Scanning-electron-microscopic studies on the pathogenesis of exencephaly and cranioschisis induced in the rat after neural tube closure

Etiology, pathogenesis and prevention of neural tube defects

Spina bifida, anencephaly, and encephalocele are commonly grouped together and termed neural tube defects (NTD). Failure of closure of the neural tube during development results in anencephaly or spina bifida aperta but encephaloceles are possibly post-closure defects. NTD are associated with a number of other central nervous system (CNS) and non-neural malformations. Racial, geographic and seasonal variations seem to affect their incidence. Etiology of NTD is unknown. Most of the non-syndromic NTD are of multifactorial origin. Recent in vitro and in vivo studies have highlighted the molecular mechanisms of neurulation in vertebrates but the morphologic development of human neural tube is poorly understood. A multisite closure theory, extrapolated directly from mouse experiments highlighted the clinical relevance of closure mechanisms to human NTD. Animal models, such as circle tail, curly tail, loop tail, shrm and numerous knockouts provide some insight into the mechanisms of NTD. Also available in the literature are a plethora of chemically induced preclosure and a few post-closure models of NTD, which highlight the fact that CNS malformations are of hetergeneitic nature. No Mendelian pattern of inheritance has been reported. Association with single gene defects, enhanced recurrence risk among siblings, and a higher frequency in twins than in singletons indicate the presence of a strong genetic contribution to the etiology of NTD. Non-availability of families with a significant number of NTD cases makes research into genetic causation of NTD difficult. Case reports and epidemiologic studies have implicated a number of chemicals, widely differing therapeutic drugs, environmental contaminants, pollutants, infectious agents, and solvents. Maternal hyperthermia, use of valproate by epileptic women during pregnancy, deficiency and excess of certain nutrients and chronic maternal diseases (e.g. diabetes mellitus) are reported to cause a manifold increase in the incidence of NTD. A host of suspected teratogens are also available in the literature. The UK and Hungarian studies showed that periconceptional supplementation of women with folate (FA) reduces significantly both the first occurrence and recurrence of NTD in the offspring. This led to mandatory periconceptional FA supplementation in a number of countries. Encouraged by the results of clinical studies, numerous laboratory investigations focused on the genes involved in the FA, vitamin B12 and homocysteine metabolism during neural tube development. As of today no clinical or experimental study has provided unequivocal evidence for a definitive role for any of these genes in the causation of NTD suggesting that a multitude of genes, growth factors and receptors interact in controlling neural tube development by yet unknown mechanisms. Future studies must address issues of gene-gene, gene-nutrient and gene-environment interactions in the pathogenesis of NTD.

The two sites of fusion of the neural folds and the two neuropores in the human embryo

BACKGROUND

Since reports on a pattern of multiple sites of fusion of the neural folds in the mouse appeared, it has been widely assumed that a similar pattern must be valid for the human. In the absence of embryological evidence, claims have been made that such a pattern can be discerned by classifying neural tube defects.

METHODS

The neural folds and tube, as well as the neuropores, were reassessed in 98 human embryos of Stages 8-13; 61 were controlled by precise graphic reconstructions.

RESULTS

Careful study of an extensive series of staged human embryos shows that two de novo sites of fusion of the neural folds appear in succession: alpha in the rhombencephalic region and beta in the prosencephalic region, adjacent to the chiasmatic plate. Fusion from Site alpha proceeds bidirectionally (rostrad and caudad), whereas that from beta is unidirectional (caudad only). The fusions terminate in neuropores, of which there are two: rostral and caudal. Highly variable accessory loci of fusion, without positional stability and of unknown frequency, may be encountered in Stage 10 but seemingly not later, and their existence has been known for more than half a century.

CONCLUSIONS

Two sites of fusion (a term preferred to closure) of the neural folds and two neuropores are found in the human embryo. No convincing embryological evidence of a pattern of multiple sites of fusion, such as has been described in the mouse, is available for the human. The construction of embryological details from information derived from other species or from the examination of later anomalies is liable to error. Neural tube defects are reviewed and although they have been considered on the basis of five, four, or three sites of fusion, interpretations based on two sites can as readily be envisaged.

