Figure 1. Multisite closure of the neural tube in the mouse embryo. Schematic summary of the successive initiation events of mouse neurulation (Closures 1, 2 and 3), the direction of spread of closure from the initiation sites (solid arrows) and the sites of completion of closure (anterior, hindbrain and posterior neuropores). The tail bud region (red shading) is the site of secondary neurulation which follows immediately after closure of the posterior neuropore. The main types of neural tube defect that result from failure of the different components of neurulation are indicated by the dotted arrows. Modified from Copp AJ and Bernfield M (1994) Etiology and pathogenesis of human neural tube defects: Insights from mouse models. Current Opinion Pediatrics 6, 624–631.

Figure 2. An embryological classification of neural tube defects. See text for explanation.

Figure 3. The spectrum of human and mouse neural tube defects. (a) myelomeningocele in a human neonate; (b) myelocele extending from thoracic to sacral levels in a neonate; (c,d) craniorachischisis in a human fetus from side (c) and back view (d). Most of the brain and spine are open; (e) hydrocephalus, with marked expansion of the cranial vault; (f) occipital encephalocele in which a large portion of the brain, contained within a meningeal sac, has herniated through a defect in the posterior part of the skull; (g) hairy skin lesion overlying an asymptomatic spina bifida occulta, which could only be detected by abdominal radiography; (h) exencephaly (arrow) and open spina bifida (arrowhead) in an E15.5 mouse embryo homozygous for the curly tail mutation; (i) open spina bifida in an E18.5 curly tail; Vangl2 double mutant mouse fetus. Note the close similarity between this mouse defect and human myelocele in (b); (j) craniorachischisis in an E15.5 mouse fetus homozygous for a mutation of Celsr1. The neural tube is open between the arrows, affecting most of the brain and the entire spine; Modified from: (a,c–e) Copp AJ (2005) In: Meyers RA (ed.), Encyclopedia of Molecular Cell Biology and Molecular Medicine 9, 119–138. Wiley-VCH: Weinheim; (h,j) Copp AJ, Greene NDE and Murdoch JN (2003) The genetic basis of mammalian neurulation. Nature Reviews. Genetics 4, 784–793; (i) Stiefel D, Meuli M, Duffy P and Copp AJ (2003) Tethering of the spinal cord in mouse fetuses and neonates with spina bifida. Journal of Neurosurgery (Spine) 99, 206–213.

Figure 4. Normal and abnormal initiation of neurulation (Closure 1) in the mouse embryo. Scanning electron microscopy of: (a) whole E8.5 embryo demonstrating the approaching neural folds approximately half way along the body axis, indicating the incipient Closure 1 event. This event occurs at the boundary of the hindbrain and cervical regions. (b,c) Sections transverse to the body axis as indicated by the red line in (a). A wild-type embryo (b) shows bending at a compact midline hinge point, and straight lateral neural folds. A homozygous loop-tail (Vangl2) mutant embryo (c) shows disturbance of the midline which is broader than normal, without a compact bend in the neural plate. Although the neural folds elevate normally, they are not able to appose and fuse in the midline, leading to craniorachischisis in mutant embryos. Abbreviations: am, amnion; hnf, hindbrain neural folds; tnf, thoracic neural folds and ys, yolk sac. Modified from: (a) Copp AJ, Brook FA, Estibeiro JP, Shum ASW and Cockroft DL (1990) The embryonic development of mammalian neural tube defects. Progress in Neurobiology 35, 363–403; (b,c) Greene NDE, Gerrelli D, Van Straaten HWM and Copp AJ (1998) Abnormalities of floor plate, notochord and somite differentiation in the loop-tail (Lp) mouse: a model of severe neural tube defects. Mechanisms of Development 73, 59–72.

Figure 5. Molecular regulation of dorsolateral bending in mouse neural tube closure. The morphology of neural tube closure changes from (a) high spinal to (b) low spinal regions of the mouse embryo. In the upper spine, the neural plate bends only in the midline, at the median hinge point (MHP), whereas in the low spine bending occurs at dorsolateral hinge points (DLHPs). (c) Summary of the molecular interactions regulating DLHP formation. In the upper spine, DLHPs are absent because of unopposed inhibition by BMP2. Although transcription of Noggin is stimulated by BMP2 at all levels of the body axis, Shh expression from the notochord is strong in the upper spine, inhibiting Noggin expression. In the lower spine, Shh influence is reduced, Noggin expression is deinhibited and antagonizes the inhibitory influence of BMP2, allowing DLHPs to form. Yellow triangles: MHP; red triangles: DLHPs; green arrows: stimulatory interactions; red lines: inhibitory interactions and dashed lines: inactive influences. (c) Modified from Ybot-Gonzalez et al. (2007a).

Figure 6. Cellular and morphogenetic processes required for neural tube closure in the future brain. Cranial neural folds in the midbrain of the mouse embryo elevate with a biconvex morphology (a). Subsequently, dorsolateral bending occurs (b), causing the tips of the neural folds to converge on the midline, ensuring fusion and completion of cranial neurulation. Mesodermal expansion appears the most important morphogenetic factor in the initial elevation of the neural folds (a). At later stages, cranial neurulation requires additional processes including a functional actin cytoskeleton, successful initiation of neural crest emigration, precisely regulated programmed cell death and continuation of regulated neuroepithelial cell proliferation, with postponement of neuronal differentiation until after closure is complete (b). Defects in each of these processes is seen in one or more of the gene knockout mice with NTDs. In each case, cranial neural tube closure fails and embryos develop exencephaly/anencephaly. Modified from Copp AJ, Greene NDE and Murdoch JN (2003) The genetic basis of mammalian neurulation. Nature Reviews. Genetics 4, 784–793.