"Neuroschisis" in human embryos

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.

Development of anencephaly and its variants

Extreme variants of anencephaly in two human embryos of the same stage, namely 22 (54 days), shed new light on problems such as craniocerebral interrelationships and the timing of developmental events. Embryo X had a chondrocranium that possessed features typical of a holoacranial anencephalic skull and an extremely well-preserved brain, in which some of the neural tracts were comparable to those in a normal control. On the other hand, embryo Y of the same stage had a completely degenerated brain, although the chondrocranium was more nearly normal and represented the precursor of a meroacranial skull. A comparison of the two cases seems to indicate a certain independence between skull and brain. Moreover, it appears possible that the disturbances are related primarily to the skeletal, and only secondarily to the nervous, component. Comparisons with experimental data allow the conclusion that the maldevelopment involves mostly paraxial mesenchyme and little or no disturbance of neural crest. The timing of the mesenchymal defect is probably as early as stages 8 and 9 (18-20 days). This is also the time at which mesenchymal defects can result in failure of the neural tube to close.