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 mult-site 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. We conclude that (1) multi-site NT closure provides the best explanation for NTDs in humans; (2) closure sites are most likely controlled by separate genes expressed during embryogenesis, and variations in rate and location of closures would make embryos more susceptible to environmental and other factors; (3) homologies between the mouse and human genomes may allow linkage studies to be done for families with recurrence of NTDs; and (4) recurrence risks and associated anomalies vary between closure sites, so that more accurate genetic counselling can be given based on the location of the NTD(s) in the proband(s). © 1993 Wiley-Liss, Inc.