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.

Variable patterns of malformation in the mouse fetal hydantoin syndrome.

Controversy over the existence of a fetal hydantoin syndrome continues in medical literature despite numerous recent clinical studies describing additional patients with a characteristic pattern of abnormalities. Resistance to its acceptance as a clinical entity seems to stem from the variability of the component malformations seen in this syndrome. To examine this variability in a controlled experimental situation, we utilized data obtained in previously reported studies of a mouse model of the fetal hydantoin syndrome. In the mouse, prenatally exposed fetuses had congenital anomalies similar to those observed in the human syndrome. In terms of overall frequency of malformation there were no differences among three inbred mouse strains. However, when considering the individual rates of the 11 most common malformations, considerable differences were noted among the strains. These strain differences in the pattern of malformations appear to be related to genotypic differences in susceptibility to specific malformations. These results provide one possible explanation for the variability observed in the human fetal hydantoin syndrome.

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)

Auditory function in Duane's retraction syndrome.

We obtained audiograms and auditory brainstem responses from 44 patients with Duane's retraction syndrome to assess the incidence and nature of hearing deficit. Of 44 patients, seven (15.9%) had evidence of hearing impairment. Three (6.8%) subjects had a temporary conductive hearing loss because of middle ear fluid, and another patient had hearing loss from Crouzon's disease. The remaining three (6.8%) patients demonstrated sensorineural hearing deficit. This hearing impairment was attributed to a cochlear lesion and not to a pontine lesion. We believe that the frequency of sensorineural hearing loss in these patients warrants hearing screening programs similar to those used for infants in neonatal intensive care units.

Central nervous system anomalies associated with meningomyelocele, hydrocephalus, and the Arnold-Chiari malformation: reappraisal of theories regarding the pathogenesis of posterior neural tube closure defects.

Complete gross and microscopic neuropathological examinations of 25 children who died with meningomyelocele, the Arnold-Chiari malformation, and hydrocephalus revealed a wide range and frequency of associated central nervous system malformations. The most remarkable of these anomalies were hypoplasia or aplasia of cranial nerve nuclei (20%), demonstrable obstruction of cerebrospinal fluid flow within the ventricular system (92%), cerebellar dysplasia (72%), a disorder of migration of cortical neurons (92%), fusion of the thalami (16%), agenesis of the corpus callosum (12%), and complete or partial agenesis of the olfactory tract and bulb (8%). The anomalies associated with posterior neural tube closure defects can no longer be considered secondary, but rather must be considered part of a spectrum of malformations caused by an unidentified primary insult to the central nervous system. The frequency and pattern of brain malformations associated with neural tube defects of some children with meningomyelocele suggest that such malformations may seriously affect intellectual outcome.

Prioritizing candidate reproductive/developmental toxicants for evaluation.

To provide a rational method for the timely evaluation of possible reproductive/developmental toxicants, a prioritization scheme was developed by the California Department of Health Services to select chemicals for consideration by the Proposition 65 Scientific Advisory Panel. Initially, four ascertainment methods were used to identify and construct a master list of 164 candidate agents. Using two criteria, the potential for human exposure and the perceived reproductive/developmental hazard as judged by an ad hoc committee of experts, 42 candidates from the master list were identified as priority agents. For practical purposes, the 15 priority agents with the highest rankings will be given the highest priority in the review process. Limitations in the prioritization method used and refinements to be incorporated in an annual update are described.

Autologous cultured fibroblasts as cellular therapy in plastic surgery.

Autologous cultured fibroblasts serve as injectable protein repair systems for correction of acne scars, rhytids, and other facial scars. The system uses the patient's own cultured fibroblasts to correct contour deformities over time.

The fetal alcohol syndrome in mice: maternal variables.

CBA, C3H, and C57 female mice maintained on a diet of 20 percent ethanol-derived calories prior to and throughout gestation were mated in a diallele cross. Prenatal death, malformations, and fetal weights were directly related to maternal blood alcohol levels, indicating a maternal effect. Fetal abnormalities and maternal blood alcohol levels varied with maternal strain (CBA > C3H > C57) and were inversely related to maternal alcohol dehydrogenase activity. Microsomal ethanol oxidizing systems induction was directly associated with increased fetal abnormalities, being greatest in CBA females. These results indicate that liability for the pattern of malformation observed in this syndrome is dependent on maternal blood alcohol levels, which are determined by the rate of maternal alcohol metabolism as well as the amount of maternal alcohol consumption.

Shiverer: an autosomal recessive mutant mouse with myelin deficiency.

Shiverer is an autosomal recessive trait in the mouse characterized by early generalized tremors that become prominent in the hindquarters with age. Seizure behavior begins after weaning and increases in frequency during the animal's shortened lifespan. The most prominent pathological feature is a deficiency of myelin and myelin basic protein in the central nervous system.

