous embryos (about 10%). Abnormal brain development and tail formation occur in homozygous mutants and are likely to be secondary to the neural tube defects. Disruption of F52 in mice therefore identifies a gene whose mutation results in isolated neural tube defects and may provide an animal model for common human neural tube defects.

Family care related to alpha-fetoprotein screening

A family's coping repertoire may be challenged when the result of a standard prenatal maternal serum alpha-fetoprotein screening is not within a normal range. Abnormal screening results can cause families to experience anxiety and confusion at a time when they may need to make difficult decisions. The nurse's role is to provide information and emotional support and to coordinate health care services for optimal family coping.

Severe defects in the formation of epaxial musculature in open brain (opb) mutant mouse embryos

The differentiation of somite derivatives is dependent on signals from neighboring axial structures. While ventral signals have been described extensively, little is known about dorsal influences, especially those from the dorsal half of the neural tube. Here, we describe severe phenotypic alterations in dorsal somite derivatives of homozygous open brain (opb) mutant mouse embryos which suggest crucial interactions between dorsal neural tube and dorsal somite regions. At Theiler stage 17 (day 10.5 post coitum) of development, strongly altered expression patterns of Pax3 and Myf5 were observed in dorsal somite regions indicating that the dorsal myotome and dermomyotome were not differentiating properly. These abnormalities were later followed by the absence of epaxial (dorsal) musculature; whereas, body wall and limb musculature formed normally. Analysis of Mox1 and Pax1 expression in opb embryos revealed additional defects in the differentiation of the dorsal sclerotome. The observed abnormalities coincided with defects in differentiation of dorsal neural tube regions. The implications of our findings for interactions between dorsal neural tube, surface ectoderm and dorsomedial somite regions in specifying epaxial musculature are discussed.

Multisite neural tube closure in humans

We present evidence for multisite NT closure in humans with representative examples of types of NTDs that would be expected if NT closure in humans is similar to experimental mice models. We determine that the majority of NTDs can be classified by the multisite closure model. Further evidence for multisite closure of the NT is apparent in previous epidemiological studies, recognized monogenic disorders, and environmental and teratogenic exposures. Previous reliance on the single-site closure model has resulted in grouping of anomalies, obscuring evidence for multisite NT closure, etiological heterogeneity, varying recurrence risks, and site-specific effects of environmental factors. The NTDs have been previously referred to as being multifactorial, due to multiple genes and environmental factors. Etiological heterogeneity has been demonstrated previously as well. Classification of NTDs by closure site will be beneficial in better defining etiologies and environmental susceptibilities. Similarly, it is apparent to us that genetic variations in closure sequence, rate, and location are most likely monogenic and result in affected embryos being more susceptible to specific environmental factors, such as the effect of folic acid deficiency. Individual closure sites are most likely under the control of specific embryonically expressed genes, whose monogenic nature may not be apparent postnatally. For the disorders such as Meckel-Gruber syndrome and Walker-Warburg syndrome, the monogenic etiology for NTDs in affected individuals is apparent because of associated malformations. There are three important implications of this study: The first is that monogenic mouse models will be helpful in investigating the pathogenesis of NTDs in humans. The homologies between the mouse and human genome may allow linkage studies to be done in some families who have recurrence of NTDs. Second, in order to have useful results from studies of NTDs, NT anomalies need to be accurately described, either by the classical nomenclature (eg, meroacranium) or by referring to the corresponding closure site involvement (eg, closure 2 defect). Special attention needs to be addressed to those NTDs that do not appear to fit into a discrete closure site (eg, midthoracic spina bifida cystica) or laterally displaced NTDs, since they may be due to other etiologies. With improved nutrition, particularly folic acid treatment, specific etiologies for the remaining NTDs may become more apparent. Finally, recurrence risks for NTDs may vary between families based on the closure site affected, and whether or not associated anomalies are present.

