Figure 2: Points of closure in the mouse embryo and phenotypes of failure of closure of the various points along the neuraxis. (a) Schematic figure illustrating the multiple points of closure of the neural tube, directions of closure, and the different locations of neuropores in the developing embryo. (1), site of closure (1) which occurs at the level of somite 3 in the 6-7-somite embryo. Closure (1) is the initiation event of neurulation. Closure then progresses caudally and is completed by closure of the posterior neuropore (PNP) at the 29-30-somite stage of development; (2), second closure site at around the 10-somite stage; (3), closure (3) site which begins soon after closure (2). Arrows depict spreading of neural tube closure to neighbouring regions with completion of anterior neuropore closure soon after initiation of closure (3) and closure of the hindbrain neuropore at the 18–20-somite stage. (b) Phenotype resulting from failure of closure (1): craniorachischisis; (c) phenotype resulting from failure of closure (2): exencephaly; (d) phenotype of failure of the caudal wave of spinal closure, leading to an enlarged PNP and later development of spina bifida. (A), posterior neuropore; (B), branchial arches; (C), developing heart; (D), hindbrain; (E), midbrain; (F), forebrain; ANP: anterior neuropore; HNP: hindbrain neuropore.