The fetal alcohol syndrome in mice: an animal model.

CBA and C3H female mice were maintained on liquid diets--Metrecal plus ethanol--containing 15-35% ethanol-derived calories. These diets, which resulted in alcohol blood levels of 73-398 mg/100 ml blood in nonpregnant females, were the sole sustenance for the females for at least 30 days before and throughout gestation. Females were killed on day 18 of gestation and offspring examined for skeletal and soft tissue anomalies. Prenatal death and maldevelopment increased with the level of alcohol intake. Deficient occiput ossification, neural anomalies, and low fetal weight occurred with low ethanol diets, and cardiac and eye-lid dysmorphology with higher ethanol diets. This pattern of malformations, which exhibited both a dose-response effect and strain differences in susceptibility, indicated that chronic maternal alcoholism is embryolethal and teratogenic in mice.

Issues in regulatory protection of reproductive health in the workplace.

Provisions of federal laws that protect reproductive health in the workplace and information on recent federal actions that seek to enhance such protection are reviewed. California's Birth Defects Prevention Act and its Proposition 65, regulatory programs that specifically address reproductive toxicity, also are described.

Hyperthermia-induced exencephaly in mice: effect of multiple exposures.

Pregnant LM/Bc female mice were given a 10-minute hyperthermic exposure in a 43 degrees C waterbath during the period of neural tube closure. On day 15.5 of gestation, the females were killed, and the fetuses were examined for exencephaly. Following a single treatment on day 8.0, 8.5, 8.75, or 9.0 of gestation 1.7, 13.6, 2.9, and 0.8% of the respective fetuses displayed exencephaly. With two treatments, one on each of gestational days 8.5 and 8.75, or three treatments, one on each of gestational days 8.5, 8.75, and 9.0, the percentage of exencephalic fetuses increased to 28.3 and 59.3%, respectively. The increased response to multiple treatments was not due to an increase in the number of susceptible embryos but rather was due to the increased number of treatments. The results of this study suggest that with increasing numbers of treatments, the embryo's ability to recover from the hyperthermic exposure is lessened, resulting in an increase in exencephaly.

Intermittent pattern of neural tube closure in two strains of mice.

Development of the neural tube is often described as a continuous process that begins in the cervical region of the embryo and proceeds both rostrally and caudally. Examination of neural tube closure in the cranial region of LM/Bc and SWV/Bc mice revealed an intermittent pattern with four distinct areas of closure. Closure I begins at the level of somites 1-3 and proceeds bidirectionally. Closure II is initiated at the prosencephalic-mesencephalic border and also proceeds bidirectionally. Closure III is unidirectional, beginning adjacent to the stomodeum and proceeding caudally to meet closure II. Finally, closure IV takes place over the rhombencephalon where it meets closure II to complete rostral neural tube closure. In these two strains of mice anterior neural tube closure progressed as somite number increased. However, the SWV strain required a longer gestational time to develop equal numbers of somites and therefore to complete closure. In light of the intermittent pattern of closure observed in mice, the development of the rostral nervous system in other mammals, including humans, should be reconsidered.

Mouse fetal hydantoin syndrome: effects of maternal seizures.

To test the effect of maternal seizure disorders on prenatal structural development, the mouse neurological mutant quaking (qk) was used. Administering phenytoin in the drinking water of females homozygous for the quaking allele reduced the frequency of tonic-clonic seizures typical for this mutant from a background rate of 2.06 to 0.34 seizures per mouse day. As the seizure frequency decreased, the percentage of fetuses exhibiting abnormalities associated with the mouse fetal hydantoin syndrome increased from 0 to 77. As in previous studies with the mouse syndrome, the increase in malformations was associated with increased maternal serum phenytoin levels. The results of this study indicate that the maternal serum phenytoin level, and not the maternal seizure disorder, is the etiologic agent responsible for the malformations observed in this syndrome.

Investigation of the teratogenic effects of exercise on pregnancy outcome in mice.

Swiss Webster female mice were arbitrarily assigned to a heavily exercised group (HE), a moderately exercised group (ME), or a sedentary group (S). Exercised groups were subjected to a progressive treadmill training routine, 6 days a week (60 min per day) for a total of 9 wk. Following mating after 6 wk of training, treatment groups continued to exercise at preconceptual intensities. Pregnant mice were sacrificed on the 19th day of pregnancy, and the fetuses were recovered. A positive training effect was demonstrated by a significant increase in succinate dehydrogenase activity in the gastrocnemius muscles of exercising dams. The numbers of implants, resorptions, live fetuses, mean fetal weight, and developmental stage were unaffected by the exercise treatment. A detailed fetal examination revealed no significant skeletal or gross tissue abnormalities in any of the experimental groups.