Prenatal craniofacial development: new insights on normal and abnormal mechanisms

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

Sex, neural tube defects, and multisite closure of the human neural tube

While neural tube defects (NTD) overall have a female sex bias, this does not apply to all sites along the neuraxis. The findings regarding sex and NTD in a series of midtrimester fetuses are reviewed, and then analysed in terms of the recent hypothesis that during embryogenesis of the human neural tube there are multiple closure sites, rather than a single zipping up process. Females more often than males tend to have craniorachischisis, spina bifida involving the thorax, the holoacrania form of anencephaly, anencephaly and cervical spina bifida and encephalocoeles, while males more often than females have spina bifida affecting the lower spine. Meroacrania occurs equally in both sexes. Other sources indicate that there is a male bias in frontoethmoidal encephalocoeles. Since sex seems to be a factor that is differentially associated with lack of closure of specific areas of the neural tube, it would seem to support the notion that there are multiple closure sites in the human neural tube. However, no association was found between a particular sex and either the type of NTD which have an isolated abnormality or those NTD associated with developmental abnormalities of other body systems.

Effects of folate deficiency on embryonic development

While there is strong evidence that folate deficiency including the use of anti-folate drugs in early pregnancy is teratogenic and may lead to a range of serious abnormalities of the developing fetus including intrauterine death, the mechanism(s) for these effects have not yet been delineated. In neural tube defects, there is increasing evidence that marginal folate status exacerbates the effect of an underlying genetic defect in the mother, the fetus, or both. An abnormal relationship between the ingestion of folate and the folate levels in red blood cells has been found in women who have given birth to infants with neural tube defects. Periconceptional folate supplementation has been shown to give effective protection against the development of neural tube defects. The mechanism of the prevention is as yet unknown. However, folic acid will not prevent all cases of neural tube defect. Moreover, neither determinations of periconceptional vitamin profiles (Mooij et al, 1993) nor determinations of methylmalonic acid and homocysteine levels will detect all women at risk. Anencephaly and spina bifida can be identified prenatally by detection of excessive levels of alpha-fetoprotein in amniotic fluid and maternal serum and by ultrasonographic scanning (Wilson and Rudd, 1993). Both genetic counselling and prenatal diagnosis should be offered to women who are recognized to be at high risk. Research into the underlying biology of neural tube defects is of major importance. The identification of underlying genetic defects would allow for genetic testing and better counselling of families at risk for the occurrence of a neural tube defect.

Targeted disruption of the GATA3 gene causes severe abnormalities in the nervous system and in fetal liver haematopoiesis

GATA-3 is one member of a growing family of related transcription factors which share a strongly conserved expression pattern in all vertebrate organisms. In order to elucidate GATA-3 function using a direct genetic approach, we have disrupted the murine gene by homologous recombination in embryonic stem cells. Mice heterozygous for the GATA3 mutation are fertile and appear in all respects to be normal, whereas homozygous mutant embryos die between days 11 and 12 postcoitum (p.c.) and display massive internal bleeding, marked growth retardation, severe deformities of the brain and spinal cord, and gross aberrations in fetal liver haematopoiesis.

Periconceptional folic acid supplementation as a social intervention

Periconceptional folic acid supplementation has been shown to decrease the first occurrence of isolated neural tube defects (NTDs) by as much as 50%, and to decrease the recurrence risk for NTDs by more than 70%. The possible mechanisms of vitamin supplementation in the prevention of NTDs are discussed, as are the current recommendations for reproductive-age women. Further, the limitations of dietary and pharmacological recommendations with regard to patient compliance as well as the possibility of grain fortification are reviewed.

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.