Induction of cytokine expression in the brain macrophages/amoeboid microglia of the fetal rat exposed to a teratogen

Using in situ hybridization, RT-PCR, lectin histochemistry and immunohistochemistry, this study examined the time course expression and cellular localization of various cytokines including tumor necrosis factor-alpha (TNF-alpha), interleukins (IL-1, IL-6) and transforming growth factor-beta (TGF-beta) in fetal rat brain after a maternal injection of the teratogen cyclophosphamide (CP). Eight hours after CP injection, there was a marked increase in brain macrophages (BM)/amoeboid microglia (AM) in different areas of the fetal brain as determined by lectin histochemistry. Concomitant to this was the induction in mRNA level of TNF-alpha, which was progressively increased with time. TGF-beta mRNA was undetectable until 24 h had elapsed. Expression of IL-1 and IL-6 was undetectable at all stages. In situ hybridization combined with immunohistochemistry has shown the localization of TNF-alpha in BM/AM and neurons. Present results suggest that both TNF-alpha and TGF-beta are involved in the progression of neural damage in the fetal brain induced by the teratogen.

Apparent lability of neural tube closure in laboratory animals and humans

Neural tube defects (NTDs), a set of structural abnormalities affecting the brain, spinal cord, and the skeletal and connective tissues that protect them, are common malformations among humans and laboratory animals. The embryogenesis of the neural tube is presented to convey the complexity of the phenomenon, the multiplicity of requisite cellular and subcellular processes, and the precise timing of events that must occur for successful neural tube development. Interruption, even transitory, of any of these intricate processes or disruption of an embryo's developmental schedule can lead to an NTD. The population distribution of human NTDs demonstrates that genetic predisposition functions in susceptibility to NTDs. Data from animal studies support these concepts. NTDs are common outcomes in developmental toxicity safety assessments, occurring among control and treated groups. Numerous agents have caused increased levels of NTDs in laboratory animals, and species with shorter gestational periods appear more prone to toxicant-induced NTDs than those with longer gestations. Data from post-implantation whole embryo culture, although not predictive of human risk, are useful in studying neurulation mechanisms and in demonstrating the importance of maintaining embryonic schedules of development. We conclude that the concept that NTDs are produced by only a few toxicants that selectively target the developing nervous system is untenable. Rather, the combination of the time in gestation that an agent is applied, its dose, and its ability to disrupt critical processes in neurulation leads to NTDs. We further conclude that, because of both the relatively high prevalence and the multifactorial nature of NTDs, the mere occurrence of an NTD is insufficient for inferring that the defect was caused by an exogenous agent.

Chlorambucil-induced postclosure exencephaly and axial skeletal abnormalities in rat fetuses

Open neural tube defects (NTD) are reported to arise from failure of the embryonic neural folds to close or rupture of a previously closed neural tube. The critical period for dysmorphogenesis of the neural tube by either mechanism is of clinical importance. We had previously reported that single doses of cyclophosphamide (CPA), administered to pregnant rats resulted in reopening of the closed neural tube. The objective of the present study was to determine if other antineoplastic drugs also had similar ability to cause rupture of the closed neural tube. Therefore, single doses of chlorambucil dissolved in bicarbonate buffer were administered to Wistar rats on one of gestation days (GD) 11 through 14 (i.e., well after closure of the neural tube) and fetuses were examined on GD 20. It was observed that the window of susceptibility extended from GD 11 through 14 and that a single dose of 10 mg/kg of the drug was most effective. The affected fetuses had the prosencephalon and mesencephalon parts of the brain protruding through a wide opening of the cranial roof. The exposed brain tissue was hemorrhagic and covered by a thin, transparent and often porous membrane. The cranial vault was missing. Numerous malformations of the axial skeleton and of several nonneural organs were found to accompany the brain defect. Electron microscopic and light microscopic studies revealed extensive cell death, fragmentation, and phagocytosis of dead cells and vacuolization of the neuroepithelium (NE) within 12 h of the drug treatment. Cell death per se was not pronounced in the cranial mesenchyme (ME). The vacuoles in the NE coalesced into small cavities. The ME failed to proliferate adequately and organize itself into the cranial vault primordium. Hemorrhage and disorganization of the NE was progressive. The fluid that escaped into the interstitium seemed to extend into the cysts that developed in the ME externally and into the ventricular system internally. The edema of the ME thus might have contributed to disruption of precursor mesenchymal tissue and consequently to malformation of the cranial bones. Large hematomas and cysts also developed on the basal aspect of the crumbling brain vesicles and appeared to lift the neural tissue out of the shallow cranial cavity. As previously reported for CPA, these data provide evidence for the ability of chlorambucil to cause postclosure exencephaly in rat fetuses for a susceptible period that is considerably long.