neural tube closure

Four separate initiation sites for neural tube (NT) fusion have been demonstrated recently in mice and other experimental animals.

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, the majority of NTDs can be explained by failure of fusion of one of the closures or their contiguous neuropores.

It has been hypothesized 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:
 familial NTDs with Sikh heritage (closure 4 and rostral 1)
 Meckel-Gruber syndrome (closure 4)
 Walker-Warburg syndrome (2-4 neuropore, closure 4).

Environmental and teratogenic exposures frequently affect specific closure sites:

  folate deficiency: closures 2, 4, and caudal 1
 valproic acid: closure 5 and canalization

References

 Van Allen MI.Multisite neural tube closure in humans.Birth Defects Orig Artic Ser. 1996;30(1):203-25. PMID: 19125329

 Van Allen MI, Kalousek DK, Chernoff GF, Juriloff D, Harris M, McGillivray BC, Yong SL, Langlois S, MacLeod PM, Chitayat D, et al. Evidence for multi-site closure of the neural tube in humans. Am J Med Genet. 1993 Oct 1;47(5):723-43. PMID: 8267004