Cerebellar dermoid tumor and occipital meningocele in a monozygotic twin: clues to the embryogenesis of craniospinal dysraphism

A case of monochorionic/monoamnionic twin with discordant occipital developmental malformations is presented. One female twin appeared to have an occipital meningocele with cerebellar aplasia and died immediately after birth. The other twin presented with signs and symptoms of raised intracranial pressure at the age of 7 months. Severe hydrocephalus was present due to an infected intracerebellar dermoid tumor with a contiguous occipital dermal sinus. The clinical and pathological characteristics are described and the different theories concerning twinning, embryogenesis, and dysmorphology in relation to neural tube defects are discussed. Analysis of the features of these monozygotic twins indicates that a meningocele is not a post-neurulation disorder but results from deficient neurulation, probably due to mesodermal insufficiency.

Role of formate in methanol-induced exencephaly in CD-1 mice

Mouse embryos develop exencephaly when dams are exposed by inhalation to high concentrations (> or = 10,000 ppm) of methanol on gestational day 8 (GD8; copulation plug = GD0). The present study examined the role of formate, an oxidative metabolite of methanol, in the development of methanol-induced exencephaly in CD-1 mice and cultured mouse embryos. The pharmacokinetics and developmental toxicity of sodium formate (750 mg/kg by gavage), a 6-hr methanol inhalation (10,000 or 15,000 ppm), or methanol gavage (1.5 g/kg) in pregnant CD-1 mice on GD8 were determined. Gross morphological evaluations for neural tube closure status in embryos or exencephaly in near-term fetuses were performed. Decidual swellings and maternal plasma were analyzed for methanol and formate. The mean (+/- S.E.M.) end-of-exposure plasma methanol concentration was 223 +/- 23 mM following the 6-hr, 15,000 ppm methanol inhalation. There were no changes in blood or decidual swelling formate concentrations under any of the methanol exposure conditions. Peak formate levels in plasma (1.05 +/- 0.2 mM; control 0.5 +/- 0.3 mM) and decidual swelling (2.0 +/- 0.2 mM; control 1.1 +/- 0.2 mM) from pregnant mice (GD8) given sodium formate (750 mg/kg, po) were similar to those observed following a 6-hr methanol inhalation of 15,000 ppm (plasma = 0.75 +/- 0.1 mM; decidual swelling = 2.2 +/- 0.3 mM) but did not result in exencephaly.(ABSTRACT TRUNCATED AT 250 WORDS)

Diprosopia revisited in light of the recognized role of neural crest cells in facial development

OBJECTIVE

The aim of this study is to compare the theory of embryogenesis of the face with human diprosopia. This peculiar form of conjoined twinning is of great interest because 1) only the facial structures are duplicated and 2) almost all cases have a rather monomorphic pattern. The hypothesis is that an initial duplication of the notochord leads to two neural plates and subsequently duplicated neural crests. In those conditions, derivatives of the neural crests will be partially or totally duplicated; therefore, in diprosopia, the duplicated facial structures would be considered to be neural crest derivatives. If these structures are identical to those that are experimentally demonstrated to be neural crest derivatives in animals, these findings are an argument to apply this theory of facial embryogenesis in man.

METHODS

Serial horizontal sections of the face of two diprosopic fetuses (11 and 21 weeks gestation) were studied macro- and microscopically to determine the external and internal structures that are duplicated. Complete postmortem examination was performed in search for additional malformations.

RESULTS

The face of both fetuses showed a very similar morphologic pattern with duplication of ocular, nasal, and buccal structures. The nasal fossae and the anterior part of the tongue were also duplicated, albeit the posterior part and the pharyngolaryngeal structures were unique. Additional facial clefts were present in both fetuses. Extrafacial anomalies were represented by a craniorachischisis, two fused vertebral columns and, in the older fetus, by a complex cardiac malformation morphologically identical to malformations induced by removal or grafting of additional cardiac neural crest cells in animals.

CONCLUSION

These pathological findings could identify the facial structures that are neural crest derivatives in man. They are similar to those experimentally demonstrated to be neural crest derivatives in animals. In this respect, diprosopia could be considered as the end of a spectrum, whereas the other end is agnathia-holoprosencephaly complex. This assumption has to be discussed, but we want to draw attention to the fact that diprosopia must not be considered as a curious form of conjoined twinning, but as a major means of bringing us a better knowledge of the facial embryogenesis in man.

Multiple sites of anterior neural tube closure in humans: evidence from anterior neural tube defects (anencephaly)

OBJECTIVE

Anterior neural tube closure in humans is thought to occur via a continuous process, culminating in the closure of the anterior neuropore. Recent studies have demonstrated that, in some species, the process is discontinuous, with four separate sites of closure initiation. In this study, we tested the hypothesis that humans, like mice and other experimental animals, have multiple sites of anterior neural tube closure.

METHODS

Twenty human fetuses and neonates with open anterior neural tube defects were identified. The rostral and caudal boundaries of each defect was localized on a model cranium upon which was superimposed the four sites of anterior closure characterized in the mouse.

RESULTS

Of the 20 cases, 7 (35%) defects involved the frontal region, 7 (35%) were limited to the parietal region, 4 (20%) to the occipital region, and 2 (10%) involved both the parietal and occipital regions. These defects clustered into discrete regions, corresponding to sites of closure in the mouse model. The location of the defects fell into two categories; those occurring at the junction of two closures, and those occurring within a single closure.

CONCLUSION

The results of this study support the hypothesis that humans, like other species, have multiple sites of anterior neural tube closure. Furthermore, the data provide evidence for two mechanisms leading to anterior neural tube defects: one resulting from the failure of a closure to occur, and the second from the failure of two closures to meet. The findings provide insight into the variations observed in the location, recurrence risk, and etiologies of anterior neural tube defects in the human population.

Ultrasound backscatter microscope analysis of early mouse embryonic brain development

The history of developmental and genetic analysis in the mouse has made it the model of choice for studying mammalian embryogenesis. Presently lacking is a simple technique for efficiently analyzing early mouse mutant phenotypes in utero. We demonstrate application of a real-time imaging method called ultrasound backscatter microscopy for visualizing mouse early embryonic neural tubes and hearts. This method was used to study live embryos in utero between 9.5 and 11.5 days of embryogenesis, with a spatial resolution close to 50 microns. Ultrasound backscatter microscope images of cultured embryos made it possible to visualize the heart chambers. This noninvasive imaging method was also used for analyzing a neural tube defect. The midhindbrain deletion associated with a null mutation of the Wnt-1 protooncogene was easily recognizable on ultrasound backscatter microscope images of 10.5- and 11.5-day embryos. Computer-generated volumetric renderings of the neural tube cavities were made from three-dimensional image data. This allowed a much clearer definition of the Wnt-1 mutant phenotype. These imaging techniques should be of considerable use in studying mouse development in utero.

Further evidence for an intermittent pattern of neural tube closure in humans

Evidence is presented to support the recent suggestion that human neural tube closure is similar to that observed in mice, and comprises several regionally distinct closure sites rather than being a simple zipping up process. Seven subjects, each with more than one neural tube defect (NTD), are described. Comparative studies of the location of the lesions in relation to the closed parts of the neural tube imply that such NTD could only have come about if there were intermittent closure of the neural tube.

Genesis and prevention of spinal neural tube defects in the curly tail mutant mouse: involvement of retinoic acid and its nuclear receptors RAR-beta and RAR-gamma

A role for all-trans-retinoic acid in spinal neurulation is suggested by: (1) the reciprocal domains of expression of the retinoic acid receptors RAR-beta and RAR-gamma in the region of the closed neural tube and open posterior neuropore, respectively, and (2) the preventive effect of maternally administered retinoic acid (5 mg/kg) on spinal neural tube defects in curly tail (ct/ct) mice. Using in situ hybridisation and computerised image analysis we show here that in ct/ct embryos, RAR-beta transcripts are deficient in the hindgut endoderm, a tissue whose proliferation rate is abnormal in the ct mutant, and RAR-gamma transcripts are deficient in the tail bud and posterior neuropore region. The degree of deficiency of RAR-gamma transcripts is correlated with the severity of delay of posterior neuropore closure. As early as 2 hours following RA treatment at 10 days 8 hours post coitum, i.e. well before any morphogenetic effects are detectable, RAR-beta expression is specifically upregulated in the hindgut endoderm, and the abnormal expression pattern of RAR-gamma is also altered. These results suggest that the spinal neural tube defects which characterise the curly tail phenotype may be due to interaction between the ct gene product and one or more aspects of the retinoic acid signalling pathway.

Patterns of lectin binding during mammalian neurogenesis

Temporospatial changes in surface carbohydrates of neuroepithelial cells were analysed by means of lectin histochemistry in normal mouse embryos subsequent to closure of the neural tube. The lectins used were concanavalin A (con A), soybean (SBA), Maclura pomifera (MPA), peanut (PNA), wheatgerm (WGA), succinylated wheatgerm (sWGA) and Limax flavus (LFA). Although labelling was obtained with all of the lectins, the most striking temporospatial differences occurred with con A which in the early embryos (9-10 somites) labelled the basal and intercellular surfaces, but not the luminal surfaces of the neuroepithelial cells, whereas in the older embryos (26-30 somites), con A showed light luminal surface labelling. A midventral wedge of cells in the floor of the neural tube in the older embryos also exhibited more intense labelling with con A, WGA, and sWGA than with the other lectins. In addition, comparisons of lectin localisation were made between the closed neural tube in normal embryos and the open neural folds in the loop-tail (Lp) mutant mouse in which the neural tube fails to close. Although similar temporospatial patterns in lectin localisation occurred as in normal embryos, the retention of lectin labelling associated with rounded putative neural crest cells that remained sequestered in the apices of the open neural folds, along with an attenuation of the luminal reaction in the older abnormal embryos, suggest that during normal mammalian development closure of the spinal neural folds may be important for the timely exit of neural crest cells as well as for eliciting changes in the luminal surfaces of the neuroepithelial cells.

Diabetic embryopathy: possible pathogenesis

Despite recent emphasis upon improved metabolic control during early diabetic pregnancy, the offspring of insulin-dependent diabetic women continue to have a 2- to 4-fold increased risk of congenital malformations. We recently evaluated the affected offspring of 4 insulin-dependent diabetic women. All had abnormal ears in association with vertebral defects. Our analysis of the structural defects of these infants and a review of the literature suggest that the pathogenesis of some cases of the diabetic embryopathy may involve a primary insult to developing somite mesoderm and associated cephalic neural crest cells.

Maternal hyperhomocysteinemia: a risk factor for neural-tube defects?

The maternal vitamin status, especially of folate, is involved in the pathogenesis of neural-tube defects (NTDs). Maternal folate administration can prevent these malformations. The precise metabolic mechanism of the beneficial effect of folate is unclear. In this study we focus on homocysteine accumulation, which may derive from abnormalities of metabolism of folate, vitamin B12, and vitamin B6. We studied nonpregnant women, 41 of whom had given birth to infants with NTDs and 50 control women who previously had normal offspring. The determinations included the plasma total homocysteine both in the fasting state and 6 hours after the ingestion of a methionine load. In addition, we measured the fasting blood levels of folate, vitamin B12, and vitamin B6. The mean values for both basal homocysteine and homocysteine following a methionine load were significantly increased in the group of women who previously had infants with NTDs. In nine of these subjects and two controls, the values after methionine ingestion exceeded the mean control by more than 2 standard deviations. Cystathionine synthase levels in skin fibroblasts derived from these methionine-intolerant women were within the normal range. Our findings suggest a disorder in the remethylation of homocysteine to methionine due to an acquired (ie, nutritional) or inherited derangement of folate or vitamin B12 metabolism. Increased homocysteine levels can be normalized by administration of vitamin B6 or folate. Therefore, we suggest that the prevention of NTDs by periconceptional folate administration may effectively correct a mild to moderate hyperhomocysteinemia.